A series of apocarotenes with 5 to 11 conjugated double bonds were synthesized and all-trans isomers were
isolated using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra were obtained
in 77 K glasses. As previously noted for other polyenes and carotenoids, fluorescence spectra of the apocarotenes
exhibit a systematic crossover from S1(2Ag) → S0(11Ag) to S2(21Ag) → S0(11Ag) emissions and a sharp decrease
in fluorescence yields with increasing conjugation. The apocarotene spectra have sufficient resolution to
accurately locate the dominant vibronic bands of the S0(11Ag) → S2(11Bu) and S1(21Ag) → S0(11Ag) transitions,
thus leading to an accurate catalog of S1 and S2 electronic energies as a function of conjugation length. We
also have obtained the low-temperature absorption and fluorescence spectra of several model polyenes and
diapocarotenes. Comparisons between these series allow a systematic exploration of the influence of terminal
cyclohexenyl rings on the energies of carotenoid S1 and S2 states. In addition, these preliminary studies indicate
that the nature of the terminal double bond has a significant influence on nonradiative decay processes in
longer carotenoid systems. Implications regarding the use of energy gap law extrapolations to estimate the
21Ag energies of long carotenoids are discussed.
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