Vibronic coupling in polyenes: High resolution optical spectroscopy of a simple Schiff base

High resolution fluroescence and fluroescence excitation spectra of a model polyene Schiff base have been obtained. 4.2 K spectra of N-(2, 4, 6, 8, 10-dodecapentaenylidine)butyl amine in n-tetradecane exhibit well-resolved electronic spectra (ππ*) from a single mixed crystal site. The major vibronic features, in common with those of polyene hydrocarbons, are due to combinations of symmetric carbon–carbon stretching vibrations. These are dominated by a double bond mode which decreases from 1589 to 1544 cm−1 upon electronic excitation. This decrease contrasts the 150–200 cm−1 increases seen in analogous polyene hydrocarbons. This difference is discussed in terms of vibronic coupling in a model previously applied to the hydrocarbons.