S₁ and S₂ States of Apo- and Diapocarotenes

Abstract

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(21Ag) → 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.