S1 and S2 states of apo- and diapocarotenes

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 S-1(2(1)A(g)) --> S-0(l(1)A(g)) to S-2(2(1)A(g)) --> S-0(1(1)A(g)) 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 S-0(1(1)A(g)) --> S-2(1(1)B(u)) and S-1(2(1)A(g)) --> S-0(1(1)A(g)) transitions, thus leading to an accurate catalog of S-1 and S-2 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 S-1 and S-2 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 2(1)A(g) energies of long carotenoids are discussed.