[Springer Series in Materials Science] Magneto-Science Volume 89 || Dynamic Spin Chemistry

6 Dynamic Spin Chemistry

The field of dynamic spin chemistry, founded in the 1970s, is the most established area in magneto-science. In dynamic spin chemistry, a magnetic field affects the transitions of electron spin states of a short-lived reaction intermediate, called a radical pair, resulting in intermediate lifetime and product yield changes. The magnetic phenomena are interpreted semi-quantitatively or quantitatively in terms of quantum chemistry. The silent feature of spin chemistry is that magnetic fields ranging from a very weak field of ca. 0.0001 T to a very high field of ca. 30 T significantly affect chemical processes in different ways.

In this chapter, MFEs on photochemical reactions, photo-electrochemical reactions and spin-dependent processes in crystals and related phenomena are described.

6.1 Magnetic Field Effects in Photochemical Reactions

Magnetic field effects (MFEs) have attracted much attention from many scientists since studies of MFEs on chemical reactions began in the 1970s. ^ The effects have been explained in terms of the radical pair (RP) mechanism by which the spin dynamics of a short-lived radical intermediate is affected by an external magnetic field. Most of the studies have been carried out in relatively low magnetic fields (< ca. 1 T). Recently, however, using a newly developed technique easily generating high magnetic fields, many interesting MFEs at room temperature have been observed in a high magnetic field range of up to 30 T. ^ As a result, a new phenomenon of reversal of MFE has been observed and interpreted by the spin-lattice relaxation mechanism. Since the studies carried out in low magnetic fields have previously been described in the literature, ^ several investigations of a new effect observed in high magnetic fields (1-14 T) on the primary photochemical process in solution, which have recently been conducted by the author's group, is mainly introduced in this chapter.

250 6 Dynamic Spin Chemistry

6.1.1 Theory

A theory very often utilized for explaining MFEs on photochemical reactions'^^ is called the RP mechanism. The MFE in the RP mechanism is usually explained by interplay between two electron spins of radical intermediates generated by photochemical reactions and an external magnetic field. After the generation, the RP populates on two electron spin states of singlet (S) and triplet (T), where the latter splits into three sublevels, T+, To and T_, in the presence of a magnetic field (Fig. 6-1-1). The initial population between them depends on the reaction and reactional condition. Once it is generated, the RP participates in the S-T spin interconversion followed by deactivating toward the starting material and final products. In such a RP deactivation scheme, the S-T spin interconversion depends on the external magnetic field, resulting in the appearance of an MFE on the radical deactivation rate and/or product yield in photochemical reactions. This is a simple explanation of the RP mechanism.

As shown in Fig. 6-1-1, however, the energy gap, 27, between the S and T states arising from the exchange interaction between the two electron spins influences the S-T spin interconversion. Whereas the RP cannot undergo the S-T spin interconversion because of the large 27 immediately after generation, it can undergo the interconversion due to 27 ~ 0 after the diffusion of two spins. Therefore, significant MFEs are

RP generation by photochemical reaction

A* + •B

Escape radicals

1 A-B,A-A, B-B

Escape products A-B.A-B'

Cage products

Inter-radical distance, R

Fig.6-1-1 Energies of singlet (S) and triplet (T+. To, T„) states of a RP consisting of radicals A» and -B and their pathways in solution. 1/ < 0 is assumed.

6.1 Magnetic Field Effects in Photochemical Reactions 251

observed when 2 7 - 0 where the two spins retain sufficient inter-radical distance. In the 2 7 - 0 condition, the S-T spin interconversion is governed by the magnetic field-dependent submechanisms of isotropic hyperfine coupling (HFC), S-T_ level crossing, isotropic A^, and relaxation mechanisms.'^^ In the HFC mechanism MFE is based on inhibition of the HFC-determined intersystem crossing (ISC) between S and T states by the Zeeman splitting of the T sublevels. In the S-T_ level crossing mechanism the MFE appears by acceleration of the so far ineffective S-T spin interconversion due to completely non-zero 27 by S and T_ energy level matching upon the Zeeman splitting. In the isotropic Ag mechanism MFE is caused by the enhanced S-To ISC resulting from the magnetic field-accelerated dephasing of the two electron spins in the precessing vector model.

6.1.2 Relaxation Mechanism

In high magnetic fields beyond about 100 mT, spin-lattice relaxation (SLR) of the RP becomes important as a RP deactivation process.'"*^ In the case of RPs made of light elements of hydrogen, carbon, nitrogen, etc., three magnetic field-dependent interactions, namely, anisotropic hyperfine (6hf) interaction, anisotropic Zeeman (5g) interaction, and electron dipole-dipole (dd) interaction, are generally associated with SLR. Each of these interactions affects the RP deactivation with increasing magnetic field, causing MFEs. The two former interactions are operative independently at each independent radical, whereas the latter functions between the two electrons on the component radicals. Based on the selection rule, both 5hf and 6g interactions induce all four SLR processes of T+-To, T_-To, T+-S, and T--S, whereas the dd interaction only induces the two SLR processes of T+-To and T--To. Therefore, under the assumption that To-S ISC is fast when 27 ~ 0, the RP decay rate constant (ICR?) (a reciprocal of the RP lifetime TRP) is generally expressed as a combination of the rate constant (/:(SLR)) due to this relaxation mechanism and the rate constant for other magnetic field-independent processes as

fep=l/TRP=i^(SLR) + T=(l/2)i{)t,(6hf) + A:/(8g)} + (dd) + /:T ( 0

Here, ki{5hf) and A:,(6g) denote SLR rate constants due to the 5hf and 5g interactions operating on each radical /, respectively, ^(dd) the SLR rate constant due to the dd interaction operating between radicals 1 and 2, and kj the rate constant of magnetic field-independent deactivation processes such as the escape process from a micellar cage.

ExpUcit expressions for A:/(5hf), ^/(5g), and )t(dd) are, respectively,'"^^

k{5hf) = y'Hôcrr,J(\ + coh,r) (2)

ki(5g) = 5-\2Kyh-Hg':g').P'Bh../i\-^coh.r) (3)


/:(dd) = / ' / /dd 'Tdd / (1 + CO'TM' ) (4)

where y denotes the magnetogyric ratio of an electron, (g': g') an anisotropic g-tensor of a radical, and H\oc and Hdd locally fluctuating magnetic fields yielded by the 5hf and dd interactions, respectively. TC, and CO represent the rotational correlation time for the tumbling Brownian motion of a radical and the Larmor frequency, respectively.

As for the 5hf interaction, H\oc includes an anisotropic hyperfine tensor and thereby depends on the kind and number of related nuclear spins. This is the origin of magnetic isotope effects (MIEs) frequently observed in high magnetic fields. Further, since co = yB, ki(Shf) decreases upon an increase in B. Hence, the RP controlled by this interaction increases its lifetime as B increases. In general, this phenomenon is known to appear when ^ = 0.1 -2 T.

As for the 5g interaction, the effect depends on the term (g' : g') whose origin is the spin-orbit-coupling (SOC) interaction of an unpaired electron on a radical. It is theoretically suggested that the effect is larger when an electron is present on a heavier atom, e.g., carbon < nitrogen < oxygen. Contrary to the 5hf interaction, the RP governed by this interaction decreases its lifetime as B increases, since there are B terms not only in the denominator (co = yB) but also in the numerator on the right-hand side of Eq. (3). It has been found that the lifetime decrease tends to appear in magnetic fields higher than about 2 T where this interaction replaces the 5hf and dd interactions in determining the RP lifetime.

Lastly, as for the dd interaction, Hdd can be rewritten using an inter-radical distance R as

//dd' =(3/10)((2;r)--/2'Y' <^"' >) (5)

where <R~^> is the mean value of R'^. This relation indicates that the dd interaction is dependent on R and plays an important role in SLR, especially in the case of a small R. Like the 5hf interaction, the RP dominated by this interaction increases its lifetime as B increases. This interaction is effective up to ca. 2 T. The distinction between the dd and 6hf interactions is usually difficult, judging from the experimental point of view. Only magnetic isotope substitution makes it possible since there is no MIE in the dd interaction.

6.1.3 MFEs Controlled by the RP Mechanism

A. MFEs of Chain-linked Biradicals in Homogeneous Solution a. Triplet BRs MFEs on photochemical reactions have been studied widely in homogenous and micellar solutions. However, a biradical (BR) where two radicals are linked by flexible chemical bonds is often used since the linkage causes a large MFE by suppressing dissociation of the radicals.


An interesting example among MFEs on the distribution of reaction products is presented here. Usually, MFEs are studied by monitoring the effect on reaction quantum yield and/or a lifetime (a rate constant) in which the former has a practical aspect of application of the MFEs to syntheses. In a photo-redox reaction in methylene-chain-linked molecules (4-02N-CioH6-0-(CH2).-NHC6H5, AT = 8-12) containing nitronaphthoxy and anilino groups/' ^ the yield of escape product via triplet BR ( BR) of n = 8 increases from 39% to 63% in the presence of 0.64 T, whereas that of cage product via singlet BR ('BR) decreases from 61% to 37%, indicating a dominant reaction pathway switched by an external magnetic field. When n ^ 10, the yield of cage product predominates in the presence and absence of the field, i) Reversal of MFEs As for MFEs on BR lifetimes, many " BRs have been investigated in homogenous solutions.' " About ten years ago, a high magnetic field beyond 10 T drew attention for exploring new MFEs. In ''BR of the anthrasemiquinone radical (AQH-)-C02-(CH2)i2-OCO-xanthenyl radical (X-) generated by photo-induced hydrogen abstraction of anthraquinone (AQ) from xanthene (X) in AQ-C02-(CH2)i2-OCO-X,'^ the BR lifetime increases steeply from 0.2 jis at 0 T to a maximum lifetime of 4.0 |LIS at 2 T, then decreases gradually to a lifetime of 3.2 |LIS at the highest magnetic field of 14 T. This new MFE, a reversal of MFE, occurs at ca. 2 T. This is the first observation of a reversal of MFE in a high magnetic field. The MFE up to ca. 2 T can be explained by the HFC mechanism and the 5hf-and dd-induced relaxation mechanisms, while the MFE observed above 2 T can be explained in terms of the 5g-induced relaxation mechanism which accelerates the SLR among spin sublevels by applying a magnetic field of a few tesla. From the analysis based on the 6g mechanism, the correlation time has been estimated to be a few picoseconds.

In order to clarify the mechanism of the reversal of MFE, other BRs showing a large decrease in lifetime have been sought. ^BR of the phenothiazine radical cation (Ph-^)-(CH2),rviologen radical cation (V- ) generated by photo-induced electron transfer from Ph to V ^ in a rotaxane-type a-cyclodextrin inclusion complex of Ph-CCH:) -V^ has exhibited the most significant reversal of MFE.^^^ Fig. 6-1-2 shows the magnetic field dependence (MFD) of the BR lifetime. The lifetimes of n= 12 are 0.14 |LIS (0 T), 6.6 lis (1 T), and 2.2 |is (13 T). The lifetime at 13 T is 33% of that at 1 T. As a result of simulation with Eq. (1) using parameters available so far, it has been found that the magnitude of both the anisotropic g-value, (g' : g'), and the correlation time TC is important for reproducing the reversal. The observed MFD of the lifetime has been well reproduced when ( / : g') = 10" and TC = 1 ps. This short correlation time of 1 ps or so is the key parameter for reproducing the reversal of MFE as well as the case of AQH-C02-(CH2)i2-OCO-X-. On the other hand, the carbazole


8 ,

B

^ 4

n=8 '1=10

6h

2 CQ

_ _ _ j ^ , • . ^ .

5 10 15 0 5 10 15

Magnetic field / T Magnetic field / T

•+ 1 4 ? • • ^ S N-CH.CH,) -CH:-N'V<fN-CH:CH,CH3

OQ I Ph»*-(CH2VV«*(Ai=8,10,12)

oi ^ 0 5 10 15

Magnetic field / T

Fig.6-1-2 Effects of high magnetic field on the BR lifetimes of a-cyclodextrin complexes of Ph-*-(CH2)„-V-^ in water.

radical cation (Cz-'')-(CH2)i2-V-^ does not exhibit such a large reversal, directly indicating that the moiety of phenothiazine of Ph-^-(CH2);,-V-'' plays an important role in the extent of reversal. Very recently, the contribution of the 5g interactions due to Ph- in Ph- -(CH2).-V- and V-"" in Cz-^-(CH2)i2-V-'' has been verified with calculated anisotropic g- and hf-tensors by the ab initio molecular calculation program Gaussian03W. ^ The origin of the short correlation time is not very clear at present. However, a very similar reversal of MFE has been observed not only in " BRs in homogenous fluid solution but also in many ^RPs in micellar solution. The micro-environment surrounding BRs in homogenous fluid solution should be very different from that surrounding RPs in micelles. Therefore, it seems most reasonable to consider local motion such as torsion around a C-N bond linking an alkane to phenothiazine as the origin of the short correlation time. Chain length (AI = 8, 10 and 12) dependence is also observed in Fig. 6-1-2. As the length decreases, the lifetimes in the absence and presence of the magnetic field increase and decrease, respectively.^^ The increase is interpreted in terms of deceleration of ISC caused by the exchange interaction of 27 of BR in close proximity. The decrease is explained by enhancement in both the SOC-induced recombination to Ph-n-W^^ and the dd-induced interaction in SLR. Similar chain length


dependence has been observed in the xanthone ketyl radical (X0H-)-C02-(CH2)„-OCO-X-'' '^ and benzophenone ketyl radical (BPH-)-0-(CH2).-0-diphenylaminyl radical (DPA-).'^^ In XOH-C02-(CH2)„-OCO-X- (n = 3, 5, 6, 8, 12 and 16), moreover, chain length dependence in the high magnetic field above ca. 2 T has been observed. The BR lifetimes remain constant and short regardless of the field in shorter chains of z = 3 and 5, whereas in longer chains of n = 6, 8 12 and 16 the lifetimes are longer than in the short chains, and show an apparent decrease with increasing magnetic field. This difference has been explained by the degree of contributions of the SOC-induced recombination and the 8g-induced SLR. In the shorter chain the thereby enlarged SOC-induced recombination, which is magnetic field-independent, is superior to that of the 5g-induced SLR, while the 5g-induced SLR operates effectively in place of SOC weakened due to the long distance between the component radicals. The chain length dependence has also been measured in BPH-0-(CH2);,-0- DPA- (n = 2, 4, 8 and 16). In n = 4, 8 and 16 the BR lifetime in each chain commonly shows a steep increase due to both the HFC and 6hf-and dd-induced SLR mechanisms followed by a gentle decrease due to the 5g-induced SLR mechanism. However, the lifetime in the shortest chain of n = 2 shows a decrease with a shallow dip at around 2 T. This dip has been attributed to the S-T_ level crossing.

Furthermore, recently, an MFE based on a new mechanism has been measured in photo-induced intramolecular electron transfer reaction from zinc tetraphenylporphyrin (ZnP) to viologen in ZnP-0-(CH2)n-V^^ (n = 4, 6 and 8). ^ By reducing the chain length connecting the component radicals, the BR lifetime lengthens owing to the appearance of 27, as noted above, keeping a single component. In the shortest chain BR of ZnP-^-0-(CH2)4-V- , however, the lifetime apparently consists of two components above about 5 T, the shorter lifetime of which is dependent on the magnetic field (310 ns at 7.6 T decreased to 140 ns at 13 T.) whereas the longer one is independent of it. These intriguing MFEs for the two lifetimes have been explained in terms of the 5g interaction in both SLR and the spin-spin relaxation in the high magnetic field. That is, the MFEs have been interpreted by the SLR mechanism adding MFD of the 8g-induced spin-spin relaxation in the To-S transition decelerated as a result of the appreciable To-S energetic separation due to the exchange interaction in the short chain of n = 4. This is the first interpretation for this unique MFE. ii) Magnetic isotope effects in high magnetic fields Magnetic isotope effects (MIEs) on the BR lifetime in the high magnetic field have been examined in detail through investigations of BRs generated from benzophenone (BP)-0-(CH2).-0-benzhydrol (BH), BP-C02-(CH2)i2-0-diphenylmethane (DPM), BP-C02-(CH2)i2-0-phenylethanol (PE), and xanthone (XO)-C02-(CH2),2-OCO-XH. The ^BR of BPH-0-(CH2).-0-BPH- generated by photo-induced hydrogen abstraction of BP from BH in


OH

10 (a)

0(CH:)„0-

BPH«-0-(CH:)„-0-BPH» (/7=3,4,6,7,8,10,12)

«tf*»SSg8g|

10 (b)

®© ^

4 6 8 10 12 Magnetic field / T

14 1 2

Magnetic field / T

Fig.6-1-3 Magnetic field and isotope effects on the lifetimes of BR composed of two BPH-s. '^C-BPH-0-(CH2),2-0-BPH-'-C ( # ) , '-C-BPH-0-(CH2)i2-0-BPH-''C(#), ' 'C-BPH-O-(CH2)i2-0-BPH-"C (O)- (a) High field region, (b) Low field region.

BP-0-(CH2).-0-BH is composed of two equivalent BPH-. This BR is suitable for analysis of MIEs since the theoretical analysis of the lifetimes becomes simple by reducing the number of parameters. Fig. 6-1-3 shows the MFD of the BR lifetimes of isotope-substituted BPH.-0-(CH2)i2-0-BPH- in benzene. ^ The MIEs are observed in BPH- with '^C-substitution at a benzylic carbon below 4 T, whereas no MIEs are present above 4 T or at 0 T. The lifetimes of the BR whose hydrogen at phenyl rings are substituted by ^H exhibit MFD very similar to that for '^C-BPH-O-(CH2)i2-0-BPH.-'^C. It is notable that the MIEs are observed in much higher fields compared to RPs in micellar solution (< 2 T).

The MIEs in 0.1-4 T can be explained by the 6hf-induced relaxation mechanism, where the SLR rates among spin sublevels are influenced by the anisotropic HFC interaction. It should be noted that this mechanism is different from the conventional MIE in which S-T ISC in BR is influenced by the isotropic HFC. Analogous MIEs are obtained for BRs generated from BP-C02-(CH2)i2-0-DPM and BP-C02-(CH2)i2-0-PE.' ' The most remarkable effect of 70-80% is observed in 0.2-0.7 T between the lifetimes of ''C-BPH.-C02-(CH2)i2-0-DPM.-''C and ^^C-BPH.-C02-(CH2)i2-0-DPM.-'^C. Furthermore, by applying Eqs. (l)-(5) to the MIE data, contributions of the 6hf and dd interactions can be evaluated separately. As a result, the values of//ioc(' C)^-//ioc(' C)% TC, Hdd' and Tdd for BPH.-0-(CH2)i2-0-BPH- have been obtained to be 0.69 mT^ 8.1 ps, 4.7 mT, and 25 ps, respectively. The correlation time for the 5hf interaction


of BPH- is about 8.1 ps whereas that for the dd interaction is 25 ps, from which the radical-radical distance is estimated to be about 1.3 nm. These are reasonable from the consideration of their molecular sizes. To the best of our knowledge, this is the first time that two correlation times have been obtained separately from the MFD of the BR lifetimes. The values for diphenylmethyl radical (DPM-) and hydroxyphenylmethyl radical (PE-) have also been obtained to be 10 and 12 ps, respectively.^^ The TC for BPH% DPM-, and PE- is around 10 ps, whereas that for XOH- is 20 ps. ^ The TC value increases with molecular size, in accordance with the expectation from the conventional Stokes-Einstein-Debye equation of TC = {AI?>)Kphl{kT) where r is the radius of a molecule, b. Singlet BRs Systematic studies have been carried out on phenanthrene (Phen) and A ,A -dimethylaniline (DMA) chain-linked compounds.'^^ Upon excitation of Phen, an electron transfer from DMA to Phen occurs to form singlet ionic BR (*IBR), which is equilibrated with fluorescent singlet exciplex. Magnetic fields operate on the spin interconversion from 'IBR to the triplet IBR (ÎBR) to affect the fluorescence intensity and lifetimes of the exciplex. Also, the solvent polarity influences the magnitude of MFE by changing the mutual stability of IBR and exciplex in the solvent polarity. The equilibrium between them shifts to 'IBR in polar solvent, resulting in significant MFE.

Figure 6-1-4 shows the MFD of the exciplex fluorescence intensity ratio /B//O in the Phen-(CH2)n-0-(CH2)2-DMA system in DMF, where h and /o are the fluorescence intensities in the presence and absence of magnetic field B, respectively.*^ *' For short-chain molecules small dips are observed in very low fields. A dramatic increase in the ratio occurs in low fields (~ 0.2 T) then decreases gradually (up to 9 T). The fluorescence mean lifetimes exhibit analogous MFD.

While the MFE in which the fluorescence intensity and lifetime increase in low fields is attributable to the isotropic HFC mechanism, the dips are attributed to the S-T_ level crossing. From the magnetic fields of dips, the S-To energy gaps (|2y|) are obtained to be a large value of 180 mT at short length n = 4 and a small one of 7.7 mT at long length /i = 8, as expected.

The exciplex fluorescence intensities of Phen-(CH2)n-0-(CH2)2-DMA decrease at higher fields (> 1 T), as shown in Fig. 6-1-4(b). In the case of ^BRs the effect of higher magnetic field above ca. 2 T is chiefly described by relaxation due to the 6g mechanism. However, in this case of IBRs the effect is interpreted in terms of the Ag mechanism, since the SLR is too slow to compete with other decay processes from the singlet state.' ^

Applying a simplified model to this BR kinetics involving the fast equilibrium between the exciplex and *IBR, the Ag value has been obtained from fast and slow lifetimes to be 0.000018 ± 0.000004 (n = 10) and


CH,

Phen-(CH:),-0-(CH:):-DMA (n=4,6,7,8,10,12)

0.0 0.1 0.2 0.3 0.4

Magnetic field / T

0.5 0.6 0 4 6 Magnetic field / T

Fig.6-1-4 Magnetic field and methylene chain length n dependence of the exciplex fluorescence intensities of Phen-(CH2)n-0-(CH2)2-DMA in DMF, /o and h being the intensities in the absence and presence of a magnetic field B. (a) In low magnetic fields of up to 0.6 T. (b) In high magnetic fields of up to 9 T.

0.000025 ± 0.000006 (n = 12) in Phen-(CH2).-0-(CH2)2-DMA, which agree with the theoretical value. Similarly, the Ag values have been obtained to be 0.000022 (Phen-(CH2)n-0-DMA), 0.000033 (Phen-(CH2)io-DMA), 0.000012 (Pyrene-(CH2)3-C02-(CH2)i2-0-(CH2)2-DMA), 0.000029 (Pyrene-CH2-0-(CH2)io-A^-methylaniline), and 0.000019 (Pyrene-CHs-O-(CH2)i2-0-(CH2)2-DMA).'

B. MFEs of RPs in Micellar Solution Since the interior of a micelle is usually hydrophobic, nonpolar or less polar organic molecules solubilized in the micelle do not escape quickly from the micellar interior to the polar water phase. Therefore, micelles are popularly used to extend lifetimes of RPs, although the chemical and physical properties of micelles of micellar size, shape, aggregation number and location of solutes inside a micelle are highly complex. Since MFE on the primary photochemical process of BP in micellar solution was reported by Sakaguchi and his collaborators in 1980,''^ many photochemical reactions of aromatic carbonyls have been reported in micellar solution to discuss the kinetics and dynamics of the reactions and the mechanisms of MFEs. ' ^ a. Influence of micelles and chain length of solutes on MFEs For instance, in photo-induced hydrogen abstraction of the excited triplet AQ in micellar solution, the influence of micelles on MFEs has been


o • oOOon

AAAî

HDTCl

Brij35 AQH#

SDS

5 10 Magnetic field / T

15

Fig.6-1-5 Magnetic field effects on the lifetimes of RPs photo-generated from anthraquinone in Brij35 (O), HDTCl (# ) , SDS (A) micellar solutions.

elucidated in the high magnetic field of up to 14 T. ^^^ Fig. 6-1-5 shows MFDs of the lifetimes of ^RPs comprising AQH- and a counter radical (R-) generated on each surfactant: SDS, HDTCl and Brij35. The lifetime in all solutions more or less increases up to 2-3 T, then decreases in the higher field. The increase in RP lifetimes in the low fields is attributable to the depletion of RP T-S transitions due to SLR (the 5hf- and/or dd-induced SLR mechanisms), while the decrease in RP lifetimes above 2-3 T is ascribed to the relaxation due to the ^-anisotropy (the 6g-induced SLR mechanism). The order of the increments in RP decay rate constants above 2-3 T is SDS < HDTCl < Brij35. This largest increment in Brij35 is understandable because the radical is generated on a carbon in the repeating chain unit of -O-CH2-CH2-. Since the carbon is definitely adjacent to an oxygen having the larger SOC interaction, the 8g anisotropy in Brij35 is larger than in HDTCl and SDS. The cases of BPs in the three surfactants also showed similar micelle dependence and effect of hydrophobicity. "^^

On the other hand, the holding time of RP kept inside the micellar core, which is directly related to the magnitude of MFE as described above, can be controlled by the hydrophobicity of the solute. In a series of AQH.-C02-(CH2)„-rCH3 (n = 2, 3, 4, 6 and 8) in Brij35, the longer the chain is, the larger the MFE on the RP lifetime.^' ' ^ From detailed analyses based on the SLR mechanism taking into account the magnetic field-independent process such as escaping of the radicals from the Brij35 micelle, it has been shown that the greatest inhibition of the escape is responsible for the largest MFE in AZ = 8. Analogous influence on MFE has been observed in BPH.-0-(CH2)._i-CH3 (« = 1, 2, 3, 4, 6 and 8).'^


The above influences have also been confirmed in the case of photo-induced intermolecular electron transfer between electron donors of N-ethylcarbazole (ECZ) or A^-nonylcarbazole (NCZ) and an electron acceptor of tetracyanobenzene (TCNB) in three kinds of micelles.^^ The relative yields of the escaped radical cation increase by 30 (SDS), 90 (HDTCl) and 50% (Brij35) in the case of RP comprising NCZ-" and TCNB-" throughout the high magnetic fields of 1 -14 T. On the contrary, in the case of RP of ECZ- and TCNB-" the yields are 30 (SDS), 5 (HDTCl) and 5% (Brij35). This difference clearly shows that the hydrophobicity of solutes is important to obtain a large MFE also in the yield, b. Influence of microviscosity and magnetic isotope on MFEs As SLR depends on the correlation time of the radical Brownian motion, influence of microviscosity on the RP lifetime governed by the SLR mechanism has been examined by adding MgCh to an SDS micellar solution of BR The microviscosities in SDS micelles have been estimated to be 0.0165, 0.0256 and 0.0430 Nsm"' at [MgCb] = 0, 0.05 and 0.1 mol dm"^ respectively.•^' Together with the increase in concentrations of MgCl2, the lifetime of the RP composed of BPH- and R- in SDS micellar solution has increased by about 30% at 4 T. Further, magnetic isotope effect (MIE) and influence of microviscosity on MIE have been studied in the micellar solution. MIE has been observed by 20-60% in 0.04-0.93 T, as reported,"^^ but no MIE above 2 T has been observed. As for the influence of microviscosity on MIE, the magnitude of MIE at 0.00165, 0.0256 and 0.0430 Nsm"^ has been obtained to be 23, 20 and 17%, respectively, at 0.48 T It has been found that there is the tendency for MIE to decrease in accordance with increase in microviscosity of the media. This effect could be explained by the 5hf-induced SLR mechanism including an argument that the correlation time of a relaxation becomes larger in more viscous solutions.

C. MFEs of Other Reaction Systems Several interesting MFEs have been detected in other photoreaction systems. ' ^^ One of them is the MFE investigated in the photo-induced electron transfer reaction from polyalkylamine dendrimers to BP in the aqueous solutions. The dendrimer is well known as a single and structurally authorized molecule having unique molecular weight, and hence simplified reaction kinetics and dynamics can be expected compared with a micelle. Moreover, dendrimers large in size usually have a suitable pocket (cage) capable of involving guest molecules. In the basic dendrimer solutions of BP, the third, fourth and fifth generations of the dendrimer have shown MFEs to increase (ca. 15% in 0.6-14 T) in the yield of BP anion radicals generated (the RP mechanism).'^^ On the other hand, in bis-carbene generated from photo-excitation of m-phenylenebis (phenylmethylene) in rigid matrix at 77 K, fluorescence intensity and


lifetime of the ^/5-carbene have together decreased with increase in the magnetic field, which has been explained by magnetic field-induced mixing of substates of the bis-carhene excited state.' ^ As a rigid system without a solvent, furthermore, MFE on the charge-transfer fluorescence and transient photocurrent has recently been investigated within 100 mT in a 1,2,4,5-tetracyanobenzene-doped poly(A^-vinylcarbazole) film.' ^ The MFE has been explained by a combination of two types of mechanisms, namely, the HFC and level crossing mechanisms. Detailed consideration using the stochastic Liouville equations has proved the stepwise hole-hopping mechanism.

Titanium oxide (Ti02) is well known as a photocatalyst that converts solar energy to electric energy by mineralizing materials through the redox reaction. Therefore, a certain extent of MFE can be expected in the redox reaction in which an electron participates. In an experiment in which a mixed solution of an aqueous H2PtCl6 solution and methanol suspending Ti02 powder is irradiated with a Xe lamp, the volume of hydrogen gas generated has been measured in zero and several high magnetic fields of up to ca. 14 T.' ^ Fig. 6-1-6 shows the MFD of the ratio (VB/VO) of the gas volume (VB) at the high magnetic field (B) toward that (Vo) at a zero field. The volume decreases by 85% at 4 T and 50-80% at 14 T. At present, the MFE on this T1O2 redox reaction seems ascribable to the Ag mechanism in RP consisting of a hole and an electron separated on the TiOz-Pt semiconductor. The ^RP has two deactivation pathways, the recombination of a hole and an electron and ISC interconversion to " RP. If it is assumed that an initially populated state is 'RP and an efficient generation of a hydrogen gas is via 'RP, then the Ag mechanism would reduce the gas volume as a result of enhanced ISC to " RP as a sink. As an opposite MFE, an increase of ca. 10% in the yield of acetone generated in the

m^-irw^ 80 - " • • • ^

^ 60 -

^ 40 -

20 -

0 — 0 3 6 9 12 15

Magnetic field / T

Fig.6-1-6 Magnetic field dependence of the ratio ( VB/VQ) of the generated hydrogen gas volume ( VB) at the field (B) toward that (Vo) at zero field. Each point is the average of three runs of experiments.


decomposition of butyl alcohol on TiOi has recently been observed at 1.5 T. ^ These mutually opposite MFEs might be attributable to the type and/or hole size of TiOa. In any case, since the study on TiOa has just started, many characteristics of the TiO: suspension and experimental factors must be investigated for the purpose of establishing MFEs in the field of semiconductors.

6.1.4 Conclusion

By exploring MFEs in a magnetic field range of up to 15 T, it has been discovered that reversal in MFEs occurs on a general basis. In ^BR and ^RP, this has been attributed to the 5g-induced SLR mechanism with a very short correlation time of a few picoseconds. In ^BR, it has been ascribed to the Ag mechanism. These MFEs have become universally known facts in photochemical reactions at present, and the investigation of MFEs has entered the next stage of exploration where new phenomena are expected to be found, for example, by combining MFEs observed thus far on photochemical reactions with the technique of high magnetic field-induced orientation and levitation of materials.

References

1. For example, Dynamic Spin Chemistry (S. Nagakura, H. Hayashi, T. Azumi, eds.), KodanshaAViley, Tokyo (1998) and references therein.

2. H. Hayashi, Y. Sakaguchi, M. Wakasa, Bull. Chem. Soc. Jpn.. 74, 773 (2001) and references therein.

3. Y. Tanimoto, Y. Fujiwara, Handbook of Photochemistry and Photobiology, Volume 1, Inorganic Photochemistry (H. S. Nalwa, ed.), American Scientific Publishers, USA (2003) Chapt. 10 and references therein.

4. H. Hayashi, S. Nagakura, Bull. Chem. Soc. Jpn.. 57, 322 (1984). 5. R. Nakagaki, M. Hiramatsu, K. Mutai, Y. Tanimoto, S. Nagakura, Chem. Phys. Lett.,

134, 171 (1987). 6. M. Mukai, Y. Fujiwara, Y. Tanimoto, M. Okazaki, / Phys. Chem., 97, 12660 (1993). 7. Y. Fujiwara, T. Aoki, K.Yoda, H. Cao, M. Mukai, T. Haino, Y. Fukazawa, Y.

Tanimoto, H. Yonemura, T. Matsuo, M. Okazaki, Chem. Phys. Lett., 259, 361 (1996). 8. Y. Mouri, Y. Fujiwara, T. Aoki, H. Yoshida, K. Naka, Y. Aoki, H. Yonemura, S.

Yamada, T. Haino, Y. Fukazawa, Y. Tanimoto, Bull. Chem. Soc. Jpn., 78, 804 (2005). 9. Y. Fujiwara, T. Aoki, T. Haino, Y. Fukazawa, Y. Tanimoto, R. Nakagaki, O.

Takahira, M. Okazaki, J. Phys. Chem. A, 101, 6842 (1997). 10. H. Cao, Y. Fujiwara, T. Haino, Y. Fukazawa, C.-H. Tung, Y. Tanimoto, Bull. Chem.

Soc. Jpn., 69, 2S0\ (1996). 11. H. Cao, K. Miyata, T. Tamura, Y. Fujiwara, A. Katsuki, C.-H. Tung, Y. Tanimoto, J.

Phys. Chem. A, 101, 407 (1997). 12. Y. Sakaguchi, S. Nagakura, H. Hayashi, Chem. Phys. Lett., 72, 420 (1980). 13. Y. Fujiwara, K. Yoda, T. Tomonari, T. Aoki, Y. Akimoto, Y. Tanimoto, Bull. Chem.

Soc.Jpn.,12, 1705(1999). 14. Y. Fujiwara, K. Yoda, T. Aoki, Y. Tanimoto, Chem. Lett., 1997, 435. 15. Y. Fujiwara, Y. Taga, T. Tomonari, Y. Akimoto, T. Aoki, Y. Tanimoto, Bull. Chem.

Soc. Jpn., 74, 237 (2001).

6.2 Magnetic Field Effects in Photosensitive Electrodes 263

16. Y. Akimoto, Y. Fujiwara, Y. Tanimoto, Chem. Phys. Lett., 326, 383 (2000). 17. Y. Tanimoto, Y. Akimoto, Y. Fujiwara, M. Mukai, T. Takui, T. Kinoshita, K. Itoh,

Bull, Chem. Soc. Jpn., 74, 2325 (2001). 18. F. Ito, T. Ikoma, K. Akiyama, A. Watanabe, S. Tero-Kubota, / Phys. Chem. B, 109,

8707 (2005). 19. M. Kamochi, Y. Fujiwara, Y. Tanimoto, Annual Meeting on Photochemistry 2003,

Shimane, November 2003, Abstr. No. 3P27 (in Japanese). 20. M. Wakasa, S. Suda, H. Hayashi, N. Ishii, M. Okano, J. Phys. Chem. B, 108, 11882

(2004).

6.2 Magnetic Field Effects in Photosensitive Electrodes

The reaction mechanism of photochemical reactions in condensed phase has been elucidated by magnetic field effects on reaction kinetics or yields.'"^^ As a consequence, magnetic field is expected to provide a novel means of controlling photoinduced electron transfer and the succeeding processes including photoelectrochemical reactions.

Magnetic field effects on photocurrents in organic films were reported very early by Sokolik and Frankevich.^* The magnetic field effects were explained in terms of a kind of Ag mechanism operating in electron-hole pairs (exciton) where singlet-triplet transitions are assumed to occur by the difference in g-values between electron and hole. The photoconductivity in organic films increased in the presence of a magnetic field. Magnetic field effects on the photoconductivity of poly-A^-vinylcarbazole films were also reported by Itaya and coworkers.^^ The photocurrent increased with increase of magnetic field in lower magnetic fields (< 0.1 T) and reached saturation in higher magnetic fields (> 0.1 T). Similar magnetic field effects were also observed in the presence of dimethylterephalate as the doping agent. Magnetic field effects on the photoconductivity in an organic polymer (a poly-phenylene vinylene derivative) with Ceo as the dopant were examined by Frankevich and coworkers. ^ The photocurrent increased with increase of magnetic field in lower magnetic fields (< 0.15 T) and reached saturation in higher magnetic fields (> 0.15 T). However, no magnetic field effects on the photoinduced discharge rates were observed in the case of Cao-doped poly-N-vinylcarbazole. ^ Thus, a number of magnetic field effects on photoconductivity in organic films have been reported.

Magnetic field effects on photoelectrochemical reactions in photosensitive electrodes were expected to provide a useful method for verifying the complicated mechanisms of the reactions and promote research on the application of magnetic field effects for molecular devices. However, very little work on magnetic field effects for such use has been reported.


6.2.1 Magnetic Field Effects in Photosensitive Electrodes Modified with Donor-Acceptor Linked Compounds

Yonemura and coworkers have studied magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes modified with zinc-teraphenylporphyrin-viologen linked compounds (ZnP(n)V(n = 4,6,8)) as a donor-acceptor linked compound, as shown in Fig. 6-2-1.^'^^ Modified electrodes were prepared by depositing the mixed monolayer with ZnP(n)V and arachidic acid (1 : 10) on ITO electrodes by the standard Langmuir Blodgett method. The magnetic field effects on the photoelectrochemical measurements were carried out using a three-electrode cell. The photocurrents of the modified electrode were measured under various applied potentials (0-0.5 V vs. Ag/AgCl) in the presence of excess triethanolamine (TEOA) as a sacrificial electron donor under a nitrogen atmosphere. Electric field was applied parallel to the magnetic field to avoid the influence of magneto-hydrodynamic (MHD) force.

Photoirradiation with visible light (> 400 nm) upon the ZnP(8)V-modified electrode afforded anodic photocurrents. Similar results were observed in ZnP(4)V- and ZnP(6)V-modified electrodes. The photocurrent action spectra of the modified ZnP(A2)V-electrodes were in good agreement with the absorption spectra of the porphyrin moiety of ZnP(A2)V in LB films. The result strongly indicated that the photocurrents of the ZnP(n)V-modified electrodes were attributable to the excitation of the porphyrin moiety of ZnP(n)V.

In the presence of a magnetic field, the photocurrent clearly increased (Fig. 6-2-1). The magnetic responses were reproducible (over 20 times). The same magnetic field effects were observed when the direction of the magnetic field was reversed. In addition, similar magnetic field effects

V N N^, _ _ . -^ •ZnP(4)V < Zn X—K ^0-(CH.)„-N+ — +N (CH.) CH. ^ „ r..^!w

^N N-\ ^ " - w " •ZnP(6)V ' ^ '' '- ' " 15 - A ZnP(8)V . • 1 A

ZnP(A2)V (/i=4,6,8) '^ t • •

^ 10- A •

0-(CH:)s-N*-(CH,). Br

ZnP(8)AB 0

0 0.2 0.4 0.6 H/T

Fig. 6-2-1 Molecular structures of porphyrin derivatives (ZnP(n)V and ZnP(8)AB) and magnetic field effects on the Q values of ZnP(8)V( A )-, ZnP(6)V(« )-, ZnP(4)V(^)-modified electrodes at 0 V vs. Ag/AgCl.


hv

w i

V .) '^ K .

Magnetic

field

effects

'ZnP' — V-*

T

'ZnP' -- V-*

•(ZnP- — V*-)

kes. 1 Êlectrode

i

ZnP- — V-

+TEOA

ZnP — V-

Fig. 6-2-2 Reaction scheme of photoelectrochemical reaction of ZnP(/i)V-modified electrodes.

were obtained when the electric field was perpendicular to the magnetic field. Therefore, the magnetic field effects on photoelectrochemical reaction in the modified electrodes are not influenced by the direction of the electric field to the magnetic field. These results indicated that the magnetic field effects on the photocurrents did not come from MHD force.

The magnitude of magnetic field effects on the photocurrent is expressed as follows:

e = (/(H)-7(0))/7(0) X100 (1)

where 7(H) and 7(0) are the photocurrent in the presence and absence of magnetic fields, respectively. The Q value (%) increased with increase of the magnetic field in lower magnetic fields (< 0.3 T) and became constant in higher magnetic fields (> 0.3 T) (ca. 15% in the case of the ZnP(8)V-modified electrode), as shown in Fig. 6-2-2.

The Q values due to the ZnP(8)V- and the ZnP(6)V-modified electrodes were twice that due to the ZnP(4)V-modified electrode. The magnetic field effects on the lifetimes of radical pairs with ZnP(8)V and ZnP(6)V are larger than on those with ZnP(4)V in solution.'^^ The influence of spacer chain length on the magnetic field effects was in good agreement with that on the lifetimes of biradicals. Therefore, the magnetic field effects on the photocurrents can be explained by the relaxation mechanism in radical pairs.^ "

As a reference system, another modified electrode was prepared using the porphyrin derivative without the viologen moiety (ZnP(8)AB), as shown in Fig. 6-2-1. The photocurrent action spectrum of the ZnP(8)AB-modified electrode was similar to those of ZnP(Az)V-modified electrodes. However, no magnetic field effects on photocurrents were observed in the case of the ZnP(8)AB-modified electrode. These results indicate that the


magnetic field effects in ZnP(A2)V-modified electrodes can be ascribed to the photogenerated biradical.

The reaction scheme of the photoelectrochemical reaction of ZnP(n) V-modified electrodes is summarized in Fig. 6-2-2. The magnetic field effects on photocurrents in ZnP(A2)V-modified electrodes as photosensitive electrodes were clearly ascribed to the photogeneration of the triplet biradical (ZnP" -V^) via the intramolecular electron-transfer process at the ITO electrode surface. The intersystem crossing process of the triplet biradical (/:isc in Fig. 6-2-2) will become the rate-determining step for the biradical to decay via reverse electron transfer to the ground state. In the presence of higher magnetic fields, the intersystem crossing process is controlled by relaxation from triplet sublevels to the corresponding singlet (relaxation mechanism).^^^^ As the intersystem crossing process was suppressed with increase in magnetic field, the reduced viologen (V^) in the triplet biradical was better able to transfer the electron to the ITO electrode. As a consequence, the photocurrents increased in the presence of a magnetic field, as shown in Fig. 6-2-1. In other words, the magnetic field effects on the photocurrents provide experimental evidence indicating that the photoelectrochemical responses of ZnP(«)V-modified electrodes can be ascribed to the sequential electron-transfer process via the viologen moiety as a mediator between the porphyrin moiety and the ITO electrode.

The influence of applied potential (E /V vs. Ag/AgCl) on the magnetic field effects in ZnP(8)V-modified electrode was also examined. The photocurrents increased with increase in the applied potential, indicating that the rate constant (k^sc in Fig. 6-2-2) of the electron transfer between the reduced viologen (V^) moiety and the ITO electrode increased as the applied potential became more positive. On the contrary, the Q values of the ZnP(8)V-modified electrode at 0.5 T decreased with increase in the applied potential. In other words, the magnitude of the magnetic field effects could be controlled by the applied potential. The rate of the intramolecular reverse electron transfer in ZnP(8)V is controlled by the rate (kisc in Fig. 6-2-2) of the intersystem crossing process. The photocurrent generation is proportional to A:esc/( esc + isc). The kesc value should increase with increasing applied potential in the positive direction, while the A:isc value is independent of the applied potential. The influence of the magnetic field on the photocurrent decreased with increasing applied potential in the positive direction. Thus the Q value must decrease as the applied potential becomes more positive. On the basis of these observations, the magnetic field effects on photoelectrochemical reactions in photosensitive electrodes can be controlled by the applied potential.

6.2.2 Magnetic Field Effects in Photosensitive Electrodes Modified with Semiconductor Nanoparticles

Semiconductor nanocrystals have unique properties which are different


from those of bulk crystals due to quantum size effects. A number of synthetic methods have been reported and their characteristics have been intensively studied by various sepectroscopic methods^' '' ^ In addition, electrodes modified with semiconductor nanoparticles (Q-CdS (cadmium sulfide)) were fabricated by immobilizing them on a self-assembled monolayer of hexanedithiol prepared on the gold electrode as reported by Bard and coworkers.'^^ Chazalviel reported the magnetic field effects on photoelectrochemical reactions of bulk semiconductor (GaAs) by the use of circularly polarized light.' ^ The magnitude of the magnetic field effects was very small and the magnetic field effects were explained in terms of the classical models for electrochemical transfer at the semiconducting photoelectrode. However, magnetic field effects on the photoelectrochemical reactions of photosensitive electrodes modified with semiconductor nanoparticles were not reported.

Yonemura and coworkers have studied the magnetic field effects on photoelectrochemical reactions of photosensitive electrodes modified with semiconductor (CdS) and diluted magnefic semiconductor (Cdi-xMn^S) nanoparticles (Q-CdS and e-Cdi-xMn^S). '* ' ^ Q-CdS and Q-Cdi-xMuxS were prepared by the use of dioctyl sodium sulfosuccinate (AOT) reversed micelles as reported previously by Steigerwald. ' ^ The size of Q-CdS or Q-Cdi-xMuxS was controlled by the water-to-surfactant ratio (W value = [H20]/[A0T]) in heptane soluUon. The relative amount of Mn " ions compared to Cd ^ ions in the semiconductor is defined as Q-Cdi-xMuxS or Cdi.xMnxS, where X = [Mn'^]/([Cd'1 + [Mn'^]), assuming that all Cd'% Mn ^ and S " in the mixed solution formed the foregoing particles. Self-assembled monolayers (SAMs) of 1,6-hexanedithiol were prepared by immersing gold electrodes in an ethanol solution of the 1,6-hexanedithiol. Electrodes modified with Q-CdS and Q-Cd].xMnxS were fabricated by immersing the SAM-electrodes in an AOT heptane solution including Q-CdS and g-Cdi-xMnxS dispersion.

Absorption spectra of Q-CdS with various W values (1.0-7.0) were observed in heptane. As the W value decreased, both the absorption onset and absorption peak were blue-shifted. For example, in the case ofW = 4.0, the peak absorption was observed at 372 nm (3.33 eV), corresponding to a particle diameter of ca. 3 nm. These results were caused by quantum size effects.^ ' ^^ In the case of Q-Cdi-xMuxS, both the absorption onset and absorption peak showed similar blue-shift as the W value decreased. These results were also ascribed to quantum size effects." ''

Photoirradiation of the Q-CdS (W = 5.4)-modified electrode afforded stable anodic photocurrents. The photocurrent action spectrum in the Q-CdS-modified electrode was in good agreement with the absorption spectrum of Q-CdS in heptane. The results strongly indicated that the photocurrents could be ascribed to the photoexcitation of Q-CdS on the gold electrode surface.


In the presence of a magnetic field (0.5 T), photocurrents appreciably decreased. The magnitude of magnetic field effects on the photocurrent is also evaluated by the Q-value as defined by Eq. (1). The Q-value in the Q-CdS (W= 5.4)-modified electrode was ca. -3 % at 0.5 T by Eq. (1). Similar magnetic field effects were also observed in the case of the Q-CdS {W = 4.0) or {W - 3.0)-modified electrodes with different diameters of nanoparticles. Opposite magnetic responses were observed in comparison with the ZnP(n)V-modified electrodes, as shown in Fig. 6-2-1. The results are probably due to the different mechanism of magnetic field effects between the semiconductor nanoparticles (2-CdS) and the ZnP(/2)V-modified electrodes. As a reference system, an electrode modified with large CdS particles was also examined. In the CdS-modified electrode, stable photocurrents were observed in the anodic direction. The photocurrent action spectrum of the CdS-modified electrode was red-shifted more than that of the g-CdS-modified electrode. This result is consistent with the quantum size effects described above in the absorption spectra. However, no magnetic field effects on photocurrents were obtained in the CdS-modified electrode. Comparing the Q-CdS- and the CdS-modified electrodes, the quantum size effects (quantum confinement effect) in Q-CdS are most likely responsible for the magnetic field effects observed in the Q-CdS-modified electrodes.

In a diluted magnetic semiconductor and its nanoparticles, a variety of unusual magnetic and magneto-optical properties due to exchange interaction between the band electrons and the magnetic ions have been reported.^^ ^^ When diluted magnetic semiconductor nanoparticles were used instead of semiconductor nanoparticles, enhancement of the magnetic field effects on the photocurrents in photosensitive electrodes was expected. Photoirradiation of the (2-Cdi .vMuxS-modified electrode also afforded stable anodic photocurrents as similar to the Q-CdS modified electrodes, as shown in Fig. 6-2-3. The Q value in the Q-Cdi^MnxS (X = 0.2, W= 3.0)-modified electrode was ca. -8% at 0.76 T by Eq. (1.) Larger reduction {Q = ca. -8 %) of photocurrent from the Q-Cdi-xMn^S (X = 0.2, W = 3.0)-modified electrode was observed compared with that {Q - ca. -3 %) from the Q-CdS (W= 3.0)-modified electrode. As a reference system, the electrode modified with large Cdi-^Mn^S (X = 0.2) particles was also examined. Although stable photocurrents were generated in the anodic direction, no magnetic field effects on the photocurrents could be observed in the Cdi-xMn; S-modified electrode. The results are fairly consistent with those of the Q-CdS-modified electrodes showing the presence of the quantum size effect (quantum confinement effect) on the reduction of photocurrents induced by the magnetic field. Magnetic interactions of Q-Cdi-; MnxS were found to increase with decreasing particle size, as reported by Pileni and coworkers.'^^ Thus, based on these observations, the large magnetic field effects on photocurrents observed in the (2-Cdi.;^MnxS-


(2-Cd|.xMnxS-modified electrode

SAM

I

Au electrode

cy-^ : 1,6-Hexanedithiol o ^ : A O T

( J iCdivMnvS nanoparticle

0.76 T < ^

off

n n n

vj

OT

f l I I T i i I

j j on I j VJ

5 10 Time / min

•8 C <

15

Fig. 6-2-3 Magnetic field effects on the photocurrents of the Q-CdixMnxS {X = 0.2, W = 3.0)-modified electrode.

modified electrodes are most likely ascribable to the quantum size effect (quantum confinement effect) and exchange effects in the exciton states due to the incorporated Mn ^ ions in the Q-CduxMuxS.

Magnetic field effects on exciton emission were reported by Oka and coworkers and in diluted magnetic semiconductor microcrystallites in a Si02 glass support/^^ Red-shift and enhancement of the emission band were observed in the presence of high magnetic fields. Therefore, the magnetic field effects on photocurrents observed in this study may be ascribed to the magnetic field effects on lifetimes of exciton and/or surface states in the Q-CdS- and Q-Cdi.xMnxS-modified electrodes.

The magnetic field effects on photocurrents observed in the electrode modified with semiconductor nanoparticles (Q-CdS and Q-Cdi-xMuxS) are most likely explained by the electron-hole pair mechanism.' ^ The reaction scheme of the photoelectrochemical reaction of electrodes modified with semiconductor particles is summarized in Fig. 6-2-4. In the close electron-


\e t I h)

Close contact singlet

electron-hole pair

Ground state

S-T Conversion

Separated singlet

electron-hole pair Magnetic

field

effects

V t t h)

Close contact triplet

electron-hole pair

\e t t h)

Separated triplet

electron-hole pair

e f + f h

Free charge carriers

Photocurrent

Fig. 6-2-4 Reaction scheme of the photoelectrochemical reaction of electrodes modified with semiconductor particles.

hole pair, the distance between the electron and hole is short, and the exchange interaction of the electron-hole pair is not negligible. In the separated electron-hole pair, the distance between the electron and hole is long, and the exchange interaction of the electron-hole pair is negligible. Singlet (S)-triplet (T) conversion of the electron-hole pair occurs in the separated electron-hole pair. The free charge carriers decrease in the presence of a magnetic field, since the S-T conversion is suppressed by the magnetic field. In the case of semiconductor nanoparticles, the lifetime of the electron-hole pair (exciton) is long enough to be perturbed by the magnetic field, while the lifetime is too short to be influenced by the magnetic field in the case of semiconductor large particles. The quantum confinement effect is analogous to the cage effect in the radical pair mechanism. ' ^

Konno and coworkers have studied high magnetic field effects (~8T) on photoelectrochemical reactions of photosensitive electrodes modified with poly(A^-methylpyrrole).'^^ The poly(A^-methylpyrrole)-modified electrodes were fabricated by the electrochemical polymerization of A-methylpyrrole. In acetonitrile solution, the initial photocurrent increased with increase in magnetic field from 0 to 7 T. On the other hand, in dichloromethane, magnetic dependence of the photocurrents was observed not only in the initial but in the second and third photocurrent responses as well. In addition, the photocurrents increased by repetition of measurement. In the repeated measurements, strong magnetic fields accelerated the increasing photocurrent as compared with that in a zero magnetic field. The result cannot be explained by the MHD effect and is


probably ascribable to the change in polymer orientation due to the magnetic field accompanied by the photocurrent generation. The mechanism of the magnetic field effects is different from the radical pair or electron-hole pair mechanism as described above.

Wakasa and coworkers have recently studied the magnetic field effects on photocatalytic reactions with ultrafine TiOz particles.^^^ Magnetically induced acceleration of the photocatalytic reaction was observed. Opposite magnetic responses on the reaction were observed in comparison with those in the photoelectrochemical reaction in the electrodes modified with semiconductor nanoparticles described above (Fig. 6-2-4).

The magnetic field effects in photosensitive electrodes can be expected to lead to an epoch-making means of reaction control involving photoelectrochemical processes and related systems such as highly functional nanomaterials.

References

1. U. E. Steiner, T. Ulrich, Chem. Rev., 89, 51(1989). 2. H. Hayashi, Y. Tanimoto, Dynamic Spin Chemistry (S. Nagakura, H. Hayashi, T.

Azumi, eds.), Chapters 2 and 3, Kodansha-Wiley, Tokyo/New York (1998). 3. Y. Tanimoto, Y. Fujiwara, Handbook of Photochemistry and Photobiology Volume 1:

Inorganic Photochemistry (H. S. Nalwa, ed.), Chapt.lO, American Scientific Publishers (2003).

4. I. A. Sokolik, E. L. Frankevich, Usp. Fiz. Nauk, 111, 261 (1973). 5. K. Okamoto, N. Oda, A. Itaya, S. Kusabayashi, Chem. Phys. Lett., 35, 483 (1975). 6. E. Frankevich, A. Zakhidov, K. Yoshino, Y. Maruyama, K. Yakushi, Phys. Rev. B,

53,4498(1996). 7. Y. Wang, A. Suna, J. Phys. Chem. B, 101, 5627 (1997). 8. H. Yonemura, K. Ohishi, T. Matsuo, Chem. Lett., 1996, 661. 9. H. Yonemura, K. Ohishi, T. Matsuo, Mol. Cryst. Liq. Cryst., 294, 221 (1997).

10. H. Nakamura, A. Uehata, A. Motonaga, T. Ogata, T. Matsuo, Chem. Lett., 1987, 543. U . S . Ogawa, F. -R. F. Fan, A. J. Bard, J. Phys. Chem., 99, 11182 (1995). 12. M. L. Steigerwald, A. P. Alivisatos, J. M. Gibson, T. D. Harris, R. Kortan, A. J.

Muller, A. M. Thayer, T. M. Duncan, D. C. Dougalss, L. E. Brus, / Am. Chem. Soc, 110,3046(1988).

13. J. -N. Chazalviel, J. Chem. Phys., 83, 140 (1985). 14. H. Yonemura, M. Yoshida, S. Mitake, S. Yamada, Electrochemistry^ 67, 1209 (1999). 15. H. Yonemura, M. Yoshida, S. Yamada, Studies in Surface Science and Catalysis, 132,

741 (2001). 16. N. Feltin, L. Levy, D. Ingert, M. P. Pileni, J. Phys. Chem. B, 103, 4 (1999). 17. Y. Yanata, K. Suzuki, Y. Oka, J. Appl. Phys., 73, 4595 (1993). 18. E. L. Frankevich, A. A. Lymarew, I. A. Sokolik, F. E. Karasz, S. Blumstengel, R.H.

Baughman, H. H. Horhold, Phys. Rev. B, 46, 9320 (1992). 19. A. Konno, I. Mogi, K. Watanabe, J. Electroanal. Chem., 507, 202 (2001). 20. M. Wakasa, S. Suda, H. Hayashi, N. Ishii, M. Okano, / Phys. Chem. B, 108, 11882

(2004).


6.3 Spin Probe and Spin Trapping Studies on the Magnetic Field Effects on Chemical Reactions in the Nanospace of MCM-41

6.3.1 Magnetic Field-dependent Photoreactions in MCM-41 Nanospace

Many experimental data have been accumulated for the magnetic field effects on the chemical reactions that produce a radical pair as the intermediate. They are systematically understood under the so-called radical pair mechanism, which postulates a pair of radicals as the reaction intermediates. When the radicals keep pairing for a period longer than their spin-lattice relaxation times, the observed magnetic field effect is usually large.'^ If the intermediate radicals are neutral, this long pairing time is possible only for the systems composed of the reactant groups which are linked by a flexible chain group or for those incorporated in a nanospace. In the latter case a micelle is frequently employed as the donor of the reaction space. The long spin lattice relaxation times make it possible to change the state of electron spins of the radical pair by the ESR technique to control the reaction. ^ This new phase of research broadens the field and is called "spin chemistry." ' On the other hand, chemistry in the nanodimension has become popular due to the development of new experimental techniques. In addition the discovery of new mesoporous materials,^^ such as M41S, has made it possible to conduct research on physicochemical processes in nanospace from the most basic point of view. Therefore, it is a natural consequence to observe the magnetic field effect on a chemical reaction in the nanochannel of MCM-41, which is a representative mesoporous material that provides a new, regularly ordered, stable and well-defined nanospace.^*

Figure 6-3-1(A) shows the magnetic field (j o) dependences in the product yield of the photoreduction of xanthone (XO) in alcoholic solutions. A schematic view of the flow system specially designed for the photolysis in the MCM-41 nanochannel is given as Fig. 6-3-1(B). Here a laser pulse excites XO to the triplet state, which then abstracts a hydrogen atom from xanthene (XH2) added as the hydrogen donor, and a pair of intermediate radicals XH and XOH are produced. In the present case bixanthyl (XH-XH) was detected chromatographically as one of the products which was formed from the coupling reaction of the two XH radicals. A large magnetic field effect was found on the photoreaction in an alcoholic solution flowing in a quartz column packed with MCM-41.^^ The effect is higher than 25% at 500 mT, when 2-propanol and MCM-41 with nanochannels whose diameter is 2.5 nm were employed as the solvent and the filler of the reaction column, respectively. The magnetic field effect appears to increase steadily with increase in the magnetic field (Fig. 6-3-1(A)). The effect was reduced when the diameter of the nanochannel

6.3 Dynamics of Molecules in the Nanospace 273

S 10

0

(B)

- ^"^ o (c) / ^•^^ —U— " ^

o (d)

/ / o

0 100 200 300 400 500

fio / mT

Fig. 6-3-1 (A) Magnetic field effect on the yield of bixanthyl for the photoreduction of xanthone (XO, 1 mM) in the presence of xanthene (XH:, 3 mM) in either 2-propanol (a, b) or ethanol (c, d) in a column packed with MCM-41 whose channel diameter is 2.5 nm (a, c) or 3.4 nm (b, d). [Reproduced from M. Okazaki et al., Phys. Chem. Chem. Phys., 4, 1202 (2002)] (B) The flow apparatus is specially designed to allow the reactant solution to flow through the quartz column (a) packed with MCM-41 powder (b) under a magnetic field. The solution is irradiated by a UV laser through a window of the ESR cavity (c). [Reproduced from M. Okazaki et al.. Appi Magn. Reson., 23, 436 (2003)]

became larger and/or ethanol was employed as the solvent.^'^^ To investigate the mechanism the spin-trapping reaction yield was monitored for the same reaction system^^ employing phenyl-r-butylnitrone (PBN) as the spin trap. From the result we confirmed the following: transient free radicals are really formed during the reaction and the concentration of free radical intermediate increases with increasing magnetic field. These results suggest that the radical pair mechanism is appropriate in explaining the magnetic field effects. In fact, quenching of the TEMPO (2,2,5,5-tetramethylpiperidine-1-oxyl) radical, added in the same reaction system, is dependent on the magnetic field as expected by the radical pair model. ^ In summary, the magnetic field effect in the photoreaction observed in the nanochannel of MCM-41 is due to the modulation of the spin-state conversion rate for the radical pair intermediate by the applied magnetic field. This spin conversion is caused by the spin-lattice relaxation of the component free radicals of the pair. Since the relaxation time is assumed to


Nanotube of

MCM-4I

A=355 nm I Solvent cluster

r ^ KM

1 > ^ Flow

direction

' \

(\-t {) xo„.

0 t W

7 t ^ R O H ^

p°«€ 'W

> ([ ^/- \ /7H

^x\\H\hh

>-"Q ^ 0

Time

v

Fig. 6-3-2 A model for the mechanism of the magnetic field dependence in the photoreduction of an alcoholic (ROH) solution of xanthone (XO) in the presence of xanthene (XH2) in the MCM-41 nanochannel. Photo-excited xanthone (XO*) abstracts a hydrogen from alcohol and the radical pair (XOH-R'OH) in the triplet state is formed, which is then converted into the pair (XOH-XH) through a hydrogen transfer reaction from the donor XH.. When the radical pair does not recombine in the nanochannel, the two radicals lose correlation of their positions. [Reproduced from M. Okazaki et al., Phys. Chem. Chem. Phys., 4, 1204 (2002)]

last a few microseconds the two intermediate radicals should keep pairing for more than several microseconds in 2-propanol during the flow in the column packed with MCM-41.

Figure 6-3-2 depicts a model for the mechanism of the magnetic field dependence in the present system.^ ^ Since the radical pair is separated by the solvent molecules which do not diffuse rapidly as in bulk solution, the recombination probability becomes dependent on the magnetic field. We assume that the long lifetime of the radical pair becomes possible because the collective nature of the solvent molecules specially endowed in the nanochannel prevents the two pairing radicals from separating. The rapid flow of the solution molecules in the nanochannel can also be explained by the collectivity of the solvent molecules, which may reduce the momentum transfer in the colinear direction from the flowing molecules to the nanochannel. Thus the correlation between the positions of two solute molecules may increase, i.e., the radical pair keeps pairing for a long time, more than several microseconds. Thus the pair of radicals does not diffuse away before recombination or flowing out of the nanochannel.

6.3.2 A New View of the Physicochemical Processes in Nanosystems

The following two observations should be noted, (i) An alcoholic solution flows in the nanochannel of MCM-41 under the conditions usually employed in liquid chromatography. This is unexpected since Poiseulle's law^^ predicts a flow rate much less than that expected in the above experiment, (ii) The intermediate radicals born as a pair should keep pairing for a period longer than several microseconds. This is also


unexpected since the usual diffusion rate constant predicts the separation of these within submicroseconds. Although these phenomena have been tentatively explained by the collectivity of the solvent molecules, they must be confirmed through other experiments and discussions of the results. The dynamics of the solution molecules in the nanochannel of MCM-41have been studied by several research groups by the pulsed-field-gradient NMR (PFGNMR) method. ' ^^ Hansen's group conducted an experiment in which the diffusion rate drastically changed by the surface modification of the nanochannel.'^^ Stallmach et al. observed that the rate of self-diffusion of benzene molecules in the nanochannel of MCM-41 exceeds that in the bulk and presented a model in which molecules diffuse through in the gas phase.^'^ These results indicate that chemical and physical processes in the nanochannel of MCM-41 are not described exactly by the classical rules for the bulk system. In the following sections the above anomalies of the molecules in the nanochannel are discussed in more detail. Below are the structures of the three aminoxyl radicals which appear frequently in the following sections.

4 H3C CH3 5 ^ - - ^ ^ ^ . 3

H.C^''''^'

h o 0 6

DTBN TEMPO TEMPOL

12) 6.3.3 Detection of Liquid Flow through MCM-41 Nanochannels

Figure 6-3-3(A) shows the ESR spectra observed for the solution of DTBN, TEMPO and TEMPOL ((a), (b) and (c), respectively) flowing in a quartz column packed with MCM-41 powder together with the simulation for the DTBN spectrum (d). The flow apparatus is the same as that shown in Fig. 6-3-1(B). Relative amplitudes of the three hyperfine components of the first derivative spectra (a) and (b) of Fig. 6-3-3(A) for DTBN and TEMPO, respectively, are 0.92-0.93, 1.0 and 0.8 from the lower field line. From the simulation we obtain the anisotropy parameter M = tx^Jh of 15 for the rotational diffusion of the DTBN radical, where TL, indicates the rotational correlation time along the L-molecular axis (L = x, y or z).' ^ Since the solution also exists between the MCM-41 particles, the spectrum is constituted of the two components from the radicals in the two regions, but the exchange rate between these two may be rapid enough since the line shape is symmetrical. The volume inside the nanochannel is a little larger than that between the particles, so the spectrum for the solution inside the nanochannel must be more anisotropic than that observed here.'" ^ This will be discussed again in the last paragraph of this section (see Fig. 6-3-4). Since the amplitude of the low field line of spectrum c) of Fig. 6-


(A) (B)

(e)

0 120 240 360 480 600

r/s

Fig. 6-3-3 (A) ESR spectra of DTBN (a), TEMPO (b), and TEMPOL (c) in 2-propanol at a concentration of 1.0 X 10^ M flowing at a rate of 0.39 ml min"' in the column packed with MCM-41. (d) a simulation of (a). [Reproduced from M. Okazaki et al., Appl. Magn. Reson., 23, 437 (2003)] (B) Time dependence of the ESR amplitude of the nitroxide radical in 2-propanol flowing through the column (solid) and its first derivative (dotted): 0.7 ml of the solution is injected as a rectangular pulse to the flow of the pure solvent just before the column is packed with MCM-41. (a), (b) and (c) are the spectra for the DTBN, TEMPO and TEMPOL solutions at a flow rate of 0.39 ml min', respectively, and (d) and (e) are the spectra for the DTBN solution at 0.25 and 0.64 ml min"', respectively. [Reproduced from M. Okazaki et al., Appl. Magn. Reson., 23, 441 (2003)]

3-3(A) is larger than the others, the TEMPOL radical prefers the rotation around the x axis, which is the direction of H-bonding for the OH group of TEMPOL. Fig. 6-3-3(B) shows the time profile of the ESR signal amplitude when a certain volume (0.7 ml) of the spin probe solution flowed into the column packed with MCM-41.'^^ From the flow profiles we conclude that: (i) The period of time for the spin probe molecules to pass through the ESR cavity is nearly equal to that calculated from the flow rate and the injected solution volume, (ii) As the time profiles (a), (d) and (e) show, the time duration of the spin probe signal is approximately inversely proportional to the flow rate. Since it has already been proven that the aminoxyl radicals mostly exist in the nanochannel, the above results show that the solution actually "moves through" the nanochannel of MCM-41 at a pressure usually employed for liquid chromatography (< 10 MPa). The modification in the time profile by the different spin probe radicals may be due to their interaction mainly with the surface of the


MCM-41 particles, which is covered with Si-OH groups. Since the DTBN molecules may avoid the small spaces between the MCM-41 particles due to a rather hydrophobic character, the ESR amplitude reaches maximum in a short time. On the other hand, since the TEMPOL radicals may get into the small spaces between the microparticles, it takes a little longer time before the ESR amplitude reaches a maximum value. In the decreasing phase of the ESR signal, the long tail for the TEMPOL radical, as trace (c) shows, can also be explained in the same way. Recently, a group suggests that the water molecules diffuse across the wall of the nanochannel of MCM-41. ' ^^ However, we believe that reassessments of their diffusion data should be made taking into account the collective nature of the molecules in the nanochannel.'^^

6.3.4 Spin Probe and NMR Studies on the Dynamics and Distribution of Solution Molecules in the MCM-41 Nanochannel

Since the magnetic field effects have been observed for the reactions in a solution that flows in an open nanochannel, the dynamics and distribution of the molecules must be studied precisely from a basic point of view to interpret the new status of the solvent molecules in the nanochannel. The four spectra (a)-(d) in Fig. 6-3-4(A) are the ' C NMR spectra of the mixture solution of 2-propanol and cyclohexane in which MCM-41 is suspended.' ^ The NMR spectra for the same solution in the Nucleosil-50 suspension (e) and clear solution (f) are also shown. Since all the peaks of spectra (a) - (d) appear as single broad lines of the pure Lorentzian type, these molecules must exchange positions at a rate of several hundred times per second between the nanospace and the space among the particles, since the latter should give sharp lines like spectrum (f). This kind of rapid diffusion through the nanochannel cannot be treated by the Stokes model. The system should give a much broader spectrum if the solution stays long in the nanochannel.

To obtain further information on the dynamics of molecules in the nanochannel, the solution ESR spectrum of a spin probe, DTBN (di-t-butylnitroxide), incorporated in the nanochannel of MCM-41 was observed. Fig. 6-3-4(B) shows the ESR spectrum of a 2-propanol solution in the nanochannel (a) and its simulation spectrum (b).' ^ In addition to these, spectrum (c) is that observed in the absence of MCM-41, and spectrum (d) is for DTBN in a mixed solvent composed of cyclohexane and 2-propanol at a ratio of 3:1 in the nanochannel. ESR spectrum (a) is a three-Une one which is similar to that in the bulk solution (c) except for the relative peak heights among the three lines and their linewidths. Since the ESR spectrum (a) appears broader than spectrum (c), the mobility of DTBN should be slightly lower in the nanochannel. The averaged correlation time (TC) of the anisotropic magnetic interaction for the simulated spectrum (b) was 8.0 x 10~'* s, considerably longer than the


(A)

66 64 62 28 26 24 22 <7/ppm

Fig. 6-3-4 (A) Proton-decoupled ' C NMR spectra of the solutions containing 2-propanol and cyclohexane at various ratios in the MCM-41 layer (a-d) and spectra for the reference (e, f). Spectra a, b, c and d are those at the volume fraction of cyclohexane, FCHX, 1/8, 3/8, 5/8, and 7/8, respectively. Reference signals: e, in the layer of Nucleosil-50 employed instead of MCM-41; f, the clear solution part. Spectra a-d were observed within 1-2 days after sample preparation. MCM-41 used in all observations was produced as the same lot. [Reproduced from M. Okazaki, K. Toriyama, J. Phys. Chem. B, 107, 7655 (2003)] (B) ESR spectra of DTBN (1.0 X 10^ M) under various conditions: (a) 2-propanol solution in the nanochannel of MCM-41, (b) simulation for (a), (c) clear 2-propanol solution, (d) in a mixed solution with cyclohexane and 2-propanol at a ratio of 3:1 in the nanochannel of MCM-41. [Reproduced from M. Okazaki, K. Toriyama, J. Phys. Chem. B, 107, 7656 (2003)]

value in the bulk/^^ Since the spectral simulation was successfully made assuming a preferential rapid rotational diffusion of the radical along the longest molecular axis connecting the two t-butyl carbons, the DTBN molecule may be in a cyrindrical space. We consider this to be a cage-like space temporarily made by the solvent molecules. Although the nanochannel provides a cyrindrical space, the diameter is too large to directly affect the mode of rotational diffusion of the solute molecule. It may be true, however, that the solute distribution has some inhomogeneity in the nanochannel. When a hydrophobic solvent, e.g., cyclohexane, is employed, the NO group of the DTBN molecule may interact strongly with the surface, and an ESR spectrum for an immobilized nitroxide radical (not shown) is observed. When a small amount of 2-propanol was added to this system, the ESR spectrum changed to a spectrum similar to (d). This means that 2-propanol strips the DTBN molecules off the surface


of the nanochannel. These observations indicate that the Hquid molecules are distributed inhomogeneously in the nanochannel.

Since the molecular dimension of cyclohexane is slightly smaller than that of DTBN, TR (rotational correlation time) for the intramolecular magnetic interactions of cyclohexane must be less than that for DTBN. Thus, the homogeneous linewidth of the NMR spectrum should be as narrow as that in the bulk solution even if the liquid is in the nanochannel of MCM-41. Therefore, the broad linewidth of Fig. 6-3-4(A) should be due to inhomogeneous line broadening. Two mechanisms are considered: 1) anisotropy in the diamagnetic susceptibility of MCM-41 particle; 2) magnetic interaction with the hydrogen nuclei on the neighboring molecules. If the molecules move collectively, the second interaction is not averaged out during the time the cluster is wandering in the nanochannel. Since the linewidths for the system in Nucleosil are only a few Hz, the latter may be the main cause of the broad linewidths in Fig. 6-3-4(A).

6.3.5 Spin Probe and NMR Studies on the Phase Separation of the Molecular System in the Nanochannel of MCM-41

The inhomogeneous distribution and phase separation of simple fluids have been studied in the micropore.'^^ Phase separation of rather complex systems including organic solutions as well as long-chained n-alkanes has been observed in the nanochannel of MCM-41.'^^^^ A spin probe ESR study has been made on the dynamics of 2-propanol and water molecules in the nanochannel of MCM-41 at various temperatures. In the former system, 2-propanol is separated into two phases: one with molecules immobilized in the ESR time scale and the other with mobile molecules, even at temperatures more than 40 degrees higher than the bulk melting point. In the case of water, on the other hand, only the "immobilized" molecule was detected at a temperature as high as 313 K. In both systems, the DTBN molecule undergoes highly anisotropic rotational diffusion to reduce resistance from the solvent molecules in the nanochannel. These results are explained using a model in which the spin probe molecule is incorporated in a cage made by the intermolecular network intensified in the nanochannel. ^

High-resolution solid-state ^^C NMR spectra of long-chained n-alkanes, which were filled in the liquid state into the nanochannel of MCM-41, have been observed. ^^ Each spectrum has two sets of peaks: one from very flexible molecules and the other from molecules with no significant flexibility, which are characterized by the carbon spin-lattice relaxation times of less than 1 s and that of longer than 10 s, respectively. ^^ This phenomenon is ascribed to the phase separation of these alkanes in the nanochannel, i.e., the amorphous and crystalline phases, respectively. The latter phase of long-chained n-alkanes in the nanochannel have characteristics similar to those of the bulk crystals, which change


alternatively between triclinic and orthorhombic following the change in the carbon number in the molecule from even to odd, respectively. These features of the long-chained w-alkanes in the nanochannel were confirmed by observing the ESR spectra of the free radicals produced by y-irradiation. ^^ It must be difficult for these A2-alkane molecules located near the wall of the nanochannel to form a regular crystal, thus the crystalline and the amorphous phases may be formed in the central and the peripheral portions, respectively, in the nanochannel.

6.3.6 Conclusion

The magnetic field affects considerably the yield of photoreduction of xanthone in an alcoholic solution flowing in a column packed with MCM-41 powder. This effect has been explained by the magnetic field-dependent spin-lattice relaxation times of the intermediate radicals that form the radical pair. Since this mechanism assumes a long lifetime of the pair, diffusion of molecules in the nanochannel must be suppressed below a very low level. At the same time the solution should flow in the nanochannel at a rate almost the same as that for the flow between the MCM-41 granules. These phenomena for the alcoholic solutions, i.e., rapid flow but slow diffusion, are contradictory in the bulk, but can be interpreted by the concepts of "collective flow" and ''collective diffusion" in the nanochannel. These have been confirmed by spin probe ESR and NMR observations for these systems. In addition, two novel phenomena in the nanochannel of MCM-41 have been noted briefly: phase separation of a mixture solution composed of 2-propanol and cyclohexane, which are missible with each other in the bulk, and phase separation of pure long-chained n-alkanes.

References

1. S. Nagakura, H. Hayashi, T. Azumi, eds.. Dynamic Spin Chemistry, Magnetic Controls and Spin Dynamics of Chemical Reactions, KodanshaAViley, Tokyo/New York (1998).

2. M. Okazaki, ibid., Chapt. 8. 3. J. L. Casci, Advanced Zeolite Science and Applications (J. C. Janzen, M. Stocker, H.

G. Karge, J. Weitkamp, eds.), pp. 329-356, Elsevier, Amsterdam (1994). 4. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359, 710

(1992). 5. M. Okazaki, Y. Konishi, K. Toriyama, Chem. Phys. Lett., 328, 251 (2000). 6. Y. Konishi, M. Okazaki, K. Toriyama, T. Kasai, / Phys. Chem. B, 105, 9101 (2001). 7. M. Okazaki, K. Toriyama, K. Oda, T. Kasai, Phys. Chem. Chem. Phys., 4, 1201

(2002). 8. For example, W. J. Moore, Physical Chemistry 3rd Ed., Chapt. 4, Prentice-Hall,

Englewood Cliff (1962). 9. G. H. Soriand, B. Hafskjold, O. Herstad, J. Magn. Reson., 124, 172 (1997).

10. E. W. Hansen, F. Courivaud, A. Karisson, S. Kolboe, M. Staecker, Microporous and Mesoporous Materials, 22, 309 (1998).

6.4 From Spin Dynamics in Ionic Pairs to Softening of Crystals 281

11. F. Stallmach, A. Graeser, J. Kaerger, C. Krause, M. Jeschke, U. Oberhagemann, S. Spange, ibid., 44-45, 745 (2001).

12. M. Okazaki, K. Toriyama, Y. Sawaguchi, K. Oda, Appl. Magn. Reson., 23, 435 (2003).

13. M. Okazaki, K. Kuwata, / Phys. Chem.. 88, 4181 (1984). 14. M. Okazaki, K. Toriyama, / Phys. Chem. B, 107, 7654 (2003). 15. F. Stallmach, J. Korger, C. Krouse, M. Jeschke, U. Oberhagemann, J. Am. Chem.

Soc, 122, 9237 (2000). 16. J. Korger, S. Vasenkov, Host-Guest Systems Based on Nanoporous Crystals (F. Laeri,

F. Schuth, U. Simon, M. Wark, eds.), pp. 255-279, Wiley-VCH, Weinheim (2003). 17. L. D. Gelb, K. E. Gubbins, R. Radhakrishnan, M. Sliwinska-Bartkowiak, Rep. Prog.

Phys., 62, 1573(1999). 18. M. Okazaki, K. Toriyama, J. Phys. Chem. B, 109, 13180, 20068 (2005). 19. K. Toriyama, M. Okazaki, J. Phys. Chem. B, 108, 12917 (2004). 20. M. Okazaki, K. Toriyama, S. Anandan, Chem. Phys. Lett., 401, 363 (2005).

6.4 From Spin Dynamics in Ionic Pairs to Softening of Crystals in Magnetic Field

Spin-dependent radical reactions controlled by diffusion in liquids are well known in spin chemistry. Moreover, there are theoretical predictions' ^ and direct experimental evidence'^"'^^ of magnetic field influence on spin-dependent reactions between structural defects in crystals (dopant ions, electrons captured into dislocation core, clusters containing few paramagnetic atoms, etc.). This work is a search for spin-dependent reactions in solids controlled by diffusion, thermal activation and plastic deformation of crystals. The ultimate aim is spin-dependent control over plasticity and luminescence that are sensitive to clusterization of paramagnetic ions in crystal lattice and wide practical applications of magnetic field effects controlling the physical properties of crystals. Since magnetic field influence on the cluster formation in a crystal is a main attribute of the spin-selective process, a crystal lattice must be nonmagnetic, keep spin polarization in radical pairs, and have comparatively fast diffusion of paramagnetic ions in the lattice. Good candidates of crystalline media are ionic crystals (NaCl, LiF, KCl, Csl, etc.) doped with Eu ions.

The Eu ^ ions coupled with vacancy provide electro-neutrality of the defect (so-called dipole). A general method for obtaining Eu" clusters is the thermally activated aggregation of single Eu~ dipoles by diffusion. Therefore, the precipitation process can be slowed down or accelerated by varying the temperature of the crystal as a whole. In contrast with liquids, diffusion in crystal lattice proceeds by limited numbers of aggregation ways and possible precipitate structures depend on the symmetry of the host material and ionic radius of the doping atoms.''* The final products of Eu aggregation in NaCl are thoroughly studied. Large clusters (precipitates) allow researchers to disdnguish them and determine their


Structure by electronic and X-ray diffraction and photoluminescence spectral ^ In contrast, the initial stages of Eu" dipole aggregation are not so clearly elucidated because the few atomic clusters are too small for diffraction methods. The initial stages of aggregation are extremely important, because they are the nucleation centers for nanocrystals

Free dipoles t . ' I % E = 0

very fast fast

i f~1—I—

• ; • i i !

0.24 eV

locked

t locked

-0J3e\f

2D precipitates along (013)

i

0.75eV

2D precipitates along (111)

Fig. 6-4-1 Possible pathways of aggregation. Circles denote Eu"^ ions; squares show vacancies. Energy values below each cluster are calculated binding energies per one dipole. Zero-energy level corresponds to energy of isolated impurity-vacancy dipoles. Compact configuration A is the most stable form of dimer. To construct the A dimer dipoles should perform several diffusion jumps in the vicinity of each other, each jump demanding more and more activation energy, so the rate of A dimer formation is very slow as compared with others. Concentration of B dimers is very low because they are composed of rare next-nearest-neighbors dipoles. The binding energies of C and D dimers are almost equal but C dimers are formed faster than D dimers due to topological reasons. Thus, at the first stage of aggregation, the concentration of C dimers is the highest. In the next step C type dimers block subsequent aggregation of dipoles, since the addition of one more dipole to the C dimer is not available for binding energy. To come out from the aggregation deadlock C dimers should be reorganized into other configurations. Dimers of types A and D are the nucleating centers of quasi-two-dimensional precipitates lying in the (013) and (111) crystallographic planes, respectively.


incorporated into the crystal lattice and they determine all subsequent aggregation pathways. Theoretical calculations''' gave several possible configurations of initial clusters in NaCl crystals (Fig. 6-4-1). Aggregation pathways branch at the stage of dimer formation (a dimer is a cluster containing two Eu ^ and two vacancies). This stage of aggregation plays a key role in the formation of big clusters because the probability of the addition of the next dipole to the dimer strongly depends on the dimer configuration. For example, the addition of dipoles to B and C types of the dimers is disadvantageous for thermodynamical reasons (Fig. 6-4-1). Thus some of the initially formed dimer configurations must be destroyed by thermal fluctuations and escape the dipoles to form other more favorable A and D dimer configurations. Dimer evolution could be sensitive to magnetic field if exchange interaction between dimer spins provides spin correlation for a long time and has a short time "window" during which energy splitting between exchanges states becomes the same order of value as the Zeeman energy. This window is initiated by thermal fluctuations and does not destroy spin correlation itself in the absence of a magnetic field because of total spin saving. During this time window one can expect switching over aggregation pathways under magnetic field action like that found in spin chemistry.

6.4.1 Spin-dependent Magnetic Field Effect in Eu ^ Clusters Being Formed during Slow Aggregation in Crystal Lattice: "Bulk Diffusion" Mode

NaCliEu (- 0.01%) crystals were used for studying the interplay between dipole aggregation and magnetic properties of crystals. Since aggregation is finished in aged crystals, thermal treatment was applied to initiate this process. Exposure of crystals for two hours at 770 K dissolves all clusters and produces separated dopant dipoles in the lattice. Rapid cooling of the annealed crystals to room temperature (quenching) causes partial freezing of high-temperature nonequilibrium state of dispersed ions and makes possible the observation of relatively slow degradation of crystal properties during growth of clusters at r = 293 K.

The SQUID magnetometer was used for measurements of magnetic moment of crystal at different aggregation stages. We studied comparatively short stages of aggregation. Average distance between dissolved separated dipoles in freshly quenched crystals is about d- 100 A at 0.01% concentration of Eu. Casual approaching of two dipoles takes time t ~ d^lD ~ 10^-10^ s (~ 100 days) at the diffusion coefficient D ~ 10" ^ m^ s"' typical for double valence dopant ions in NaCl crystals. The duration of our experiments was about this time. For this reason MEU(T)T

dependence was approximated by two contributions: x\ separated dipoles and X2 dimers of different configurations MEU = A dip + Mdim. Dipole contribution A/dip was described by the Brillouin function Bs{^):


A/dip(7-) = ^;UB5x,Bs(g) (1)

where ^B is the Bohr magneton, g = 2 the "^-factor," ks the Bohzmann constant, S = 7/2 the spin value of Eu"* ions,

g = - - and «s(g) = ^ c o t h ( ^ ^ , ) - - c o t h ( ^ j (2)

Dimer contribution A/dim must account for all possible spin states with total spin S varied in the range from 0 to 5max = 25EU = 7' ^

where g = Sgii^Blk{T + To), parameter To characterizes average exchange interaction in different dipoles. Approximation of experimental dependencies by MEU{T)T= (Mdip(r) + Md\m(T))T function was obtained for different times elapsed after quenching. Good coincidence of theoretical curves with experimental data confirms the validity of the "dipole + dimer" model and gives the time dependencies of dipole x\ and dimers X2 concentrations. In agreement with a simple prediction, the number of dipoles decreases as dimer number grows. This indicates that SQUID magnetometry is a convenient method for studying aggregation.

Initial and final states of this cluster must have different total spins. In this case change in spin state of magnetosensitive clusters under a magnetic field can be detected using the SQUID magnetometer. Since the strongest sensitivity of clusters to MF is known to arise at about ~ 50 h after quenching (Fig. 6-4-2), the sample was kept for two days at 300 K after quenching. In these experiments we took into consideration the absence of magnetic field effects in low field J? < 1 T at room temperature and in magnetic field up to 20 T at low temperature, 7 < 77 K. We used weak MF of SQUID magnetometer 0.3 T for testing of spin state and strong MF 5 T to initiate cluster transformation. The dependence MEU(T)T

was recorded in a weak MF in the temperature range 2 - 200 K. Then the crystal was slowly heated close to 300 K and kept for 40 min in the magnetometer without any disturbance. After this procedure the crystal was cooled to 2 K and dependence MEU(T)T was measured again. It was necessary to check possible deviations of MEU that might occur due to aggregation itself at high temperature. The heating of crystals itself does not produce any changes in the MEn(T)T curve (Fig. 6-4-2). Exposure of crystals in static MF B = 5 T at 300 K modifies the shape of the MEU(T)T

dependence recorded at low temperatures (Fig. 6-4-2). Similar changes were found in each of the six samples used in our experiments. Observed


3 UJ

1.0

0.9

0.8

"n ^

^

% / /

/ / 1 i-

/ /

T -

y

JL.

— I 1 1 1 ^ r-

/ /"

A V

/ v

•—

t

•" ""

J 1

- 2 - 3

- 4 - 5

- 6 J

T / K

Fig. 6-4-2 Points are experimental MÂT)T dependencies: (1) recorded during first measurement M before MF application, (2) recorded during second measurement M before MF application, (3) recorded during third measurement M after MF application. Solid lines 1-3 are approximations of M^y,{T)T dependencies by the Brillouin function for an average spin 5. Solid lines 4-6 are dependencies of MEUC7)7 calculated from the Brillouin function for: (4) separated dipoles S = 7/2, (5) dimers 5max = 7, (6) trimers 5max = 21/2.

deviations of MEU(T)T dependence are due to the magnetic properties of crystal changing under strong MF.

The Brillouin functions of dipoles (S = 7/2) and clusters with maximal spin 5max (dimer spin Smax = 7, trimer spin 5niax = 21/2) are presented in Fig. 6-4-2 by solid lines. The experimental curves are found between two Brillouin functions of the dipoles (5 = 7/2) and the dimers. Average spin approximation gives the average spin values determined before MF application 5 = 6.2 ± 0.1 and after MF application 5 = 4.8 ± 0.1. Thus, exposure of crystals in MF causes reduction of spin of magnetosensitive clusters. Closeness of the experimentally determined average spin S = 6.2 to its maximal value 5max = 7 in dimers identifies the magnetosensitive cluster as a dimer (pair of the dipoles) having parallel spins bound by exchange interaction. The discrepancy of measured spin values from the theoretical 5max = 7 can be explained by the contribution of free non-aggregated dipoles into the SQUID signal.

Another advantage of our experiments is the capability of dislocation obstacles to be photoluminescent (PL) labels. The experimental conditions established in this work allowed us to observe the effect of a "weak" magnetic field on the PL of a small number of reliably identified centers, which represent Eu^^ dipoles. Ultraviolet excitation of the Eu^^ ions


3 - - - 1

— ' /S>

n\ / So \

1 / \

1 ^ 1

1 1

1 A

M / s, \ '

200 300

Xl nm

400

Fig. 6-4-3 Excitation spectra of Eu'^ dipoles and small clusters in NaCliEu (emission band 427 nm) recorded within 90 hours after quenching: (1) crystal stored without MP, (2) crystal stored inMFB= 15 T.

entering into the clusters of such centers initiates transitions from the 4f shell to the 4f ^5d shell split in the crystal field into two tig and e^ sublevels differing in symmetry.' ^ The radiative relaxation of excited ions proceeds from the state of the eg symmetry with a high probability (~ 99%), whereas the transition from the t2g state has a high probability of radiation-less relaxation. Before the emission of optical quant the t2g -> eg transition must occur. It gives a single emission band and two bands in excitation spectra (Fig. 6-4-3). The relation between intensities of excitation bands S\/S2 characterizes the distribution between radiative and radiation-less processes strongly dependent on intracrystalline field and atomic local arrangement of Eu ^ ions.

In order to provide a way for clusters of various atomic configurations to form, the samples after quenching were held for a certain time t (from 10 min to 100 hours) at room temperature in MF 15 T Quenched crystals contain a single PL band including small cluster and separated dipole contributions. Since we cannot distinguish these contributions in the emission spectrum, the excitation spectrum was recorded several fimes after quenching during exposure of the crystal in MF. Every time a crystal was taken out from MF for excitation spectrum measurement. Exposure of a crystal to a MF with ^ = 15 T at 7 = 293 K reduces the ratio of intensities in the 5i and 2 bands of the excitation spectrum recorded at 427 nm (Fig. 6-4-3). The MF effect on 5i/52 can be observed only in the samples that were stored after quenching at room temperature for r > 50 h. This means that bulk diffusion during -- 50 h leads to the formation of magnetosensitive clusters.


6.4.2 Magnetic Field Influence on Eu * Clusters Formed near Dislocation Cores by Diffusion: "Fast Diffusion" Mode (dc/dt = const)

As the aged NaCl:Eu crystals were not subjected to any thermal treatment for a long time, most doping ions were included in large-scale precipitates. A composite structure of the emission PL spectrum was observed in unquenched crystals (Fig. 6-4-4(a)). A regime of constant rate of stress sweeping (da/dt = const) led to the redistribution of the emission intensity between various regions of the PL spectrum and to the appearance of

(a) 400 , r

300 h

=! 200

3D-EUCL

small clusters

EuCl2(111)

->

i-EuCl2(013) New! (538nm) New II . (SOOnm)

400 500

>. /nm

600

(b)

240

Fig. 6-4-4 (a) PL spectra of NaCl: Eu crystals before deformation (curve 1) and after deformation e = 10% without MF (curve 2). (b) PL spectra of NaCliEu crystals deformed to f = 10% without MF (curve 1) and in MF B = 15 T (curve 2). Decomposition of spectra into individual Gauss components (in photon energy scale), associated with different europium complexes, is shown by thin lines.


additional bands at 536 nm and 600 nm reported in reference 15 (Fig. 6-4-4(a)). Decomposition of averaged spectra on Gauss components reveals that the contribution of the dipole and small clusters decreases after deformation. The intensity of the band associated with (013) precipitates slightly grows in the absence of MF. This means that dislocation displacement causes clusterization of the dipoles and small clusters into big ones. Since the deformation process takes - 1 0 - 3 0 min we can conclude that aggregation proceeds faster in comparison with the case of bulk diffusion described in the previous section. As the acceleration of the diffusion is usually provided by the increase in the mobility of doping ions near the dislocation core, the described regime of plastic flow could be called the "fast diffusion" mode.

As one can see from Fig. 6-4-4(b), deformation-stimulated redistribution of the PL intensity is sensitive to magnetic field applied during deformation. Decomposition of the spectrum into the Gauss components reveals an increase of PL intensity in bands of free dipoles and small clusters (at 427 nm) if deformation was performed under constant MF B = 15 T. The luminescence band associated with (013) precipitates (at 485 nm) grows during deformation in a magnetic field more intensively in comparison with zero MF deformation. This means that the MF promotes generation of small clusters (at 427 nm) and (013) plane precipitates (at 485 nm) during deformation. The other bands, including the new ones at 536 and 599 nm, are not affected by the applied MF. Since the magnetic field changed deformation in the regime of destruction of precipitates we can assume that the "fast diffusion" mode was changed to the "cut off mode described in the following section. Aggregation of the dipoles in clusters during plastic flow was found by both SQUID magnetometer and EPR spectrometer.

6.4.3 Magnetic Field Effects on Eu * Clusters Generated by Moving Dislocations: "Cut Off" Mode (de/dt = const)

A so-called "hard" deformation machine providing linearly increasing deformation (de/dr = const) was used. Aged NaCl:Eu crystals were unloaded between additional portions of deformation as in the previous series to perform measurements of the PL spectrum. Comparative analysis of the absolute values of changes in integral intensities of luminescence bands during deformation allows one to conclude that processes taking place under the deformation in the d£/dr = const regime can be divided into two groups: (i) transformation of the three-dimensional EUCI3 precipitates into emitting centers at A = 536 nm and (ii) transformation of the two-dimensional plate-like EuClz precipitates into small-size clusters. Since the destruction of large clusters into small ones predominates in this series of experiments, it can be concluded that another mode of plastic flow exists. This can be called the "cut off mode. After each deformation step on


1-2% crystal was subjected to the action of a pulsed magnetic field 6 T for 10 ms, the deformation and magnetic field treatment were performed separately both to accumulate more magnetosensitive clusters and to prevent artifacts from the action of magnetic field on metallic parts of the factory-made deformation machine. Exposure of the deformed NaCliEu crystals to the magnefic field leads to a redistribution of the emission intensity between various components of the PL spectrum. Magnetosensitive clusters have been formed during the "cut off' mode of the deformation. A study of the variation of each separate component showed that the intensity of emission at 427 nm decreases, while that of the band at 485 increases. The other bands, including the new ones at 536 and 599 nm, were not affected by the applied magnetic field. Luminescence of nondeformed crystals was not sensitive to MF. Thus, plastic deformation in the "cut off mode supplies a number of magnetosensitive clusters because big clusters are destroyed by moving dislocations.

6.4.4 Spin-dependent Magnetic Field Effect on Plasticity of Crystals at Different Modes of Magnetosensitive Cluster Formation

Let us summarize the described data from the viewpoint of clusterization modes. Three different regimes of magnetosensitive cluster formation exist (Fig. 6-4-5).

A. Bulk Diffusion Mode The bulk diffusion mode can occur in the absence of dislocations and plastic flow (Fig. 6-4-5(a)). In this regime free paramagnetic ions meet each other and form magnetosensitive clusters in the bulk of crystal. As shown above, the structure of the intermediate clusters can be changed by MF due to modification of binding exchange energy caused by spin transition in clusters. Plastic properties depend on the final atomic configuration of the clusters because overcoming these clusters by dislocations is very sensitive to lattice distortion (or power of the elastic field produced by the clusters). Dislocation mobility serves as an indirect measure of magneto-induced events proceeding to the crystal lattice. After quenching of crystals there was a waiting period essential for the accumulation of magnetosensitive clusters. Similar magnetoplastic effects were found in ionic crystals containing other doping ions ^ as well as in metals' ^ and semiconductors.'^^

B. Cut Off Mode This mode produces magnetosensitive clusters by destroying large precipitates under dislocation elastic fields (Fig. 6-4-5(b)). In this case it is not necessary to perform quenching of crystals, and magnetic field effects could be observed in aged crystals both during plastic flow and after deformation. Some of the magnetoplastic effects were found during


(a) Bulk diffusion mode Dispersed dipoies Dimsrs Reconstruction of the Growth of ciusters

dimers under MF

(b) Cutoff mode Precipitate before destruction

Cutting | Destroyed precipitate

M Partial restoration under MF

/ /

(c) Fast diffusion mode Dispersed cHpoles I Capture of the dipoles • Fast aggregation of

I in dislocation core dipoles under MF

/

•/,,,•

Separation of dislocation from clusters

Fig. 6-4-5 Three different modes of "step-by-step" nucleation of magnetosensitive clusters.

deformation of unquenched crystals.'^^^^ It was shown that recovering magnetosensitivity of plastic flow after the first pulse of MF originates after some delay necessary for deformation of crystal and accumulation of magnetosensitive clusters. ^ This fact is evidence that plastic deformation itself produces magnetosensitive clusters.

C. Fast Diffusion Mode This type of mode occurs near the dislocation core (Fig. 6-4-5(c)). It is similar to the bulk diffusion mode. The main difference between them is the diffusion coefficient that is a few orders of magnified higher in the vicinity of the dislocation core than in the undistorted lattice. Since the configuration of clusters lying near the dislocation core is more important for dislocation mobility than that of clusters in the bulk, even a small part of the clusters being transformed by MF could strongly contribute to the plasticity. For observation of magnetoplasticity in this mode it is necessary


to have quenched crystals with dispersed impurity that are able to be absorbed into the dislocation core.

It is well known that plastic flow can be discontinuous (or jumplike) (Fig. 6-4-6). This phenomenon, called the Portevin-Le Chatelier (PLC) effect, is observed in NaCl:Eu crystals under definite range of impurity concentrations. The effect is caused by dynamic strain aging, associated with the formation of Cottrell impurity clouds around dislocation cores. This process is accompanied by the clusterization of individual impurity-vacancy dipoles (Fig. 6-4-6). The PLC effect in the deformation machine linearly sweeping mechanical stress manifests itself in the form of a stepped deformation curve. We observed notable change in the discontinuous flow parameters under MR The presence of a MF decreased the probability of the appearance of deformation jumps, caused chaotization of the steps size ACT and decrease of depth in the deformation jumps A£ of up to two times. Calculation of the probability density of the magnitudes of deformation jumps (Fig. 6-4-7), according to the formula p

i

^ 3

3. C o

i 1

I

} 1

, - A*

'*' Sw»«ping of fr»»dipoi*s

3 ' 5

Fast aggr«|)«tion ofth«dipolMln dislocation cors

12

-'". r. -

. - T ^ '

Swrasping of frsadipolas

""le ' 20"

^^^fiDOOO-

24

fotrce f i t

force

• • f

Fig. 6-4-6 Height of as-quenched NaCl:Eu crystal during its deformation versus time (or linearly growing stress o). This fragment of deformation curve presents two jumps of the PLC effect. The lower part of the figure shows a schematic representation of interaction between individual dislocation and impurity-vacancy Eu'^ dipoles, explaining the origin of steps on the deformation curve. At the beginning, free dislocation moves and gathers dipoles in its core. The velocity of dislocation is determined by the competition of two processes: absorption of dipoles to the dislocation core and liberation of the dislocation from the cluster cloud. When aggregation is very fast, dislocation stops due to cluster growth. Increase of external linearly sweeping stress kicks out dislocation from clusters and allows the deformation process to continue. (Clusters are much stronger obstacles for dislocation movement than dipoles.) As the external stress sweeps linearly with time, one has to wait when stress achieves the critical value needed for separation of dislocation from clusters. This is the reason for the existence of horizontal segments on the deformation curve.


(a) 0.3

0.2

0.1 h

0.0

r 1

1

• ^ - ^ H ^ ^ ^ ^ ^^_"".

A e / %

(b) 0.3

0.2

0.1 h

0.0

- I

A •

\ \

^ / ' ^ /

^ > • • " »

, , 1 . * • ; • . ' • #

M . , n t J B L • <

A e / %

Fig. 6-4-7 Probability density p of deformation jumps (A£ and its approximation (solid line) by sum of Gauss function with amplitude A\ and function AZ/AE (dotted lines): in zero MF (a), in MF B = 15 T MF (b). Insets show photos of the NaCl:Eu crystals surfaces after plastic deformation e ~ 10%: under zero MF (a), under MF B = 15 T (b).

= M(A£)/[(ei-ei_i)Motal], where N,{A£) is number of jumps in the range £\- £i-i, shows that plastic deformation of the crystals occurs by two additive flows of events: movement of correlated ensembles of dislocations and independent noise-like movement of dislocation groups of various sizes. The probability densities obtained were approximated by the sum of the Gauss function with amplitude A] and the decreasing function A2/AE. The approximating curves precisely follow the experimental data, both for the tests carried out in MF and for the reference tests (Fig. 6-4-7). One can see that the MF decreases the contribution of the correlated component of the dislocation motion and increases the noise-like component.

The spatial heterogeneity of deformation often looks like steps or shear bands on the specimen surface. Since the typical depth of


deformation jumps on deformation diagram registered in our tests was -1-10 //m, one can expect these jumps to correspond to definite spatial heterogeneity of deformation of similar scale. Observation of the surfaces by optical microscope allowed us to find alternating zones of plastic deformation with average width ~ 20 jum (inset. Fig. 6-4-7(a)). These zones were parallel to the (001) direction and perpendicular to the load axis. In crystals deformed in MF, decrease in the amount of pronounced deformation zones was observed (compare insets of Figs. 7(a) and -7(b)). A comparison shows that in crystals subjected to MF, the number of deformation bands on the surface was on average one half that in crystals not subjeced to MF.

PLC itself in the absence of MF arises as a result of the sweeping of free dipoles and their capture in the dislocation core (Fig. 6-4-6). In the dislocation core, diffusion provides fast clusterization of the dipoles into clusters that are strong obstacles to dislocation motion. Thus, after a short period of motion, the dislocation absorbs dipoles and clusters. Under the action of thermal fluctuations dislocation can overcome formed clusters. Liberation of dislocation from the cloud under the action of a magnetic field leads to the partial destruction of clusters and to the continuation of dislocation motion.

6.4.5 Conclusions

1. The initial stage of diffusion-controlled growth of Eu clusters inside the NaCl crystal lattice is found to be spin-dependent and sensitive to static and pulsed magnetic fields 5 = 5-15 T. Dimers (couple of dipoles) of several atomic structures are formed and undergo thermo-activated atomic transformation at this stage. Application of the magnetic field leads to the reconstruction of dominant type dimers into more favorable ones for subsequent growth of large clusters. Switching over aggregation pathways under magnetic field at the beginning of aggregation affects the final products of aggregation and stimulates more rapid growth of 2D precipitates.

2. The reason for the rearrangement of Eu' ^ ions belonging to dimers is alteration of the dimer spin state under magnetic field in thermo-excited positions of atoms when the covalent bond between them is stretched out and the spin transition is resonant in magnetic field. Change in the spin-electronic ground term of the Eu " pair from the high-spin state to the low-spin state modifies part of the binding energy due to exchange coupling and causes destabilization of the initial atomic configuration of the dimer.

3. Magnetosensitive clusters can be formed in NaCliEu crystals in three different ways: (a) slow thermo-activated aggregation of the dipoles in the crystal lattice, (b) cut-off of big precipitates by gliding dislocations, (c) fast aggregation of the dipoles in dislocation cores. This means that


different kinds of magnetoplastic effects have a uniform origin, i.e., dimer transformation. The experimental conditions of magnetoplastic effect observation can vary depending on the microscopic mechanisms of the magnetosensitive dimer formation.

References

1. V. Alshits, E. Darinskaya, J. Exp. Theor. Phys.. 70, 749 (1999) (in Russian). 2. M. Molotskii, R. Kris, V. Fleurov, Phys. Rew B, 51, 12531 (1995). 3. M. Molotskii, V. Fleurov, Phil. Mag. Lett., 13, 11 (1996). 4. M. Molotskii, Mat. Sci. Eng., A287, 248 (2000). 5. R. Morgunov, S. Shmurak, A. Baskakov, Y. Tanimoto, J. Exp. Theor. Phys., 97, 754

(2003) (in Russian). 6. R. Morgunov, Progress in Physics (Physics -Uspekhi), 47, 125 (2004) (in Russian). 7. V. Alshits, E. Darinskaya, M. Koldaeva, E. Petrzhik, Crystallography Reports, 48,

768 (2003) (in Russian). 8. A. Baskakov, L. Dunin-Barkovskii, R. Morgunov, Y. Tanimoto, S. Shmurak, J. Exp.

Theor. Phys., 100, 66 (2000) (in Russian). 9. Yu. Golovin, R. Morgunov, A. Baskakov, Mol. Phys., 100, 1291 (2002).

10. Yu. Golovin, R. Morgunov, C/z^m./?^v., 23, 23 (1998). 11. J. Strutt, E. Lilley, Phys. Stat. Sol., 33 (a), 229 (1976). 12. A. Cordero-Borboa, O. Cano-Corona, A. Clavel-Hemandez, E. Orozko, Phisica C,

19,7113(1986). 13. J. Anderson, G. Kido, Y. Nishina, M. Gorska, L. Kowalczyk, Z. Golacki, Phys. Rev.

B,41, 1014(1990). 14. J. Rubio, J. Phys. Chem. Solids, 52, 101 (1991). 15. Yu. Ossypian, R. Morgunov, A. Baskakov, S. Shmurak, Y. Tanimoto, Phys. Stat. Sol.,

201(a), 148 (2004). 16. Yu. Golovin, R. Morgunov, A. Dmitrievskii, Mat. Sci. Eng., 115, 345 (2000). 17. O. Dacko, V. Alekseenko, Solid State Physics, 39, 1234 (1997) (in Russian). 18. E. Darinskaya, E. Petrzhik, S. Erofeeva, J. Phys. C, 14, 12883 (2002). 19. Yu. Golovin, R. Morgunov, V. Ivanov, Solid State Physics, 39, 2016 (1997) (in

Russian). 20. L. Dunin-Barkovskii, R. Morgunov, Y. Tanimoto, Solid State Physics, 47, 1241

(2005) (in Russian).

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