Ultrafast Dynamics Study Of LDS698 In Solvents

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.