摘要
本文用时间分辨泵浦探测瞬时吸收光谱技术研究了LDS698染料分子在甲醇(methanol)、丙酮(acetone)、二甲基甲酰胺(DMF)、二甲基亚砜(DMSO)四种不同溶剂中的超快动力学过程。我们得到了两种超快弛豫过程:一种时间尺度为几百飞秒的快速弛豫过程,一种为时间尺度在皮秒量级的相对慢速弛豫过程。通过分析,对于快速弛豫过程,我们认为其应为分子内部振动能量再分配的过程(IVR),并且在甲醇溶剂中的这个快过程要快于在其他三种溶剂中,我们认为其是因为在甲醇溶剂中LDS698分子会和甲醇分子形成氢键,其形成了一种促进IVR过程的效应。对于慢速弛豫过程,我们认为是由locally excited(LE)态向twisted intramolecular charge transfer (TICT)态弛豫的过程,即TICT态的形成过程。并且除了甲醇溶剂之外的三种溶剂,弛豫时间随着溶剂粘滞系数的增大而增大。我们对TICT态在不同溶液中的转动角度进行了计算,得到转动角度和溶液的黏度系数也成正比例关系。而转动角度越大,即意味着达到这个角度的时间越长,即TICT态的形成时间越长,从而溶液的黏度系数和TICT态的形成时间成正比关系。这和我们实验中观察到的相一致。在甲醇中的慢速弛豫时间则要远大于其他三种溶剂,我们认为在甲醇溶剂中,甲醇分子之间会因为氢键而形成一种网状结构,从而阻碍LDS698分子的转动,使TICT态的形成时间增大。
With the rapid development of femtosecond lasers, several optical techniques are utilized to characterize ultrafast dynamics of complex molecules in solution experimentally, for instance, transient absorption, fluorescence depletion, fluorescence up-conversion. Ultrafast dynamics of complex molecules in solution is a topic of interest. In a solution system, a solute molecule in its excited state relax by several ultrafast dynamical processes such as intramolecular vibrational energy redistribution (IVR), diffusive solvent relaxation, twisted intramolecular charge transfer(TICT) etc.. Studies on these processes can reveal information on intra- and/or inter- molecular interaction that plays a key role for understanding solvent effect in chemistry.
In this paper, for understanding the influence of the solvent properties on forming process of TICT state of dye molecules, we have focused on the ultrafast relaxation of the exited LDS698 in several solvents with different viscosity, such as methanol, acetone, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), using a femtosecond time-resolved pump/probe transient absorption spectroscopy with a pulse width of 90 fs and wavelengths of 800 nm and 400 nm. For the study of the steady-state absorption and emission spectra, we get three characteristics. First, there is a larger Stokes shift of absorption and fluorescence spectra. Second, the spectra have a wide bandwidth without fine structure. Third, the absorption and fluorescence do not have a mirror symmetry relation. Based on these characteristics and some previous studies, after excited by the laser, there will be a charge transfer process, and forming twisted intramolecular charge transfer (TICT) state.
For the transient absorption spectra, the best fitting for the obtained curves can be achieved by using a biexponential decay function in this study. Two decay processes are observed. One is a rapid decay process on the scale of hundreds of femtoseconds and the other is a slower decay process on the picosecond scale. The fast decay accounts for an intramolecular vibrational energy redistribution (IVR) process. The time constant of the fast process in methanol is smaller than that in other solvents, which is attributable to the hydrogen-bond between the solute and solvent molecules in methanol. The slower decay process is considered as a forming TICT state from LE state, and the relaxation time depends on the viscosity of the solvents but methanol. In methanol, the time is much longer than in other solvents. It may be explained that the rotation of the extended pyridyl aminoethyl group is hindered by the stronger hydrogen-bond between the methanol molecules.
引文
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