Probing Excited-State Electronic Structure and Dynamics of Fluorescein Doped in PVA Film Using Electroabsorption and Electrophotoluminescence Spectroscopy.

Electroabsorption (E-A) and electrophotoluminescence (E-PL) spectra of fluorescein (FL) doped in poly(vinyl alcohol) (PVA) film were measured in the visible region at various concentrations of FL in PVA. By analyzing the E-A spectra, the changes in the electric dipole moment and polarizability following excitation into the S1 state are determined for each concentration. The analysis of the E-A spectra indicates the formation of H-aggregates of FL in PVA at high concentrations. The electric-field-induced changes in the radiative decay rate constant (kr) of FL in PVA, estimated from the field-induced change in the total absorption intensity, indicate that kr decreases in the presence of an electric field and that the magnitude of the field-induced change in kr decreases with increasing FL concentration. In contrast to the kr of the FL monomer, the kr of H-aggregates is considered to increase with the application of an electric field. The fluorescence lifetime of FL in the PVA film decreases rapidly with increasing FL concentration, indicating a monotonic decrease in the fluorescence quantum yield with increasing FL concentration due to the rapid increase of the rate constant of the nonradiative decay process (knr). The nonradiative process from the emitting state of FL is probably assigned to the energy dissipation to nonemissive H-aggregates, i.e., energy transfer from the emitting state of the FL monomer to the H-aggregate. The applied electric field quenches the fluorescence of FL in PVA, indicating the field-induced increase in knr. Further, the magnitude of the field-induced quenching increases rapidly with increasing FL concentration, indicating that the field-induced increase of knr increases significantly with increasing FL concentration in PVA.