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 S 1 (2 1 A g ) → S 0 (1 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.