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

Abstract

Dynamics of the excited singlet (both the S₂ and S₁) 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 S₂ and S₁ states) behavior. Although the intensity of the S₂→S₀ fluorescence is weaker than that of the S₁→S₀ 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, S₂→S₀ fluorescence and S₂→S₁ internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S₁ state. A good correlation between the lifetime of this component and the longitudinal relaxation times (t_L) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S₁ state in alcoholic solvents in the ultrafast 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 S₁ state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.