Zhou Panwang, Song Peng, Liu Jianyong, Shi Ying, Han Keli, He Guozhong
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, Liaoning, China.
J Phys Chem A. 2008 Apr 24;112(16):3646-55. doi: 10.1021/jp7120998. Epub 2008 Mar 26.
The rotational reorientation dynamics of oxazine 750 (OX750) in the first (with pump pulse at 660 nm) and a higher excited state (with pump pulse at 400 nm) in different polar solvents have been investigated using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD) spectroscopy. In both excited states, three different anisotropy decay laws have been observed for OX750 in different solvents. Only in acetone and formamide could the anisotropy decays of OX750 be described by single-exponential functions, whereas the anisotropy decays have been found to exhibit biexponential behavior in other solvents. The slower anisotropy decay observed in all of the solvents has been assigned to the overall rotational relaxation of OX750 molecules, and a quantitative analysis of this time constant has been performed using the Stokes-Einstein-Debye hydrodynamic theory and the extended charge distribution model developed by Alavi and Waldeck. In both methanol and ethanol, a faster anisotropy decay on the order of picoseconds and a slower anisotropy decay on the hundreds of picoseconds time scale are observed. The most likely explanation for the faster anisotropy involves the rotation of the transition dipole moment in the excited state of OX750 resulting from the electron transfer (ET) reaction taking place from the alcoholic solvents to the OX750 chromophore. As a possible explanation, the wobbling-in-the-cone model has been used to analyze the biexponential anisotropy decays of OX750 in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The observed faster anisotropy decays on the hundreds of femtoseconds time scale in DMF and DMSO are ascribed to the wobbling-in-the-cone motion of the ethyl group of OX750, which is sensitive to the strength of the hydrogen bond formed between the solvent and the protonation site of OX750.
利用飞秒时间分辨受激发射泵浦荧光耗尽（FS TR SEP FD）光谱，研究了恶嗪750（OX750）在不同极性溶剂中第一激发态（泵浦脉冲波长为660 nm）和更高激发态（泵浦脉冲波长为400 nm）下的旋转重取向动力学。在两种激发态下，不同溶剂中的OX750均观察到三种不同的各向异性衰减规律。只有在丙酮和甲酰胺中，OX750的各向异性衰减能用单指数函数描述，而在其他溶剂中各向异性衰减表现为双指数行为。在所有溶剂中观察到的较慢各向异性衰减归因于OX750分子的整体旋转弛豫，并使用斯托克斯-爱因斯坦-德拜流体动力学理论以及阿拉维（Alavi）和瓦尔代克（Waldeck）开发的扩展电荷分布模型对该时间常数进行了定量分析。在甲醇和乙醇中，均观察到皮秒级的较快各向异性衰减和数百皮秒时间尺度上的较慢各向异性衰减。对较快各向异性衰减最可能的解释是，由于发生了从醇类溶剂到OX750发色团的电子转移（ET）反应，导致OX750激发态下跃迁偶极矩的旋转。作为一种可能的解释，采用了圆锥内摆动模型来分析OX750在二甲基甲酰胺（DMF）和二甲基亚砜（DMSO）中的双指数各向异性衰减。在DMF和DMSO中观察到的数百飞秒时间尺度上较快的各向异性衰减归因于OX750乙基的圆锥内摆动运动，该运动对溶剂与OX750质子化位点之间形成的氢键强度敏感。
