SPECTRAL SHAPE OF DIPHENYLPOLYENE FLUORESCENCE AND MIXING OF THE S-1 AND S-2 STATES

Specific symmetry of the lowest excited states in polyenes results in unusual fluorescence properties. In the present work the influence of the S-1-S-2 state mixing on the diphenylpolyene fluorescence spectra w as considered. Analysis based on a general description of the state mi xing indicated that a change of the energy gap between the mixing stat es should influence not only the intensity, but also the spectral shap es of the forbidden S-1 --> S-0 fluorescence. This result is confirmed by analyzing the experimental diphenylpolyene fluorescence spectra, w hich exhibit solvent-dependent shapes and intensities. The theoretical model and experiments also showed that the effective S-1-S-2 gap depe nds on the detected emission frequency, and the gap is smaller if it i s determined at the high-frequency edge of the S-1 emission spectrum. Thus, the spectra cannot correspond to the Franck-Condon factor for th e S-1-S-0 transition. Since the difference between actual molecular en ergy and the S-2 state energy decreases for vibrationally excited mole cules, the radiative rate increases for vibrationally excited molecule s in the S-1 state. This provides a higher relative intensity for the emission from thermodynamically 'hot' levels in diphenylpolyenes as co mpare to compounds with allowed S-1-S-0 transitions. The 'hot' emissio n of polyenes is more sensitive to the solvent than the average intens ity of their fluorescence. (C) 1998 Elsevier Science B.V.