Spectral shape of diphenylpolyene fluorescence and mixing of the S1 and S2 states Original Research Article

Specific symmetry of the lowest excited states in polyenes results in unusual fluorescence properties. In the present work the influence of the S1–S2 state mixing on the diphenylpolyene fluorescence spectra was considered. Analysis based on a general description of the state mixing indicated that a change of the energy gap between the mixing states should influence not only the intensity, but also the spectral shapes of the forbidden S1→S0 fluorescence. This result is confirmed by analyzing the experimental diphenylpolyene fluorescence spectra, which exhibit solvent-dependent shapes and intensities. The theoretical model and experiments also showed that the effective S1–S2 gap depends on the detected emission frequency, and the gap is smaller if it is determined at the high-frequency edge of the S1 emission spectrum. Thus, the spectra cannot correspond to the Franck–Condon factor for the S1–S0 transition. Since the difference between actual molecular energy and the S2 state energy decreases for vibrationally excited molecules, the radiative rate increases for vibrationally excited molecules in the S1 state. This provides a higher relative intensity for the emission from thermodynamically `hotʹ levels in diphenylpolyenes as compare to compounds with allowed S1–S0 transitions. The `hotʹ emission of polyenes is more sensitive to the solvent than the average intensity of their fluorescence.