Optical Studies of a Simple Polyene Schiff Base: Low-Lying Electronic Levels in the Free, Hydrogen-Bonded, and Protonated Species

Article abstract of DOI:10.1021/ja00389a004

Fluorescence and fluorescence excitation spectra of a model polyene Schiff base, N-(2,4,6,8,10-dodecapentaenylidene)butylamine, have been obtained in 77 K hydrocarbon glasses.The low-energy electronic transitions of this polyene Schiff base have been studied in a variety of hydrogen-bonding and protonation environments.All spectra are well resolved and allow the accurate location of electronic origins.For all three species, i.e., free Schiff base, hydrogen-bonded Schiff base, and protonated Schiff base, the fluorescence origins are substantially red shifted (3108, 2774, and 2238 cm^-1) from the origins of the strongly allowed absorptions.These experiments show that the lowest excited singlet in each of these Schiff base species correlates with the forbidden A_g^- states observed in analogous polyene hydrocarbons.These findings are compared with current theoretical predictions and discussed with regard to their implications for the photochemistries of other polyene Schiff bases, including rhodopsin.