Radiation & Photochemistry Division, Bhabha Atomic Research Center, Trombay, Mumbai-400085, India.
J Phys Chem A. 2010 Apr 8;114(13):4507-19. doi: 10.1021/jp9107969.
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.
溶剂极性对两种新合成的氨巯基噻唑并喹喔啉染料的光物理性质的影响，一种具有柔性二苯基氨基，即 N,N-二苯基-4-[2-(噻唑并[4,5-b]喹喔啉-2-基)乙烯基]苯胺（TQ1），另一种具有刚性六氢吖啶基氨基，即（9-[2-(噻唑并[4,5-b]喹喔啉-2-基)乙烯基]六氢吖啶）（TQ2），在不同的非质子溶剂和溶剂混合物中进行了研究。从吸收和荧光光谱性质随极性的变化可以看出，染料的荧光状态具有分子内电荷转移（ICT）的特征。对于两种染料，光物理性质如荧光量子产率（Phi(f)）、荧光寿命（tau(f)）、辐射速率常数（k(f)=Phi(f)/tau(f)）和非辐射速率常数（k(nr)=1/tau(f)-Phi(f)/tau(f)）在较低和较高溶剂极性区域都显示出明显相反的溶剂极性效应，导致在中间溶剂极性以下和以上的性质发生有趣的反转。推断在较低溶剂极性区域中顺反异构化的主导作用和在较高溶剂极性区域中扭曲的分子内电荷转移（TICT）态形成是导致两种染料光物理性质观察到相反溶剂极性效应的原因。由于两种过程都导致激发染料的非辐射衰减速率增加，并且在中间溶剂极性区域这两种过程变得不那么重要，因此两种染料在中间溶剂极性下表现出最大的 Phi(f)和 tau(f)值。已经提出了合适的机制方案，并提出了定性的势能图，以解释随着所用溶剂极性的变化而观察到的结果。
