Femtosecond study on the isomerization dynamics of NK88: I. Excited-state dynamics

The molecule 3,3'-diethyl-2,2'-thiacyanine (NK88) isomerizes after irradiation with light of the proper wavelength. After excitation, it undergoes a transition, in which one or more conical intersections are involved, back to the ground state to form different product photoisomers. The dynamics before and directly after the transition back to the ground state is investigated by transient absorption spectroscopy in a wavelength region from 360~nm to 950~nm, as well as by fluorescence upconversion. It is shown that the excited-state dynamics are governed by two timescales: a short one with a decay time of less than 2~ps and a long one with about 9~ps. A thorough comparison of the experimental results with those of CIS (configuration interaction singles) and TD-DFT (time-dependent density functional theory) calculations suggests that these dynamics are related to two competing pathways differing in the molecular twisting on the excited surface after photoexcitation. From the experimental point of view this picture arises taking into account the timescales for ground-state bleach, excited-state absorption, stimulated emission, fluorescence and assumed hot ground-state absorption both in the solvent methanol and ethylene glycol.