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.