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INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, VOL. VII, 99-107 (1 975)

Excited State Kinetics and Stern-Volmer Quenching Plots in the Photolysis of

Azoisopropane at 366 nm

G. 0. PRITCHARD and F. M. SERVED10 Department of Chemistry, [Jniversity o j California, lranta Barbara, California 93706

Abstract

The photochemistry of azoisopropane is reinvestigated at 366 nm over an extended pressure range by using n-butane as an added bath gas, and over a range of temperature from 217" to 180°C. The Stern-Volmer type plot of the NS product quantum yield is interpreted in terms of the decomposition of the vibrationally excited upper singlet and triplet states, with the onset of the dissociation of the vibrationally equilibrated triplet state as the temperature is increased. The energy barrier for the dissociation of the vibrationally equilibrated first triplet state is found to be 8.8 kcal/mole. Triplet sensitization experiments with biacetyl correlate with our observations, and it is suggested that the proposed mechanism is generally applicable to the photodissociation of acyclic azoalkanes at 366 nm, based on a comparison of our data with those of Wu and Rice on hexafluoroazomethane.

Introduction

The photochemistry of the azoalkanes and their use as free radical sources has interested photochemical kineticists for several decades. A comprehensive review was presented by Collier, Slater, and Calvert in 1968 [ 11, dealing with the nature of the excited states and the primary processes involved in the photolysis of acyclic azoalkanes. However, the role that multiplicity plays in these excited state reactions is not established [l-51. Luminescence studies cannot be used as a diagnostic tool since acyclic azoalkanes do not fluoresce or phosphoresce when irradiated [ 1 , 2, 6, 71. The simplest activated molecule reaction scheme is given by [ I , 3,71

(a) A + h v - A "

(1) A * - + N f + 2R

(11) A * + A(M) -+ A + A(M)

Although not explicitly stated, it has been generally assumed that the A * molecules represent the vibrationally excited upper singlet state (&*) or that decomposition (and perhaps deactivation) occurs from a highly vibrationally excited

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100 PRITCHARD AND ‘SERVED10

ground state molecule (So**) formed via a rapid internal conversion (IC). How- ever, Calvert and Steel and their respective coworkers conclude that the experimental evidence is best explained by assuming that the A* molecules in reactions (I) and (11) are in a vibrationally excited triplet state, formed by a rapid inter- system crossing (‘chat" ISC) from the initial photochemically excited singlet state.’ The reaction scheme leads to the traditional Stern-Volmer (S-V) type equation

(A) w 2 - l = 1 + (kII/kI)[AI

The published data on acyclic azoalkane photochemistry in the 366-nm region tend to obey the S-V expression very closely [2, 7, 9-14]. All of the data were obtained in the range of 0-200 torr.2 In 1968 Wu and Rice [l6] published an important reinvestigation of the photolysis of hexafluoroazomethane (HFAM). Their data, obtained a t room temperature over an extended pressure range (up to 700 torr), displayed distinct curvature in the S-V plot. I t was concluded that two dissociating states (with different rates) were involved, which were either different electronic states (singlet and triplet) or different configurations of the same electronic state [lob, 16, 171. Probably the most definitive data exist on hexafluoroacetone (HFA) [15, 181, and the curvature in the S-V plots is un- equivocally interpreted in terms of decomposition from both the S3 and TI states, in the excitation range 265 to 334 nm. Based on the discussion of Porter and Connelly [19] on the possible primary kinetic processes of photoexcited ketone and azoalkane molecules, we have recently further evaluated the behavior of S-V plots for photochemical decomposition versus collisional quenching in these sys- tems [ZO]. We have also presented data [21] on the photolysis of HFA in the presence of a triplet quencher, and conclude that both the vibrationally excited and vibrationally equilibrated S1 states decompose. The dissociation of the latter state only becomes important as the temperature is raised.

Herein we describe a reinvestigation of the gas phase photolysis of azoisopropane (AIP) over a sufficient range of pressure to look for curvature in the S-V plots. In the absence of luminescence such plots provide an alternative method of determining the nature of the excited species, as well as the primary processes that they undergo [ZO, 211. The 366-nm photolysis of AIP has been studied previously [3, 111. Durham and Steacie [ l lb] detected some curvature in the S-V plot at about 10 torr and 100OC. The later and much more extensive data

. .

It should be noted that Steel’s group [4, 81 has observed fluoresence with cyclic azoalkanes, with dissociation from both the first singlet (S,) and triplet ( T I ) states.

ZThis statement excludes the pioneering experiments of Weininger and Rice [loa] on azoethane, who went to almost 2 atmospheres. However, the data are rather scattered and will not be considered further. Careful linear extrapolations to zero pressure often result in intercepts that deviate from unity, and this has been the subject of discussion [7, lob, 1 la, 13b, 151. The various mechanistic possibilities need not be reiterated here, and our ensuing discussion will regard them as being not significant.

PHOTOLYSIS OF AZOISOPROPANE 101

of Riem and Kutschke [ l l a ] were interpreted as linear plots with intercepts at zero concentration ((p~$")-- l greater than unity. Both studies [ 1 11 were conducted at less than 100 torr. Steel and coworkers [3] photolyzed AIP in both the gas and solution phases.

Experimental and Results

AIP was obtained chromatographically pure from Merck, Sharp, and Dohm of Canada, Limited. Azomethane was synthesized and purified by standard procedures [22, 231, and n-butane was research grade obtained from Phillips Petro- leum Company. Biacetyl was supplied by Chemical Samples Company and purified by bulb-to-bulb distillation. The mercury and grease-free apparatus and experimental procedures are described elsewhere [2 1, 231. The decomposition of AIP was kept to less than 0.1% in all experiments. At pressures above 30 torr of AIP, n-butane was used to increase the pressure in the system. The 366-nm radiation was isolated with a Farrand interference filter, having a peak wavelength at 368 nm and a half-bandwidth of 5.5 nm, with a transmission of 41%. The absorbed intensity never exceeded 15% of the incident light. Blanks were run under all conditions to correct for any dark reactions. The correction was -5-10y0 at 160°C. Azomethane was used as an actinometer with ( P N ~ taken as unity [24]. In runs were biacetyl was added as a triplet sensitizer an Oriel interference filter with a peak wavelength at 436 nm, a half-bandwidth of 9 nm, and 52% transmission was used. No detectable direct photolysis of the AIP occurred with t.his filter.

Detailed tabulations are given elsewhere [23]. I t is assumed that the quenching efliciencies of the AIP and n-CIHlo are the same. Wu and Rice [ 161 found the eficiencies of HFAM and CzFs to be virtually identical in HFAM photolysis. We used n-CrHlo, rather than (CHJ) zCHCH(CH3) 2, due to the former's greater volatility.

In the photosensitized experiments with biacetyl, no Nz was detected at 27 or 12OoC, but some sensitized Nz product was detected at 160°C (see Table I ) .

The data on AIP photolysis are presented as a S-V plot in Figure 1.

TABLE I. Biacetyl photosensitized dissociation of AIP at 436 nm and 160°C.

[AIP] x lo3 [Biacetyl] x lo3 [ C ~ H ~ ~ I x lo3 (P(N2)

1.105 1.105 0.4

1.105 1.105 6.65 0.4

The N P yield was corrected for the dark reaction. Concentrations are in mole/l. The expressed quantum yield is based on the production of N z in the photolysis of AIP at 366 nm and 16OoC, see Figure 1. The values are therefore only relative.

102 PRITCHARD AND SERVED10

Discussion

Our data on AIP, taken at room temperature over 250 torr, display curvature similar to that found by Wu and Rice [16] for HFAM at room temperature. Our quantum yields at lower pressures generally agree with the previous studies on AIP at less than 100 torr [ 111; we conclude that the data of Riem and Kutschke [ 1 la ] are best interpreted as having some curvature, with ( p ~ * O ) - l = 1 (see [ l la, Fig. 11).

We propose the following basic mechanism to account for our results, where the superscripts have their usual significance [16, 20, 211.

(a) A + h v 4 ‘A*

(1) ‘A* -+ Nz + 2R

(2) ‘ A * + M -+ lAo + M

(3) ‘ A o 4 Nz + 2R

(4) 1A* -+ 3A*

(5) 3A* 4 Nz + 2R

(6) 3A* + M -+ 3A0 + M

( 7 ) ‘A0 3 3AO

(8) 3A0 4 Nz + 2R

(9) 3AO 4 A

“Hot” ISC of ‘A* molecules is competitive with dissociation, and 3A* molecules decompose, but at a slower rate. There is no basis to assume other than the strong collision assumption for deactivation in both the singlet and triplet mani- , folds. The mechanism accounts for the shape of the S-V plots for both AIP and HFAM at room temperature [16]. As the temperature is raised the p~~

values increase for AIP in Figure 1, due to the increasing dissociation of ‘ A * and 3A* molecules, and the eventual significant onset of the dissociation of molecules from the 3A0 state. The latter dissociation, being from a vibrationally equilibrated (Boltzmann) state which is not subject to pressure-dependent depopulation, is manifested as a pressure-independent leveling off in the S-V plot; compare HFA [15, 18].3 Since HFAM is known to photolyze in solution between 0 and 120°C [25], the room temperature S-V plot would presumably level off at high enough pressures, in accord with some decomposition of molecules from the 3A0 state. AIP also photodecomposes slightly in solution at room temperature

3Reaction (3), the decomposition of the Boltzmann singlet state, is not necessary to the mechanism, and its importance cannot be assessed until triplet quenching experiments are per- formed, cf. HFA [21]. ‘A0 and 3Ao states are equivalent with regard to S-V behavior.


8.0 4 I I

6.0

4.0 - 160’C

0 2.0

I .o

-

I I I I I I 1 I I I I I

2.0 4.0 6.0 8.0 10.0 12.0

[MI x lo3 Figure 1 . Stern-Volmer plot for AIP at various temperatures. Points above [MI = 1.6 X lop3 mole/l. have n-butane added to achieve the indicated concentrations. The vertical arrows show where average data are taken to con- struct Figure 2.

with ( c p ~ ~ ~ ) - l = 40 [3]. The rates of the respective infinite pressure decomposi- tions will be largely dependent on the triplet energy barriers ET. Reaction (9) is included for completeness to allow for isomerization, and other nondissociative return processes to the ground state (e.g., IC via ISC). The fact that biacetyl triplet sensitizes the decomposition of AIP at 160°C but not at lower temperature is in accord with our observations and mechanism and the triplet energies involved. Calvert and coworkers [ 11 estimate the triplet energy level for 1 , 1’-azoisobutane a t 53 i 3 kcallmole, and presumably AIP is much the same. The energy of triplet biacetyl is about 55 kcal/mole [ 11. Similarly Calvert and coworkers [ 11 found that there was no detectable Nz formed when 1 , 1’-azoisobutane was sensitized with biacetyl between 25 and 112°C at 436 nm, even though the phosphorescence quenching showed that energy transfer was complete. We can make the reasonable assumption that the energy barriers to triplet dissociation are very similar for the two azocompounds.

We may now consider the alternative representations of the A * molecule in reaction (I). If it is a very “hot” ground state molecule formed by a rapid IC, the S-V plot would be linear, assuming that quenching (of either S1* or SO** molecules) occurs on a single collision. If the quenching of So** molecules was

‘The photoisomerization of AIP is a triplet state reaction and Steel and coworkers [3]

_______

suggest a 37r, a* rather than a 3n, a* state.


multistage, the curvature would be concave rather than convex [ 191. Theoreti- cal considerations and interpretations of the data on acyclic azoalkane photodecomposition also effectively rule out the participation of the So** state [ I , 4, 5, lob].

If the molecules in reaction (I) are "hot'' triplet state molecules, formed via a very rapid ISC, we have as the total mechanism reactions (4), (5), (6), (8 ) , and (9). The decomposition of the Boltzmann state would lead to a leveling off in the S-V curve, which only occurs at the highest temperatures. However this is not the case at low temperatures in the pressure range examined. The biacetyl sensitizing experiments further show that 3A0 molecules have insufficient thermal energy to decompose up to 120°C which should result in linear S-V plots if only 3A* molecules were decomposing. This again is clearly not the case. Multistage deactivation of 3A * molecules would lead to a concave S-V plot, which is not observed.

As we have seen in the complete mechanism, reaction (I) incorporates the simultaneous decomposition of both the vibrationally excited ' A * and 3A * states, reactions (1) and ( 5 ) , with the dissociation of the 3A0 state at higher temperatures. The mechanism also necessitates the postulate that the photochemically excited HFAM molecules are still not vibrationally equilibrated at almost one atmosphere pressure [16] in the presence of an efficient vibrational relaxer. Acetone and HFA, which are more energetic triplet sensitizers than biacetyl, sensitize the decomposition of azomethane and HFAM, respectively, at room temperature [6, 211, which occurs due to dissociation from the 3A* state. For example we observed [21] a 7570 reduction in the Nz product yield in going from 60 to 200 torr in the sensitized decomposition of HFAM by HFA at 27°C. I t should be noted that the molecules formed initially in the direct photolysis contain approximately 6 kcal/mole additional excitation energy over those in the HFA sensitized experi- ment [26], so that their lifetime may well be short enough for decomposition to occur at 700 torr. I t is known for instance that no collisional relaxation of azomethane occurs below one atmosphere IS].

A steady-state treatment on reactions ( l ) , (2), (4), (5), and (6) leads [16] to an equation of the form

(B) where a, b, and c are composite rate constants.

CPN,-' = 1 + [(WI + b[MIZ)/(1 + c[Ml)l

At sufficiently high pressures eq. (B) takes the form

C P N ~ - ' = 1 + Q / C + (b/c)[Ml (C)

Substituting the rate constants [16] we have

(D) CPX,-' = 1 + (kg + k5) / k1 + (kz/kJ[Ml

which is the simple S-V expression with an intercept > unity. The high-pressure decomposition is therefore due primarily to the dissociation of ' A * molecules,


0.7-

0.6

0.5

0.4

0.3

0.2

with the longer lived 3A * molecules being effectively quenched by collision. Ex- trapolations of the high pressure slopes of the data at 27, 120, and 140°C in Figure 1 lead to intercepts at ( c p ~ ~ O ) - l 3i 2.5, or (k4 + k5)/kl = 1.5, which is not in disagreement with kq > kl > k 5 . From their curve fitting on HFAM, Wu and Rice [ 161 also find that (k4 + k5)/kl = 1.5 with k4( 1 1.4X lo9 sec-l) > kl(8.2 X lo9 sec-l) > k5(0.90 X lo9 sec-I); the values are for room temperature. Their extrapolated intercept is ( c p ~ ~ O ) - ' > 1.75, which indicates more significant 3A * dissociation at relatively higher pressures with HFAM than for AIP.

Taking cuts at the concentrations indicated by the vertical arrows in Figure 1, the data are presented in the Arrhenius form in Figure 2. Assuming that we are looking at a vibrationally equilibrated state, the slope results in a triplet energy barrier of Er = 8.8 kcal/mole. The two lines of very small and identical slope result from the pressure-dependent depopulation of the ' A * state. The location

0 - -

-

-

-

200.C 100.C

0.8 l.r--.--. 0

N2

0. I 5 - 1 2.10 230 2.50 2.70 2.90 3.10 3.30

103/T"K

Figure 2. Arrhenius plot for AIP. The points are averaged values taken from Figure 1. o-[M] = 6.65 X mole/].; O-[M] = 10.1 X mole/l. The point at 200°C is from an individual run, and there was a large correction for Nf formed thermally. ET is the triplet energy barrier for 3A0 molecules; E s is associated with the very small temperature dependence for dissociation of the ' A * state.


of the low-temperature data points will also be a function of the photon energy input [13b, 151.

In the past linear Arrhenius plots have usually been made of the initial S-V slopes (log k11/k1 versus 1/T O K ) resulting in supposed energy barriers for reaction (I) in the range of 2-4 kcal/mole (7 , 9, 1 l a ] for experiments up to about 150°C. However, in reality the Arrhenius plots, being composites of IA*, 3A*, and 3A0

decomposition are all curved [13b, 15, 171 leading to energy barriers in excess of 5 kcal/mole in the high-temperature limit, where thermal effects are dominant.3 These can be taken to be triplet energy barriers.

The reasoning for the nonparticipation of the upper singlet state in reactions (I) and (11) hinges upon an assumed linear Arrhenius plot for the photodecomposition of 1 ,l’-azoisobutane, for which a value of k11/k1 3 1 0 4 . 8 5 e4800iR* cm3/mole is given [ 1, 21. Equating k11 with the collision frequency, a computa- tion of k~ at room temperature suggests that fluorescence (which is not observed) should be a competitive step with dissociation. We have already commented on the nonlinearity of such Arrhenius plots. Further, in the azoisobutane work [ l , 21 the data were obtained at high temperatures, between 100” and 300°C. The S-V plot is based on a very limited number of experiments at each temperature, and it would seem that the Arrhenius plot obtained from the s-V lines rep- resents more closely the high temperature limit, and the energy barrier should be taken as E T . If the datum point obtained from the data at 580°K is omitted, and this is where the correction to N:! formed thermally is the greatest, a value of ET > 6 kcal/mole is indicated (see [2, Figs. 2 and 31). The suggested “hot” triplet mechanism for AIP [3] is based upon similar grounds to that for azoisobutane [ l , 21. We interject a cautionary note against assuming linearity in S-V and Arrhenius plots and extracting lifetimes and rate constants, when in reality more than one dissociating state is involved. Extensive data over as large a pressure range as possible are required to observe possible curvature for proper mechanistic interpretations. This is the importance of the data of Wu and Rice on HFAM [16], and it is proposed that the mechanism given in the following summary is generally applicable to the photodecomposition of acyclic azoalkanes at 366 nm.

Our analysis would not be complete without referring to the other possible mechanism raised by Wu and Rice [16] to account for the S-V curvature. The energy is introduced into the molecule in a nonrandom fashion, so that IA* may represent a molecule which can dissociate before randomization of energy has occurred. A second state, ‘At, where the excess vibrational energy has been randomly distributed, then decomposes at a slower rate. However, the failure of the effective randomization of the internal energy distribution can only occur for sufficiently short-lived species, and much larger values of k l / k ~ would be anticipated than the data suggest [16, 271.

For the data on 1,l ‘-azo-n-butane [9] the 200°C S-V slope is omitted from the Arrhenius plot (see [9, Figs. 2 and 31) which would have made the curvature even more pronounced.


Summary

We present a consistent mechanism for both HFAM and AIP which postu- lates that decomposition occurs from both the vibrationally excited first singlet and triplet states: Collisional deactivation of the vibrationally excited states leads to the population of the vibrationally equilibrated states, and the decomposition of the 3A0 state becomes of increasing importance as the temperature is raised. The lack of any significant decomposition and phosphorescence of the 3A O state at room temperature suggests that radiationless return to the ground state is very rapid [ 11. The absence of fluorescence also indicates that the depopulation of the lAo state occurs via a rapid ISC to the triplet manifold.

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Received May 14, 1974 Revised August 1, 1974

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