Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature

Spectrochimica Acta Part A 56 (2000) 2669–2677

Proton transfer reaction of a new orthohydroxy Schiff basein protic solvents at room temperature

D. Guha a, A. Mandal a, A. Koll b, A. Filarowski b, S. Mukherjee a,*a Department of Chemistry, Indian Association for the Culti6ation of Science, Calcutta 700034, India

b Department of Chemistry, Uni6ersity of Wrocl*aw, Wrocl*aw, Poland

Received 4 January 2000; accepted 4 May 2000

Abstract

Ground and excited state inter- and intramolecular proton transfer reactions of a new o-hydroxy Schiff base,7-ethylsalicylidenebenzylamine (ESBA) have been investigated by means of absorption, emission and nanosecondspectroscopy in different protic solvents at room temperature and 77 K. The excited state intramolecular protontransfer (ESIPT) is evidenced by a large Stokes shifted emission (�11 000 cm−1) at a selected excited energy inalcoholic solvents. Spectral characteristics obtained reveal that ESBA exists in more than one structural form in mostof the protic solvents, both in the ground and excited states. From the nanosecond measurements and quantum yieldof fluorescence we have estimated the decay rate constants, which are mainly represented by nonradiative decay rates.At 77 K the fluorescence spectra are found to be contaminated with phosphorescence spectra in glycerol and ethyleneglycol. It is shown that the fluorescence intensity and nature of the species present are dependent upon the excitationenergy. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Proton transfer; o-Hydroxy Schiff base; Protic solvents

www.elsevier.nl/locate/saa

1. Introduction

Salicylideneaniline (SA) is a typical anil thatshows photochromism. The emission properties ofSA and some substituted derivatives have beenstudied extensively by spectroscopists in recentyears [1–10]. The emissions have been shown todepend mainly on solvent polarity, pH, hydrogenbonding character of solvent and structures of the

molecules [2,10,11]. Among such compounds theproton transfers of SA, 2-(2%-hydroxyphenyl) ben-zothiazole (HBT), 2-(2%-hydroxyphenyl) benzoimi-dazole (HBI), 2-(2%-hydroxyphenyl) benzoxazole(HBO) have been studied in details [12–14]. Themain species in the ground state of such com-pounds are intramolecularly hydrogen bondedcis–enol forms. In nonpolar solvents protonmoves spontaneously to the opposite side of thehydrogen bond rapidly by photoexcitation. This isthe origin of the large Stokes shifted emission forexcited state intramolecular proton transfer (ES-IPT) to occur. So, all these compounds are con-

* Corresponding author. Tel.: +91-33-4734971; fax: +91-33-4732805.

E-mail address: [email protected] (S. Mukherjee).

1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S1 386 -1425 (00 )00303 -6

D. Guha et al. / Spectrochimica Acta Part A 56 (2000) 2669–26772670

sidered to perform a closed reaction cycle with theenol–keto type proton transfer reaction throughcis–trans isomerization by photoexcitation [8,15–17]. Photochromism is not restricted to protonmigration through keto–enol tautomerizationonly, but also involves framework changes whichdisrupt the hydrogen bond between the oxygenand the nitrogen atom [18].

HBI is believed to exhibit two different in-tramolecularly hydrogen bonded isomers in theground electronic state. Excitation of these iso-mers leads to formation of the normal and ketotautomers. The appearance of both the emissions

seems to be dictated by the competition betweenintra- and intermolecular hydrogen bonding withsolvent molecules. o-Hydroxy Schiff bases formthe intramolecular hydrogen bonds and show in-teresting properties like thermochromism [19,20].These compounds show dual fluorescence as isobserved in the case of salicylanilides [21].Grabowska et al. [22] have studied the emissionproperties to establish the structure of the basicmolecular units which is responsible for the dou-ble fluorescence. They have shown that 2-(N-methyl(iminophenyl))-phenol is indeed such aunit.

Molecules that give rise to excited state tau-tomer by intramolecular proton transfer are usedas laser dyes, in higher energy radiation detectorsand molecular energy storage systems, as fluores-cent probes and polymer protecting agents [23–25]. A good number of Schiff bases showphosphochromism which is explained by photo-induced proton transfer. Such properties sug-gested the possibility of using these compoundsfor constructing the optical switches or opticalmemory devices [26].

In our previous paper [27] with 7-ethylsalicyli-denebenzylamine (ESBA) in nonpolar solvents, itis shown that ESIPT can take place only at 77 K.It is proposed that at room temperature at leastthree different species are present in the excitedstate, although there is no indication of ESIPT tooccur. In the present work we have studied thisnew o-hydroxy Schiff base, ESBA (Fig. 1), insome polar protic solvents. We have measured itsabsorption, emission and excitation spectra bothin the ground and transient state in water,methanol, ethanol, ethylene glycol and glycerol.The presence of the ethyl substituent results in theC–C(C2H5)�N group which introduces perturba-tions to intramolecular hydrogen bonds andmodifies the potential for the intramolecular pro-ton transfer. Koll et al. observed a strengtheningof the intramolecular O–H···N hydrogen bridgein ESBA [19]. From IR spectra it is shown thathydrogen bonds are quite stable in CCl4 solutionsand there is no free n(OH) bands in the IRspectra. It is also shown that below 100 K the IRspectrum is characteristic for very short hydrogenbonds with large dynamics of proton movementsFig. 1. Possible configurations of ESBA.

D. Guha et al. / Spectrochimica Acta Part A 56 (2000) 2669–2677 2671

within the O···H···N bridge [19]. The purpose ofthis investigation is to identify the nature of thespecies present both in the ground and excitedstate in different protic solvents and compare theresults with those obtained in nonpolar solvents.We have compared our results with the resultsobtained in the case of SA and HBT, also. Thepossible conformers of ESBA are shown in Fig. 1.

2. Experimental

Synthesis of ESBA (melting point=45°C) fromstoichiometric mixture of a particular salicylalde-hyde and benzylamine in methanol was performedwith standard procedure [28]. The solid productswere recrystallized from methanol and dried sev-eral times. All the solvents used were of spectro-scopic grade and freshly distilled before use.Room temperature absorption and emission wererecorded in JASCO 7850 and Perkin–Elmer MPF44B spectrophotometers, respectively. The fluores-cence emission and excitation spectra at 77K wererecorded on a Hitachi F-4500 fluorescence spec-trophotometer. The concentration of ESBA wasmaintained at 3–5×10−5 mol dm−3.

The relative quantum yields (ff) were deter-mined with 4-methyl-2,6-diformylphenol as refer-ence with a quantum yield (ff) of 0.11 incyclohexane [29]. The transient fluorescence life-times (t1 and t2) are measured with an NF-900nanosecond spectrometer (Edinburgh InstrumentsLtd., UK) using a pulsed nitrogen lamp (therebyusing an excitation wavelength of 334 nm) basedon the time-correlated single-photon countingtechnique. The quality of the fits over the fluores-cence decay curves was assessed by, reduced chi-square (xR

2 ) and a plot of weighted residuals. Inall the cases reported here, xR

2 =0.990.2.

3. Results and discussion

3.1. Absorption spectra

The absorption spectra of ESBA show a singleband at 322 nm in water and methylcyclohexane(Fig. 2) and can safely be assigned as due to the

Fig. 2. Absorption spectra in ethanol (a), cyclohexane (b) andin water in presence of NaOH (c); [NaOH]=2.5×10−4 moldm−3; [ESBA]=3–5×10−5 mol dm−3.

intramolecularly hydrogen bonded closed con-former of ESBA (I, Fig. 1). In ethanol, glyceroland ethylene glycol, on the other hand, a redshifted band (red band) appeared at 395 nm alongwith the 322 nm band. It has long been knownthat the absorption spectra of salicylidene aniline(SA) exhibit a band around 400 nm region in thepolar hydrogen bonding solvents. It is suggestedthat this red shifted band is due to the formationof intermolecularly hydrogen bonded complexwith the solvent molecules [21,22,30,31]. Thus, inalcoholic solvents ESBA is likely to exist as so-lute-solvent hydrogen bonded complex at 395 nmalong with the closed conformer. It is interestingto note that the 395 nm band is unstable in highlyviscous liquids like glycerol and ethylene glycol.This red band is found to disappear graduallywith time along with the light yellow colour of thesolution, with the concomitant increase of 322 nmband intensity as shown in Fig. 3. It is to bementioned here that in nonpolar solvents ESBAshowed a single band at 322 nm due to closedcis-configuration. Due to the presence of electrondonor CH3 or C2H5 group with the positive in-ductive effects, methanol or ethanol can releaseelectron towards the oxygen atom of the OH


Fig. 3. Plot of absorbance (at a fixed wavelength lf) with timefor ESBA in glycerol; lf=322 nm (a) and 395 nm (b);[ESBA]=5.6×10−5 mol dm−3.

3.2. Emission and excitation spectra at roomtemperature

The steady state emission spectra of ESBA(�10−5 mol dm−3) appeared at 470 nm regionwhen excited by 330 nm light in all the alcoholicsolvents studied here. This band is found indepen-dent of excitation wavelength (lexc). By increasingthe lexc to 360 nm another large Stokes shifted(�10 303 cm−1) band appeared at 500 nm inaddition to the 470 nm band. Moreover, only the500 nm emission band occured when excited by390 nm light as shown in Fig. 4. Thus, excitedstate intramolecular proton transfer (ESIPT) andformation of tautomer of ESBA (VI, Fig. 1) areevidenced by the large Stokes shifted emission inalcoholic solvents. That is a proton is transferredin the excited state from oxygen to nitrogen atomat a selected lexc only with a typical large Stokesshift. It is to be mentioned here that we are unableto detect ESIPT in nonpolar solvents like methyl-cyclohexane or n-hexane at room temperature onexcitation by light of any energy. Moreover, emis-sion spectra of ESBA in nonpolar solvents indi-cate the presence of three species [27]. Weobserved ESIPT in solid crystalline sample ofESBA at room temperature which is independent

group and can accept proton from ESBA. Thiswill stabilize the intermolecular complex inmethanol and ethanol. It seems quite likely there-fore, that ethylene glycol and glycerol are weakerproton acceptor than methanol and ethanol. Thiswill explain the unstability of the red band inglycerol and ethylene glycol. An examination ofthe structures of ESBA (Fig. 1) reveals that rota-tion around C�N bond is restricted and henceprobability of intermolecular bond formation isfavourable.

By the addition of aqueous NaOH in watersolution of ESBA the produced anion occured at360 nm (Fig. 2). Hence, the red band (395 nm) isnot due to anion, but the coloured form of ESBAwhich is intermolecularly bonded to alcoholic sol-vents. Lewis and Sandorfy [32] pointed out thatthe coloured form of anil in solution is the zwitte-rion with its cis configuration. On the other hand,Becker et al. [8] from their work on SA suggestedthat for coloured form more than one conforma-tion is possible. The chemical shift of OH groupin the NMR spectra of ESBA appeared at 15.2ppm both in CDCl3 and methanol-d. This indi-cates stronger intramolecular hydrogen bond withOH in ESBA. Hydrogen bonding decreases theelectron density around the proton and the pro-ton moves to lower field. Enols are usually stabi-lized by intramolecular hydrogen bonding. Theenolic proton absorbs downfield relative to thephenolic or alcoholic protons. In the case of eno-lic form proton may be found as far downfield as16.6 ppm. When the intramolecular bonding isnot involved, or is very weak, the enolic protonabsorbs in about the same region as the phenolicproton.

Fig. 4. Emission (a–c) and excitation (d,e) spectra in pureethanol at different excitation (lexc) and emission (lem) wave-lengths; lexc: 330 nm (a), 360 nm (b) and 390 nm (c); lem: 470nm (e) and 500 nm (d).


of any lexc studied here. This indicates that thestable molecular structure of ESBA in the groundstate is the cis configuration. Such a structure hasbeen experimentally shown by X-ray analysis re-cently by Koll et al. [19].

The excitation spectra of 470 and 500 nmfluorescence, in alcoholic solvents, appeared at350 and 390 nm and none can explain the absorp-tion spectra. The lexc dependent emission spectraand the different excitation spectra for differentemission indicate the presence of more than onespecies in the ground state. This also indicatesthat the species present in the ground and excitedstate are different and both the species (460 and500 nm) are formed in the excited state and noneof them originates from the ground stateconformers.

The emission spectra of ESBA in water show asingle band at 420 and 470 nm when excited by330 and 390 nm light, respectively. This indicatesthe presence of two species which are formed inthe excited state. It is noted also that by increas-ing the lexc the intensity of the emission banddecreases gradually (Fig. 4). Because the band isbroad it is quite likely that small amount oftautomeric form may be present in equilibriumwith anionic form in pure polar protic solvents.The excitation spectra of 470 nm fluorescence onthe other hand appeared at 360 nm and well agreewith the absorption spectra of anion. All theseobservations indicate the presence of more thanone different species in the excited state. In thecase of HBT, Williams and Heller [1] suggestedthe presence of as many as four configurations inthe photochemistry of HBT.

The emission spectra of ESBA in aqueous alka-line medium, using NaOH in water, or in presenceof a strong base like triethylamine (TEA) in alco-holic solvents show a single band at all the excita-tion energy. The excitation spectra of 470 nmfluorescence appeared at 360 nm and well agreewith its absorption counterpart. The 470nm emis-sion can safely be assigned as due to the anion ofESBA (V, Fig. 1). It is also noted that the inten-sity of emission band increases strongly (about sixtimes) by the added base without any change inposition of the band. It is to be mentioned herethat we are unable to detect any measurable

change in absorption spectra by added TEA. It isalso observed that the position and the intensityof emission spectra of ESBA are dependent uponthe excitation energy in all the pure protic sol-vents used here. From the possible configurationspresented in Fig. 1 it can be seen that the ESIPTis possible only in case of cis-form (I, Fig. 1). Thisshows that the stable molecular structure ofESBA in solid crystalline medium is cis-conforma-tion both in the ground as well as in the excitedstate. It is to be mentioned here that we havedetected anionic band in the excited state in allthe alcoholic solvents in presence of a strong baselike TEA which is found independent of anyexcitation wavelength.

A good number of earlier work on similarcompounds such as SA suggested the presence ofzwitterion in polar protic solvents[11,21,22,30,31,33–35]. It is suggested by Williamsand Heller [1] in the case of SA and HBT, that themolecules formed before ESIPT can be regardedsimply as vibrationally excited zwitterion. Thisreflects that the intramolecular hydrogen bond inESBA can undergo ESIPT emitting the character-istic large Stokes shifted fluorescence of phototau-tomer at relatively lower excitation energy.Accordingly, zwitterionic species of ESBA (III,Fig. 1) may also be present in the excited state.ESIPT is not observed in nonpolar solvents at anylexc at room temperature. This is probably due tothe absence of the cis-form in the excited state orcis-form is not so much populated to show ES-IPT. This seems to indicate that formation oftautomer in alcoholic solvents is probably fromcis-zwitterion or anion. The anion is formed inwater but ESIPT is not observed even in purewater. This shows that formation of ESIPT isfavourable from cis-zwitterion. This is in favourof the suggestion made by Williams and Heller[1]. Because ESIPT is observed in solid sampleand difficult to occur in trans configuration (II,Fig. 1), cis–trans isomerization seems to be per-turbed in solid media. This is probably due tosome packing arrangement in the solid crystallineform [36]. Accordingly, it may be suggested thatthe stable molecular structure of ESBA in theground state is cis-configuration. It is shown insome earlier work that photochemistry of N-ben-


zylidene aniline in solution is different from thatof solid sample [16,17,37].

The emission spectra of ESBA in presence ofNaOH or TEA in all the protic solvents show asingle band at 470 nm at all lexc. Thus, ESIPTcannot be observed in presence of a base, likeNaOH or TEA, due to stronger interaction withthe environment where the anion formation isalmost complete. This adds further support to thefact that ESIPT is from cis-zwitterion and notfrom anion.

It is shown earlier that in relatively weakerproton accepting solvents like trifluoroethanol,SA shows intermolecular hydrogen bonding tosolvent as well as ESIPT [31]. Intermolecular hy-drogen bonded species could undergo ESIPT onlyunder anti–syn isomerization around the C�Nbond if the barrier height is low. Becker et al [8]also suggested anti–syn isomerization aroundC�N bond for HBT and SA. They proposed thatproton transfer may not occur by excitation, butthe molecules are undergoing an anti–syn isomer-ization around C�N bond.

In case of ESBA anti–syn isomerization is pos-sible since there is no sulphur bridge like benzoth-iazoles. In case of benzothiazoles, anti–synisomerization is perturbed due to the presence ofsulphur bridge [8]. Zwitterion can be formed in atrans conformation which undergoes ESIPT toform an anion depending upon hydrogen bondingability of the environment. In Schiff bases thereexists the possibility of direct electronic couplingbetween acidic and basic centres of the hydrogenbond. The flow of electrons through the coupledbonds compensates the increase of atomic chargescaused by the proton transfer process. Methyl andethyl groups and benzene ring are electron donors

particularly in the excited state i.e. in more polarenvironment.

Our theoretical calculaltions at AM1 level showthat –CH2–C6H5 group present in ESBA istwisted out of plane of the molecule. By excitationthis group will rotate to release some steric strainto become more stable. Theoretical calculationalso shows that the barrier height is 18.3 kcalmol−1 in the ground state. Hence, proton cannotmove spontaneously to the other side of the nitro-gen atom and ESIPT will not occur unless thebarrier height is low. Moreover, if cis-isomer isconverted to the trans-isomer ESIPT will bedifficult. We observed ESIPT in alcoholic solventsat lower excitation energy where the barrier heightis low and the major species present is cis-zwitte-rion and in solid sample major species presentshould be the cis-isomer (I, Fig. 1). On the otherhand, main species present in nonpolar solventsafter excitation is the trans-isomer and we areunable to detect ESIPT at any lexc in nonpolarsolvents at room temperature.

3.3. Decay beha6iour of ESBA fluorescence

The fluorescence decay of ESBA is measured onthe nanosecond timescale. After deconvolution abiexponential decay curve is obtained in all theprotic solvents studied here, indicating that themeasured fluorescence decay can adequately bedescribed by a double exponential function withtwo different lifetimes (t1 and t2). The weightedresiduals appear to be better distributed when adouble exponential decay curve is fitted. The life-time values are displayed in Table 1 and onerepresentative decay curve is shown in Fig. 5.

Table 1Lifetimes (t1 and t2), quantum yield (ff) and decay rates (kf and k f%) of fluorescencea

k fnr%×10−8 (s−1)k f

nr×10−8 (s−1)k f%×10−8 (s−1)Solvent t1 (ns)ff kf×10−8 (s−1)t2 (ns)

7.7 1.90.03Water 7.54.9 (22)1.3 (78) 2.00.05 3.8 (70) 0.7 (30)Methanol 2.6 14.3 2.5 13.6

4.1 (70)0.02 16.4Ethanol 16.616.72.40.6 (30)9.111.9Glycerol 12.50.27 2.20.8 (32)4.5 (68)

0.23 4.0 (62)Ethylene glycol 1.0 (38) 2.5 10.0 9.4 7.7

a Values in parenthesis represent the percentage population of the species with the respective t values.


Fig. 5. Typical decay profile of ESBA in ethanol (lmon=470 nm). Broken line denotes lamp profile (resolution=0.096 ns/channel).Solid curve represents the best computer fit of the experimental points to a double exponential decay. The box below gives adistribution of the weighted residuals (xR

2 =0.72)

The fluorescence lifetimes combined with thequantum yield of fluorescence (ff) leads to theradiative decay rate constant (k f

r). The fluores-cence decay rates (kf=1/tf) are given by the sumof the radiative (k f

r) and nonradiative (k fnr) decay

rates. The values are displayed in Table 1. It canbe seen from Table 1 that k f

nr are dominant overthe k f

r, that is k fnr mainly represents the decay

rates. The nonradiative decay rate must play animportant role in the transfer process during thedeactivation of the excited ESBA. According to

the suggestion of Felker et al. [38] the jet-cooledspectra of MS show long progressions involvingfrequency intervals of 180 cm−1 which corre-spond to a ground-state out-of-plane bending mo-tion of the ‘ring’ that includes the intramolecularH-bond. Furthermore, the 1300 cm−1 threshold(3.7 kcal mol−1) agrees with the activation energyfor the nonradiative rate derived from studies insolution. Heimbrook et al. [39] showed that the S1

state of methylsalicylate (MS) is strongly coupledto a nonradiative decay channel, when the vibra-


tional energy exceeds �1500 cm−1. Barbara etal. suggested that the nonradiative decay pro-cesses in the derivatives of benzothiazole involvethe torsional motion [5,15]. Accordingly, it maybe said that the out-of-plane bending or torsionalmotion would be responsible for such higher val-ues of k f

nr for ESBA. The k fnr value of ESBA is

more than one order of magnitude higher than k fr.

That is kf is mainly represented by k fnr. The in-

crease in k fnr quenches the fluorescence of ESBA,

which explains the decrease of ff [40–42].The proton transfer processes of ESBA are

significantly slower than OHBA, salicylideneani-line and similar compounds [8,23] (�109 s−1).This reflects interaction with solvents and pres-ence of considerable barrier.

The decay kinetics in all the protic solvents,when monitored at their respective fluorescence,shows the relative integrated intensity for the fastand slow components to be different. The kineticsof the fluorescence is strongly dependent on theprobe photon energy. All these facts and thebiexponential decay, with two different lifetimes,indicate the presence of two fluorescence speciesin all the solvents studied here. Monitoring thedecay of the emission we have found a short-livedcomponent. According to the decay times ob-tained, we attribute the short-lived component tothe zwitterionic form or anion and the long-livedcomponent to the tautomer. Dutta et al. [16]examined the spectral properties in different non-polar and alcoholic solvents in order to get someinsight into why higher amounts of zwitterionicform of SA are found in alcoholic solvents. It ispointed out that the necessary aspect for theformation of the zwitterionic structure is thechange in phenolic group to O−. It requires thepresence of hydrogen bonding solvents dependingupon the acidity of the alcoholic proton and alsoon the dipole moment of the solvents. The dielec-tric constant of the solvents are unsuitable todetermine solvation of ions [16].

3.4. Emission spectra at 77 K

The emission of ESBA at 77 K in glycerol andethylene glycol comprises the superposition of

Fig. 6. (F), phosphorescence (P) and excitation (E) spectra ofESBA in glycerol at 77 K. lexc: 325 nm (F & P); lmon=470nm (E).

fluorescence and phosphorescence spectra asshown in Fig. 6. As the temperature is raised thephosphorescence intensity gradually decreases, be-cause of phosphorescence quenching and onlyfluorescence is observed at room temperature.Only phosphorescence is observed after using afilter (Fig. 6) with phosphorescence lifetimes 0.07and 0.11 s, in glycerol and ethylene glycol, respec-tively. It is to be mentioned here that we areunable to detect phosphorescence in nonpolar sol-vents, n-hexane and methylcyclohexane, at 77 K,rather fluorescence is observed at 500 nm due toESIPT. Lifetime values do not have any measur-able increase in nonpolar solvents compared tothat at room temperature (4.3 ns). The phospho-rescence excitation spectra in glycerol show aband at 360 nm, and a shoulder at 325 nm,indicating the presence of more than one specieseven at 77 K. The shoulder at 325 nm agrees wellwith the absorption spectra of the closed con-former of cis-ESBA (Fig. 2).

4. Conclusion

In conclusion we can outline a summary of ourresults on ESBA as below.

(1) At room temperature ESIPT is observed inhighly polar alcoholic solvents at a selected excita-tion energy from cis-configuration of ESBA. (2)In the ground state anion is detected in presenceof alkali, only in aqueous medium, and inter-


molecularly hydrogen bonded complex in alco-holic solvents. (3) Anion is formed in all theprotic solvents in presence of base.

Acknowledgements

This work is supported by the INSA-polishAcademy of Science Exchange Programme. Au-thors are grateful to the department of Spec-troscopy of I.A.C.S. for the low temperaturemeasurements and Saha Institute of NuclearPhysics, India for nanosecond lifetime measure-ments. DG thanks CSIR for providing a seniorresearch fellowship. AM thanks I.A.C.S. forproviding a junior research fellowship.

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Proton transfer reaction of a new orthohydroxy Schiff base in protic solvents at room temperature