Knowledge Bank

The high resolution ($\sim 5cm-1$ linewidth) emission and excitation spectra of several cis-trans isomers of the conjugated polyene aldehydes 2,4,6,8-decatetraen-1-al and 2,4,6,8,10-dodecapentaen-1-al in alkane matrices at 4.2K will be presented and interpreted in terms of the energy ordering of the $11A\prime \prime (n\ast \leftarrow n)$ and $21A\prime (\pi \ast \pi \ast \leftarrow \pi \pi )$ excited states. Based on molar extinction coefficients and low resolution spectra in glasses at 77K, other $workers1$ have shown that the two states are nearly degenerate in the all-trans $C10$ aldehyde, their order being sensitive to the solvent environment, whereas the $21A\prime$ state is lowest in the all-trans $C12$ aldehyde. In this paper, we will discuss the dependence of the state ordering on the geometric configuration of the polyene chain. State assignments will be made on the basis of a complete vibronic analysis of the spectra.