SPECTROSCOPY OF POLYENES .1. COMPREHENSIVE INVESTIGATION OF ABSORPTION-SPECTRA OF POLYENALS AND POLYENONES RELATED TO VISUAL CHROMOPHORES

The absorption spectra of various homologues and analogues of retinals, a total of 21, with varying number of double bonds (n) have been examined in detail under various conditions of solvent and temperature. Altogether six band systems have been identified and their oscillator strengths and transition energies are presented as functions of chain length. The origin of the transitions is discussed in the light of the results of semiempirical calculations available in the literature. The trend in the lower polyene systems (n = 2-4) where the 1(n,π*) state is seen in absorption clearly indicates that this latter state is the lowest singlet state in these systems, and is close to the Bu state in retinals and their analogues. The ∼280-nm band system in retinals and their analogues, heretofore not satisfactorily assigned, is traceable to a more intense band system in the lower homologues and is tentatively interpreted in terms of absorption of 6-s-trans conformers present in solution to the extent of ∼10% in equilibrium with distorted 6-s-cis conformers. The absorption spectra of retinones and C16 ketones indicate methyl-methyl and methyl-hydrogen steric interaction leading to geometric distortion of the polyene chain. © 1978 American Chemical Society.