Vibronic absorption spectra of naturally occurring conjugated polyenynes. Evidence for localized excitations

Abstract

The second-derivative technique has been used to measure the absorption spectra of a series of related naturally occurring polyenyes whose chromophoric fragments range from three to six conjugated groups. The reliability of the second-derivative method has been checked by comparison between the observed and calculated absorption and second-derivative spectra in terms of the vibronic theory. The spectral resolution achieved allowed the vibrational structure of the electronic transitions to be analysed. This is useful in identifying the chromophore involved in the electronic excitation. The spectra have been interpreted in terms of localized excitons and subsequent exciation transfer. These results account for the structural characteristics, steroisomerism included, of a given sequence of triple and double bonds providing a complete fingerprint of the polyenyne chromophore.