STUDIES OF THE EFFECT OF HYDROGEN-BONDING ON THE ABSORPTION AND FLUORESCENCE-SPECTRA OF ALL-TRANS-RETINAL AT ROOM-TEMPERATURE

Ultraviolet (UV)-visible and fluorescence spectra were obtained for complexes of ATR and TFA at different ratios and in four different solvents: hexane, chloroform, dichloromethane, and methanol. In the first three solvents, a large excess of TFA generates retinylic cations that absorb from 459 to 600 nm. Also, in CHCl3, Raman spectroscopy and fluorescence indicate that some aggregated species like ATR:(TFA)n, with a lambda(max) of ca. 470 nm, are present. In methanol, TFA protonates the solvent and it is CH3O+H2 which interacts with ATR so that only blue-shifted H-bonded ATR is present. From these results, it is shown that in the tautomeric equilibrium C=O. . .HOOC-(1) half arrow left over half arrow right C=O+H. . .-O2C- (2), form (1) is always favored in the ground state whatever the solvent. In the excited state in hexane and in methanol, (1) is rapidly transformed into (2). In CH2Cl2 and CHCl3, this transformation is absent so that there is no energy dissipation, with the result that the retinal complexes become more unstable.