ChemInform Abstract: Electronic Relaxation in Long Polyenes

Abstract

A series of carotenols with from 7 to 11 conjugated double bonds have been synthesized and purified by using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra have been obtained in 77 K glasses. The shorter members of this series exhibit the Stokes-shifted, S{sub 1} {yields} S{sub 0} emissions seen in previous studies of model polyenes. For carotenols with more than eight conjugated double bonds, however, the fluorescence is dominated by anti-Kasha, S{sub 2} {yields} S{sub 0} fluorescence. these findings in part can be attributed to a larger S{sub 2}-S{sub 1} energy difference and the resultant decrease in S{sub 2} {yields} S{sub 1} radiationless decay rates in longer polyenes. The precipitous crossover from S{sub 1} {yields} S{sub 0} to S{sub 2} {yields} S{sub 0} emission, however, cannot be fully accounted for by the energy gap law, which predicts only modest changes in radiative and nonradiative decay rates with increasing polyene length. The implications of large S{sub 2}-S{sub 1} energy gaps for the spectroscopy and photochemistry of {beta}-carotene and other long polyenes also are discussed.