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

Relaxation dynamics in the excited electronic state (S-1) of LDS-821 have b een studied in aprotic and alcoholic solvents with pico- and subpicosecond time resolution using pump-supercontinuum probe transient absorption techni que. 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 S-1 state-conversion of the locally excited (LE):st ate to the twisted intramolecular charge-transfer (TICT) state. A short (<1 ps) rise time of stimulated emission monitored at the shorter wavelength b and, is the signature of the barrierless skeletal stretching motion along t he "valley-like" region of the potential energy surface (PES) of the SI sta te in the Franck-Condon CFC) region to attain a metastable untwisted config uration, called the LE state. The decay time of the LE state is equal to th e rise time measured at the longer wavelength band, assigned to the TICT st ate. The linear dependence of the rate of the LE --> TICT conversion proces s on the inverse of viscosity of the solvent indicates that this low-energy barrier crossing process is accompanied by a torsional motion about the fr ee double bond of the molecule. The solvation time of the TICT state in dif ferent 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 th e 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.