Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India.
Phys Chem Chem Phys. 2018 Aug 29;20(34):22320-22330. doi: 10.1039/c8cp03590c.
The fluorescence of 5-aminoquinoline (5AQ) is quenched strongly in alcoholic solvents, in stark contrast to the enhancement in fluorescence observed earlier for its positional isomer, 3-aminoquinoline (3AQ). This phenomenon has been explained by the involvement of a highly dipolar excited state, which is manifested in the solvatochromism of 5AQ. A marked wavelength dependence of fluorescence decays of 5AQ in alcoholic solvents is ascribed to the ultrafast solvation of the highly dipolar excited state in these solvents. The resultant stabilization of this state leads to a decrease in the gap between its energy and lower lying triplet as well as ground singlet states, resulting in an efficient non-radiative relaxation and consequently, fluorescence quenching. Solvation dynamics reported for 5AQ is somewhat slower than earlier reports with coumarin probes, due to the involvement of solute-solvent hydrogen bonds, especially in the excited state of the solute. At lower temperatures, solvation is slowed down. An extreme case of this phenomenon is the absence of solvent relaxation at liquid nitrogen temperature. The fluorescence lifetime increases from tens of picoseconds at room temperature to tens of nanoseconds at cryogenic temperature, lending credence to the proposed model.
5-氨基喹啉（5AQ）在醇溶剂中荧光被强烈猝灭，与之前观察到的其位置异构体 3-氨基喹啉（3AQ）的荧光增强形成鲜明对比。这种现象可以通过涉及高度偶极激发态来解释，这在 5AQ 的溶剂致变色中表现出来。在醇溶剂中，5AQ 荧光衰减的明显波长依赖性归因于高度偶极激发态在这些溶剂中的超快溶剂化。这种状态的稳定导致其能量与较低的三重态以及基态单重态之间的间隙减小，从而导致有效的非辐射弛豫，最终导致荧光猝灭。与香豆素探针的早期报告相比，5AQ 的溶剂化动力学稍慢，这是由于溶质-溶剂氢键的参与，尤其是在溶质的激发态中。在较低温度下，溶剂化速度会减慢。这种现象的一个极端情况是在液氮温度下不存在溶剂弛豫。荧光寿命从室温下的数十皮秒增加到低温下的数十纳秒，这为所提出的模型提供了证据。
