Triplet 0-carotene

There was a marked difference in the rate of triplet energy transfer for 24 and 26. In a benzene solution of 24, the carotenoid triplet species had a rise time of ca. 2/is and decayed in ca. lO s. Concomitant with the rise of the carotenoid triplet absorbance at 550 nm, the porphyrin triplet absorbance at 440 nm decayed with a time constant of 2/js. There was no appreciable change in these parameters when 24 was dissolved in a rigid plastic matrix [73]. For 26 the triplet energy transfer was much faster. In 1981, we reported it as faster than 30 ns, which was the limit of our instrumentation [91]. Measurements with greater time resolution were made in 1983, but it remained difficult to separate the carotenoid triplet rise time from the instrument response time [73]. In any case, under conditions ranging from solution to solid plastic to a glass at 10 K, the rise time of triplet carotene in 26 was < 5 ns. 

Liddell P A, Kuciauskas D, Sumida J P, Nash B, Nguyen D, Moore A L, Moore T A and Gust D 1997 Photoinduced charge separation and charge recombination to a triplet state in a carotene-porphyrin-fullerene triad J. Am. Chem. Soc. 119 1400-5... 

The isomerization of all-trans-[3-carotene under N2 atmosphere by photosensitization action of eight chlorophyll compounds naturally present in the extracts of green vegetables was investigated by illumination with fluorescent white light (3000 lux) at 12°C to minimize the thermal degradation (O Neil and Schwartz 1995). All chlorophylls showed similar isomeric distribution and efficiency, 9-d.s- 3-carotene is the main isomer formed. On the other hand, the illumination of all-fran.s-(3-car-otene without chlorophylls indicated that the main isomer formed was 13-d.s- 3-carotcnc, probably due to the population of a different triplet state manifold by direct photolysis. 

Kuki, M., Y. Koyama, and H. Nagae. 1991. Triplet-sensitized and thermal isomerization of all-frans, 7-cis, 9-cis, 13-cis, and 15-cis isomers of (3-carotene Configurational dependence of the quantum yield of isomerization via the T1 state. J. Phys. Chem., 95 7171-7180. 

Lambert, C. and R. W. Redmond. 1994. Triplet energy level of (3-carotene. Chem. Phys. Lett. 228 495 198. 

Mortensen, A. 2001. Scavenging of acetylperoxyl radicals and quenching of triplet diacetyl by P-carotene mechanisms and kinetics. J. Photochem. Photobiol. B Biol. 61 62-67. 

Frozen 1 1 ethanol/toluene glass containing 1 mM 3-carotene and TPP Nd YAG-530 mn 4 K No transient carotene triplet-state... 

The triplet state decay rate constant (ky) of TPP is increased in the presence of the carotenoids apocarotenal and canthaxanthin but the addition of 3 Carotene seems to have little effect. In the case of TPPS it is seen that the water insoluble apocarotenal has no effect whereas the water soluble crocetin increases ky as was expected. 

In several cases, dependent on the donor, the electron transfer triplet energy transfer from the triplet state of the fullerenes to the donor was observed. For example, excitation of C6o/perylene (Pe) mixtures leads to 3Pe and C6o in a fast reaction ((1.4 0.1) X 109 M 1 s-1). The electron transfer from Pe to 3C o occurs with a rate one-third of triplet energy transfer [127]. Ito et al. investigated the photoexcitation of mixed system of C6o and (3-carotene [141], They observed triplet energy transfer from 3C o to (3-carotene in polar as well as in nonpolar solvents besides electron transfer from (3-carotene to 3C o However, the electron transfer rate constant increases with solvent polarity while the energy transfer is only less effected by the change of solvent polarity (Table 5). 

The energy gap between the two levels (3 (3-carotene + C6o and (3-carotene + + (7,o) is also dependent on the solvent polarity, becoming smaller in polar solvents (Scheme 3) [141], The decay of the 3-carotene +-absorption follows second-order kinetics suggesting that back-electron transfer occurs from C 60 [141], Similar results were observed for mixtures of fullerenes with phthalocya-nines as donors [148]. In nonpolar solvents triplet energy transfer from the triplet-excited fullerenes to the phthalocyanine becomes prominent [148],... 

The core of the PS-II reaction center has been prepared. Flash absorption showed that this core is able of efficient charge separation to form the primary radical pair which decays in about 30ns. The recombination populates the P-680 triplet state, which does not transfer to beta-carotene and can be detected by spin-polarized ESR. The yield of formation and kinetics of decay of the radical-pair have been measured in various PS-II preparations. The data are in favor of an equilibrium between the radical-pair and chlorophyll excited state in the antenna. 

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