S2 fluorescence and ultrafast relaxation dynamics of the S2 and S1 states of a ketocyanine dye

Dynamics of the excited singlet (both the S-2 and S-1) states of a ketocyanine dye, namely, 2,5-bis[(2,3-dihydroindolyl)-propylene]-cyclopentanone (KCD), have been investigated in different kinds of media using steady-state absorption and emission as well as femtosecond transient absorption spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation of KCD to its second excited singlet state, reveal dual fluorescence (emission from both the S-2 and S-1 states) behavior. Although the intensity of the S-2 -> S-0 fluorescence is weaker than that of the S-1 -> S-0 fluorescence in solutions at room temperature (298 K), the former becomes as much as or more intense than the latter in rigid matrixes at 77 K. The lifetime of the S2 state is short and varies between 0.2 and 0.6 ps in different solvents. After its creation, the S2 state undergoes two simultaneous processes, namely, S-2 -> S-0 fluorescence and S-2 -> S-1 internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S, state. A good correlation between the lifetime of this component and the longitudinal relaxation times (tau(1).) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S, state in alcoholic solvents in the ultralast time domain has been explained by the occurrence of the repositioning of the hydrogen bonds around the carbonyl group in the excited state of KCD. In 2,2,2-trifluoroethanol, a strongly hydrogen bond donating solvent, it has even been possible to establish the existence of two distinct forms of the S, state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.