Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and INAMOL, Universidad de Castilla-La Mancha, Avenida Carlos III, S.N., 45071 Toledo, Spain.
Phys Chem Chem Phys. 2018 Oct 31;20(42):27149-27161. doi: 10.1039/c8cp03185a.
We report on the excited-state behavior of proton-transfer phenanthroimidazole derivatives, such as HPPI and NMHPPI, in solutions using steady-state and femto- to nanosecond time-resolved fluorescence spectroscopies. Experimental observations are supported by theoretical calculations (TDDFT). In dichloromethane (DCM) and acetonitrile (ACN), two different paths are found for the excited-state intramolecular proton-transfer (ESIPT) reactions following two different channels. A fast and direct channel (ESIPT1) in 1-2.5 ps and a slower one (ESIPT2) in 12-15 ps, the latter being more influenced by the solvent viscosity (30 ps for HPPI and 20 ps for NMHPPI in triacetin (TAC) solutions). The slowing down of the ESIPT2 reaction is explained in terms of an intramolecular charge transfer (ICT) reaction coupled to a twisting motion to reach a more suitable conformation of the involved parts in the proton-transfer motion. The absence of OH/OD exchange effects in the ultrafast and slow proton-transfer dynamics suggests that the ESIPT reactions, which involve intramolecular and solvent coordinates, do not occur via tunneling. These results reveal new insights into the photobehavior of proton-transfer dyes, which might help in designing photosensors or lighting devices.
我们使用稳态和飞秒到纳秒时间分辨荧光光谱法研究了质子转移菲并咪唑衍生物（如 HPPI 和 NMHPPI）在溶液中的激发态行为。实验观察得到了理论计算（TDDFT）的支持。在二氯甲烷（DCM）和乙腈（ACN）中，发现 ESIPT 反应有两种不同的通道，存在两条不同的途径。在 1-2.5 ps 内存在快速且直接的通道（ESIPT1），在 12-15 ps 内存在较慢的通道（ESIPT2），后者受溶剂粘度的影响更大（在三醋酸甘油酯（TAC）溶液中，HPPI 为 30 ps，NMHPPI 为 20 ps）。ESIPT2 反应的减缓是由于 ICT 反应与扭转运动耦合，以达到质子转移运动中涉及部分的更合适构象。在超快和缓慢质子转移动力学中不存在 OH/OD 交换效应表明，涉及分子内和溶剂坐标的 ESIPT 反应不是通过隧道过程发生的。这些结果揭示了质子转移染料光致行为的新见解，这可能有助于设计光传感器或照明设备。
