Vibronic coupling in polyenes: High resolution optical spectroscopy of 2,10-dimethylundecapentaene

High resolution fluorescence and absorption spectra of 2,10-dimethylundecapentaene in 4.2 °K n-nonane matrices are presented. Analysis of these spectra indicates that the transitions between the ground and first excited state are induced by low frequency, nontotally symmetric vibrations. This is in agreement with Herzberg-Teller expectations for forbidden transitions and supports the 1Ag assignment for the lowest lying excited singlet state. Vibrational analysis of the absorption indicates increases in the carbon–carbon double bond stretching frequencies in the 1Ag state. This is unexpected given the decreases in carbon–carbon double bond orders predicted by molecular orbital calculations. Rationalization of this apparent discrepancy may be found in substantial mixings of excited state vibrations. The photochemical implications of these findings also are discussed.