SPECTROSCOPY OF POLYENES .3. ABSORPTION AND EMISSION SPECTRAL INVESTIGATION OF POLYENE SCHIFF-BASES AND PROTONATED SCHIFF-BASES RELATED TO VISUAL PIGMENTS

Abstract— A number of n‐butylamine Schiff bases of polyenals related to retinals as homologues and analogues, and their protonated forms, have been studied for absorption and emission spectral properties. The polyene Schiff bases exhibit the same general features in their absorption spectra as those of the parallel polyenals except that the lBu←1Ag and π*←n singlet transitions are at substantially higher energy in the Schiff bases (the shift being larger for the π*← n transition). The Schiff bases with short polyene chainlength (n= 2, 3 where n is the number of double bonds including C=N) do not fluoresce or phosphoresce in 3‐methylpentane in the temperature range 298–77 K. The Schiff bases with intermediate chainlength (n= 4, 5) show fluorescence at 77 K with intensity strongly dependent on the nature of solvent. The Schiff bases with relatively long chainlength (n= 5–7) show strong or moderately strong fluorescence at 77 K and very weak fluorescence at 298 K (n = 7) with intrinsic radiative lifetimes much longer than those estimated from the oscillator strength of the low‐energy, strong absorption band (1Bu←1Ag). A discussion on the possible state order and nature of the fluorescing state of the various polyene Schiff base systems is presented. Copyright © 1979, Wiley Blackwell. All rights reserved