Carotenoids excited singlet state

Jeevarajan, A. S., L. D. Kispert et al. (1996b). Role of excited singlet state in the photooxidation of carotenoids a time resolved Q-band EPR study. J. Phys. Chem. 100 669-671. 

In good electron acceptor solvents, such as carbon tetrachloride and chloroform, the photodegradation of carotenoids is significantly increased as compared to other solvents (Christophersen et al. 1991, Mortensen and Skibsted 1999), because of a direct photoinduced electron-transfer reaction from the excited singlet state of the carotenoids to the solvent, as determined by transient absorption spectroscopy (Jeevarajan et al. 1996, Mortensen and Skibsted 1996,1997a,b, El-Agamey et al. 2005), Equation 12.2 ... 

A study of a we o-tetraphenylporphyrin bearing four negatively charged, bixin-based carotenoid substituents has shown that in water at pH 9, unilamellar vesicles made up of monolayer membranes are formed [169]. In the presence of guanidi-nium porphyrin counter ions, excitation at wavelengths absorbed by porphyrins leads to photoinduced electron transfer. Spectroscopic evidence for the bixin radical cations and porphyrin radical anions was obtained. Presumably, photoinduced electron transfer from the bixin to porphyrin first excited singlet states is involved in the formation of the radical ions. 

Interporphyrin photoinduced electron transfer can of course be the basis of light-driven charge separation in more complex systems. For example, triad 42 is similar in structure to dyad 1, with the addition of a carotenoid moiety, which can serve as a secondary donor as in 35 [13]. Excitation of 42 in dichloromethane with a laser pulse at 590 nm creates two porphyrin first excited singlet states, C- Pzn-Pp and C-Pzn- Pp. As with 1, both states decay at least in part by photoinduced electron transfer to give an initial C-Pzn -Pp charge-separated state. This state can recombine to the ground state, but as with the other triads discussed above, electron donation from the carotene competes with this to yield a final C +-Pzn-Pp state with an overall quantum yield of 0.32 with 590-nm excitation. The lifetime of this final state is 240 ns. 

Excitation of the porphyrin moiety of 2 in dichloromethane solution yields the first excited singlet state, which can decay according to the pathways detailed in Figure 4. As with P-Q dyad 1, photoinitiated electron transfer competes with other decay processes to yield a C-P -Q charge-separated state. Fluorescence decay studies yielded a fluorescence lifetime r of 0.10 ns for 2 [27]. The hydroquinone form of the triad, 3, in which such electron transfer is not possible, has a fluorescence lifetime of 3.4 ns in the same solvent (see Section 111.A.). Application of Eq. (1) yields an electron transfer rate constant fcj in Figure 4 of 9.7 x 10 s", and consequently a quantum yield for this step of essentially unity. Thus, the addition of the carotenoid moiety to the molecule has had little influence upon the initial photodriven electron transfer step. 

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