Vibronic coupling in polyenes: High resolution optical spectroscopy of all-t r a n s-2,4,6,8,10,12,14-hexadecaheptaene

High resolution fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8,10,12,14-hexadecaheptaene have been obtained in n pentadecane at 10 K. The most prominent vibronic features, like those in spectra of shorter polyene hydrocarbons, are due to combinations of symmetric carbon–carbon stretching vibrations. These in turn are dominated by a double bond mode whose frequency increases from 1555 to 1782 cm−1 upon electronic excitation (1 1Ag→2 1Ag). This 227 cm−1 increase, the largest yet observed in polyene spectroscopy, can be explained by vibronic coupling between the ground and first excited singlets. The possible role of vibrational mixing due to mutual polarizibilities between carbon–carbon bonds also is discussed.