Contrasting solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl dyes in the lower and in the higher solvent polarity regions.

Solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl-thiazoloquinoxaline dyes, one with a flexible diphenylamino group, namely, N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1), and the other with a rigid julolidinylamino group, namely, (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2), have been investigated in different aprotic solvents and solvent mixtures. From the polarity dependent changes in the absorption and fluorescence spectral properties, it is indicated that the fluorescent states of the dyes are of intramolecular charge transfer (ICT) character. For both the dyes, the photophysical properties like fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)), radiative rate constants (k(f) = Phi(f)/tau(f)), and nonradiative rate constants (k(nr) = 1/tau(f) - Phi(f)/tau(f)) show clearly contrasting solvent polarity effects in the lower and in the higher solvent polarity region, causing an interesting reversal in the properties below and above an intermediate solvent polarity. It is inferred that the domination of the cis-trans isomerization in the lower solvent polarity region and that of the twisted intramolecular charge transfer (TICT) state formation in the higher solvent polarity region are responsible for the observed contrasting solvent polarity effects on the photophysical properties of the two dyes. As both isomerization and TICT state formation causes an enhancement in the nonradiative decay rate of the excited dyes and both the processes become less significant at the intermediate solvent polarity region, the two dyes show their largest Phi(f) and tau(f) values at intermediate solvent polarities. Suitable mechanistic schemes have been proposed and qualitative potential energy diagrams have been presented to explain the observed results with the changes in the polarity of the solvents used.