Carotene-porphyrin dyads

GustD, Moore TA, Moore AL, DevadossC, LiddellPA, Herman R, Nieman RA, Demanche LJ, DeGraziano JM and Gouni 1 (1992) Triplet and singlet energy transfer in carotene-porphyrin dyads the role of the linkage bonds. J Am Chem Soc 114 3590-3603... 

Li and coworkers49 reported a molecular motion of /1-carotene and a carotenopor-phyrin dyad (composed of a porphyrin, a trimethylene bridge and a carotenoid polyene) in solution. Internal rotational motions in carotenoid polyenes and porphyrins are of interest because they can mediate energy and electron transfer between these two moieties when the pigments are joined by covalent bonds. Such internal motions can affect the performance of synthetic model systems which mimic photosynthetic antenna function,... 

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. 

The electronic spectra of carotene radicals in triton X micelles produce a strong band at 850 nm (Matsuo, 1989 Hill, 1995) (Fig. 5.2.8). Porphyrin-carotene dyads, in which the carotene is supposed to donate electrons in charge separation processes, give similar absorption bands and are discussed in Chapter 6 (De-breczeny, 1997). 

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