Observing ground state vibrational coherence and excited state relaxation dynamics of a cyanine dye in pure solvents

Abstract

Using a degenerate pump probe technique at 800 nm, Ground State Vibrational Coherence (GSVC) of a cyanine dye (IR780) is explored in various solvents. Five representative solvents were studied, of which GSVC is present in three: methanol (MeOH), ethanol (EtOH) and dichloromethane (DCM). The maximum number of damped oscillations was observed in DCM solvent and the oscillations persisted for 1.2 ps. However, GSVC is absent in two solvents: acetonitrile (ACN) and dimethyl sulfoxide (DMSO). For nondegenerate pump probe studies with a 400 nm pump and a supercontinuum probe, GSVC was absent for IR780 in all of the five solvents studied. Instead these studies showed excited state absorption corresponding to S₁ → S_n absorption bands in the 450 to 650 nm range. Decay profiles for these correspond to the higher electronic states (S_n, n ≥ 2) in contrast to the degenerate case involving S₁. Complete dynamics of IR780 with 30 fs pulses are reported by invoking three time-constants to fit the kinetic traces that can, in turn, be assigned as: vibrational cooling, intermediate state time, and ground state recovery (GSR). All these dynamics exhibit noticeable solvent dependence. A plateau region is also observed in the early part of the GSR dynamics, strongly suggesting the existence of an intermediate state between the ground (S₀) and excited singlet (S₁) states.