S-1 and S-2 states of apo- and diapocarotenes

A series of apocarotenes with 5 to 11 conjugated double bonds were synthesi zed and all-trans isomers were isolated using HPLC techniques. Absorption, fluorescence, and fluorescence excitation spectra were obtained in 77 K gla sses. 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 sha rp decrease in fluorescence yields with increasing conjugation, The apocaro tene spectra have sufficient resolution to accurately locate the dominant v ibronic 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 ob tained the low-temperature absorption and fluorescence spectra of several m odel polyenes and diapocarotenes. Comparisons between these series allow a systematic exploration of the influence of terminal cyclohexenyl rings on t he energies of carotenoid S-1 and S-2 states. In addition, these preliminar y studies indicate that the nature of the terminal double bond has a signif icant influence on nonradiative decay processes in longer carotenoid system s. Implications regarding the use of energy gap law extrapolations to estim ate the 2(1)A(g) energies of long carotenoids are discussed.