Spectral shape of diphenylpolyene fluorescence and mixing of the S 1 and S 2 states

• Published in: Chemical physics Vol. 229; no. 1; pp. 75 - 91
• Main Authors: Bachilo, S.M., Bachilo, E.V., Gillbro, T.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.03.1998
• ISSN: 0301-0104
• DOI: 10.1016/S0301-0104(97)00357-1

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 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 S 1→S 0 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 S 1–S 2 gap depends on the detected emission frequency, and the gap is smaller if it is determined at the high-frequency edge of the S 1 emission spectrum. Thus, the spectra cannot correspond to the Franck–Condon factor for the S 1–S 0 transition. Since the difference between actual molecular energy and the S 2 state energy decreases for vibrationally excited molecules, the radiative rate increases for vibrationally excited molecules in the S 1 state. This provides a higher relative intensity for the emission from thermodynamically `hot' levels in diphenylpolyenes as compare to compounds with allowed S 1–S 0 transitions. The `hot' emission of polyenes is more sensitive to the solvent than the average intensity of their fluorescence.