Low-frequency vibrational dynamics of pyrene derivatives investigated by time-resolved impulsive stimulated Raman spectroscopy

Intramolecular charge transfer (ICT) dynamics of trisodium 8-aminopyrene-1,3,6-trisulfonate (APTS) in polar protic solvents were investigated using time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS). The hydrogen-bond donating ability of the solvents promotes the ICT process of APTS in the excited state. APTS exhibits two distinct excited-state Raman spectra of the locally excited and charge-transferred S1 state in TR-ISRS, where ultrafast ICT and subsequent vibrational relaxation dynamics with time constants of ∼160 fs and ∼5 ps, respectively, were observed in the formamide solution. Multidimensional structural changes of APTS, including the twist (θt) and bend (θb) of the amino group, are predicted to accompany ICT in the excited state by time-dependent density functional theory calculations. The two in-plane deformation modes of the pyrene backbone at 457 and 812 cm-1 are strongly coupled to the ICT coordinate of APTS in the S1 state. These low-frequency deformation modes of the pyrene backbone were commonly observed in the excited-state dynamics of pyrene derivatives with hydroxyl and methoxy groups, and thus, they can provide crucial information on the multidimensional structural dynamics of pyrene derivatives in numerous excited-state processes, including ICT and proton transfer.