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(2Ag) → 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.

Evidence for direct carotenoid involvement in the regulation of photosynthetic light harvesting

“…The time constants obtained at 540 and 550 nm for both spinach and Arabidopsis, where the largest differences in amplitude were observed, yielded lifetimes ranging from Ϸ7 to 11 ps. Within the experimental uncertainty, this value is identical to the intrinsic S 1 lifetime of carotenoids with 11 conjugated double bonds (23,24). Measurements of the S 1 lifetime of Zea in a variety of solvents produced a value of Ϸ9 ps (8,25), whereas recently a value of 11 ps was obtained by using a reconstituted LHCII protein with Zea as the sole carotenoid species (26).…”

Spectroscopic and Photochemical Properties of Open-Chain Carotenoids

“…This forces the ring double bond out of the plane of conjugation of the other double bonds, resulting in a blue shift of the electronic transitions. A comparison of the molecules studied here with their apo counterparts 37 indicates shifts of 1540, 1210, and 670 cm -1 for the S 0 f S 2 absorptions of the heptaene, octaene, and nonaene in 77 K EPA. The corresponding shifts for the S 1 f S 0 (0-0)'s are smaller (880, 930, and 810 cm -1 ) and relatively insensitive to the length of conjugation.…”

“…37 Although fluorescence quantum yields are sensitive to structural details, the relatively abrupt change to S 2 f S 0 emissions invariably occurs for molecules with seven or eight conjugated bonds. At first glance, the crossover might be attributed to the decrease in the rate of S 2 f S 1 internal conversion following the "energygap law".…”

“…37 The presence of a terminal double bond within a -ionylidene ring gives rise to steric repulsions between the methyl groups on the ring and the polyene chain. This forces the ring double bond out of the plane of conjugation of the other double bonds, resulting in a blue shift of the electronic transitions.…”

Intense Long-Lived Fluorescence of 1,6-Diphenyl-1,3,5-Hexatriene: Emission from the S<sub>1</sub>-State Competes with Formation of O<sub>2</sub> Contact Charge Transfer Complex

“…They are part of the light-harvesting complex, where they absorb visible light and transfer the energy to the reaction center of the photosystem. Alltrans-α,ω-diphenylpolyenes (also referred to as minicarotenes) are well established as model compounds for the bigger carotenoids such as β-carotene or lutein [1][2][3][4][5][6]. The latter absorb light and transfer the energy to the chlorophyll unit of the pigment [7].…”