Manna Arpan, Sayed Mhejabeen, Kumar Anil, Pal Haridas
Physical and Material Chemistry Division, National Chemical Laboratory , H. J. Bhabha Road, Pune 411 008, India.
J Phys Chem B. 2014 Mar 6;118(9):2487-98. doi: 10.1021/jp500281z. Epub 2014 Feb 21.
The excited-state intramolecular proton-transfer (ESIPT) process in 1,8-dihydroxyanthraquinone (18DHAQ) dye has been investigated in protic ionic liquid (PIL) solvents using photochemical measurements. The results demonstrate noteworthy modulations in both steady-state and time-resolved emission characteristics of excited normal (N*) and tautomeric (T*) forms of the dye. That the emission of T* increases unexpectedly upon increasing solvent viscosity indicates that subsequent to the initial forward ESIPT, there is also a relatively slower back ESIPT process involved for the excited dye. It is inferred that the propensity of this back ESIPT process is determined by the dynamics of the diffusive solvent relaxation, a process that is known to be strongly viscosity-dependent in ionic liquids. Evidence of both forward and back ESIPT for the dye has been obtained from femtosecond fluorescence up-conversion measurements. While an unusually fast forward ESIPT is clearly observed in all of the PILs studied, the uncommon back ESIPT process is distinctly indicated in PIL solvents having lower viscosities, certainly due to reasonably fast diffusive solvent relaxation in these solvents that causes a temporal modulation in the energies of the normal and tautomeric forms within a reasonably short time and thereby brings down the energy of N* compared to that of T*, triggering the back ESIPT process. Observation of solvent-viscosity-dependent back ESIPT is an intriguing finding for the present study as to the best of our knowledge, such a behavior has so far not been reported in the literature for the ESIPT reaction.
利用光化学测量方法，在质子离子液体（PIL）溶剂中研究了1,8 - 二羟基蒽醌（18DHAQ）染料的激发态分子内质子转移（ESIPT）过程。结果表明，该染料激发态的正常（N*）形式和互变异构（T*）形式的稳态和时间分辨发射特性均有显著调制。随着溶剂粘度增加，T的发射意外增加，这表明在初始正向ESIPT之后，激发态染料还涉及一个相对较慢的反向ESIPT过程。据推测，这种反向ESIPT过程的倾向由扩散溶剂弛豫的动力学决定，已知该过程在离子液体中强烈依赖于粘度。通过飞秒荧光上转换测量获得了该染料正向和反向ESIPT的证据。虽然在所有研究的PIL中都清楚地观察到异常快速的正向ESIPT，但在粘度较低的PIL溶剂中明显显示出不常见的反向ESIPT过程，这肯定是由于这些溶剂中合理快速的扩散溶剂弛豫，在相当短的时间内导致正常形式和互变异构形式的能量发生时间调制，从而使N的能量相对于T*降低，触发了反向ESIPT过程。对于本研究而言，观察到溶剂粘度依赖性的反向ESIPT是一个有趣的发现，据我们所知，迄今为止文献中尚未报道ESIPT反应有这种行为。
