Relaxation dynamics in the excited states of LDS-821 in solution

Abstract

Relaxation dynamics in the excited electronic state (S₁) of LDS-821 have been studied in aprotic and alcoholic solvents with pico- and subpicosecond time resolution using pump-supercontinuum probe transient absorption technique. Steady-state absorption and fluorescence as well as the time-resolved transient absorption/stimulated emission spectra and the temporal dynamics monitored at different wavelengths mainly reveal the features of a two-mode kinetic process in the S₁ stateconversion of the locally excited (LE) state to the twisted intramolecular charge-transfer (TICT) state. A short (<1 ps) rise time of stimulated emission monitored at the shorter wavelength band, is the signature of the barrierless skeletal stretching motion along the "valley-like" region of the potential energy surface (PES) of the S₁ state in the Franck-Condon (FC) region to attain a metastable untwisted configuration, called the LE state. The decay time of the LE state is equal to the rise time measured at the longer wavelength band, assigned to the TICT state. The linear dependence of the rate of the LE → TICT conversion process on the inverse of viscosity of the solvent indicates that this low-energy barrier crossing process is accompanied by a torsional motion about the free double bond of the molecule. The solvation time of the TICT state in different solvents has been seen to be nearly equal to the growth lifetime of the TICT state but much shorter than the longitudinal relaxation time of the solvent. These facts indicate that the rate of the LE → TICT process is mainly controlled by the contribution from the intramolecular modes rather than the solvation.