Agarwal, Umesh P.

Jagannath, H.

Rao, C. N. R.

Rao, D. Ramachandra

English

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CommunicationsDynamical Behaviour of ExcimersUMESH P. AGARWAL. H. JAGANNATH & D. RAMACHANDRA RAODepartments of Chemistry & PhysicsIndian Institute of Technology. Kanpur 208016andC. N. R. RAO*Solid State & Structural Chemistry UnitIndian Institute of Science. Bangalore 560012Received 8 June 1977; accepted 20 June 1977Dynamics of decay of aromatic excimers have beenexamined in different solvents employing pulsed nitro-gen laser excitation. The results suggest solventdependence of the rate constant for non-dadiative decay.THE most characteristic feature of an aromaticexcimer in solution phase is its fluorescencespectrum which occurs at a longer wavelengththan the monomer fluorescence and is observedonlv when the concentration of the aromaticcompound is sufficiently Iarge-. There have beena few studies on the quantitative aspects of fluore-scence quantum yields and decay of pyrene excimers-.The formation and decomposition processes ofexcimers can be described by the mechanism shownin Scheme 1.D* D (Non-radiative)D+D+hv'fD+D (Non-radiative)Scheme 1In the above scheme, k's are the rate constantswith kl and k2 representing the decay of the excitedmonomer molecule D*, while k5 and k6 representthe decay of the excimer D~. Kinetics of formationof excimers can be investigated by the analysisof fluorescence decay curves. In this communica-tion, we report preliminary results of such studieson the dynamical behaviour of excimers formedby aromatic molecules. The study was directedtowards examining the solvent dependence ofexcimer decay.A pulsed nitrogen gas laser with a pulse widthof 7·5 nsec and peak intensity 3371 A was employedfor the study. The method of measurement of decaycurves has been described elsewhere", Spectro-scopic grade solvents were used and solutions weredegassed by the freeze-thaw technique under pureN2-atmosphere.In Table 1, the lifetimes of excimers, 're. of afew aromatic hydrocarbons are presented alongwith the monomer lifetimes, 't"A. The lifetimesorA directly give the rate constants kl +k2 while theTABLE 1- DECAY TIMES OF AROMATIC HYDROCARBONMONOMERS AND EXCIMERsa(ki +k.)x 10-7sec-I(k6+kS)X 10-7secl't"AnsecT.nsecTEREPHTHALONITRILE1·8 25 23 4·0 4·34PYRENEH 94 71 1-06 1·40BENZOPYRENE2-1 56 18 1·78 5·55(a) Toluene solvent, concentration of hydrocarbon for't"A (monomer) ,..,10-6M and ~10"'M for 't" Measurementof 't"e is at longer wavelength than "t"A. Thus. for pyrenemonomer fluorescence maximum is at 425 nm and excimerfluorescence maximum is at 470 nm.SolventTABLE 2 - EFFECT OF SOLVENT ON EXCIMER DECAYPyrene BenzopyrenebTA Te (ki + k.) X 10-7 (k6+ks) X 10-7nsec nsec sec=! see'?Toluene 2·38 94 71 1·06 1·40Ethyl acetate 6·02 72 49 1·38 2·04Dichloromethane 9·08 110 97 0·91 1·03Isopropyl alcohol 18·31Methanol 33·6 55 51 1·81 1·96Acetonitrile 37·5 60 57 1·66 1·75"t"e (k6+kS) x 10-'nsec see'<18 5·520 5·018 5·528 3·629 3·45(a) Solvent dielectric constant.(b) In cyclohexane "t"A and T. are 100 and 35 nsec respectively.*To whom all correspondence should be addressed.738COMMUNICATIONS't"e values give kr,+k6• The values of 't"A and 't"e arequite similar in terephthalonitrile and pyrene butnot so in 1,2-benzopyrene.Earlier studies--" have shown that kl>k2 andk5>k6; also, kL>ks and k2>k6. Furthermore, k5is reported to be independent of the nature of thesolvent+", Thus, for pyrene, k5 is 1·16xl07 see"!in cvclohexane and 1·3 X 107 sec! in both acetoneand -ethanol. This implies that any solvent-depen-dence of 't"e of excimers should arise from the variationof ke with solvent. In Table 2, are listed the valuesof't"A and 't"e of pyrene and 1,2-benzopyrene in differentsolvents. It is observed that 't"A and 't"e are com-parable in the case of pyrene in solvents of varyingdegrees of polarity and both of them arise fromsingle exponential decay. The lifetimes do not varyin any systematic manner with the solvent dielectricconstant. If we take the value of ks to be constant(1·0-1·3 X 107 see-I) based on earlier studiesl,4,5 itis found that ke varies significantly from solventto solvent. The value of k6 varies by ",100% inthe series of solvents studied. The results onbenzopyrene are also similar; in this system, thevalue of (ks+ke) varies between 2·9 and 5·5 X 107sec-I again suggesting solvent-dependence of k6•The present study indicates the interesting possi-bility that the rate constant representing non-radiative decay of excimers is considerably moresolvent-dependent than their radiative decay. Itis indeed noteworthy that in a very recent reportson the photophysical processes in aromatic electrondonor-acceptor complexes, the non-radiative rateconstant has been found to increase with solventpolarity while the rate constant for inter-systemcrossing remains constant.References1. MATAGA, N. & KUBOTA, T., Molecular interactions andelectronic spectra (Marcel Dekker Inc., New York),1970.2. AGARWAL, U. P., JAGANNATH, R., RAo, D. R. & RAo,C. N. R., J. chem, Soc., Faraday Trans. II, 73 (1977),(in press).3. YOSHIHARA, K., KASUYA, T., IN ONE, A. & NAGAKURA, S.,Chem. phys. Left., 9 (1971), 469.4. BIRKS, J. B., DYSON, D. J. & MUNRO, I. R., Proc. R. Soc.,A275 (1963), 575.5. BIRKS, J. B., LUMB, M. D. & MUNRO, I. R., Proc. R. se.,A2S0 (1964), 289.6. CRAIG, B. B., RODGERS, M. A. J. & WOOD, B., J. chem,s»., Faraday Trans. II, 73 (1977), 349.739

Dynamical Behaviour of Excimers

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Dynamical Behaviour of Excimers

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