Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes

Optical absorption and emission spectra at liquid helium temperatures of solutions of all-trans-1,8-diphenyloctatetraene in single crystals of bibenzyl and polycrystalline n-paraffin matrixes are presented which show an excited singlet state at a lower energy than the 1Bu state responsible for the strong absorption of all linear polyenes. The transition from the ground state to this level has an oscillator strength of about 0.05. The location of this low energy weak transition removes a number of discrepancies between the expected and observed fluorescence properties of diphenyloctatetraene. The observed vibronic pattern is consistent with the Raman spectrum which is also reported. Substantial indirect evidence is presented which indicates that many, and perhaps all, linear polyenes have a similar ordering of excited states. A review of the available information on diphenylpolyenes is presented. A correlation of the oscillator strength of the fluorescence transition as deduced from the emission lifetime with the separation of the low energy excited states for a number of polyenes is noted. The implications for the photochemistry of linear polyenes including the visual chromophores are pointed out.