Electron tunneling in reactions involving chlorophyll and its synthetic analogues

Electron tunneling in reactions involving chlorophyll and its synthetic analogues 

For a better insight into the mechanism of the primary stages of photosynthesis it is of interest to study electron transfer reactions with the participation of chlorophyll and its synthetic analogues. As far back as 1948, a reversible reaction was discovered of photoreducing chlorophyll, Chi, in solutions containing ascorbic acid, AH, (the Krasnovskii reaction) [51] 

The electron transfer proceeded with the participation of the triplet excited state of Chi. The discovery of the reversible reaction of chlorophyll photoreduction served as a stimulus for starting systematic research on photochemical redox reactions of chlorophyll and its synthetic analogues, i.e. various metalloporphyrins. 

Metalloporphyrins, MP, represent derivatives of porphyrin, P, in which four pyrrole fragments are bound together by methine bridges (Fig. 13). The diversity of porphyrins is due to the possibility of variation for substituents R in the periphery of the porphyrin ring. A typical optical spectrum of a P solution is presented in Fig. 14. One can point out quite a number of characteristic bands in it. The most intensive short-wave peak in the P absorption spectrum (/max 400 nm) corresponds to the transition S0 -+ S2 and is referred to as Soret band. The extinction coefficient of this band is very large, as a rule, and amounts to 10 -106 M 1 cm-1. The less intensive long-wave bands of P absorption correspond to the S0 - Sx transition (bands I-IV in Fig. 14). Complexation with the metal results in a rise of the symmetry of the molecule, due to which MP molecules have only two bands in the long-wave part of the absorption spectra. Most of the metalloporphyrins are characterized by intense luminescence. The time of MP fluorescence decay (transition Si - S0) is short and amounts to 10 8 to 10 9 s. Besides the transition 

Sj - S0, the interconversion into the triplet state Tlf is also possible. The characteristic time of the phosphorescence decay of MP, on the other hand, is rather large and amounts to 10-2 s. (For a review of the physical and chemical properties of MP, see, for example, ref. 52.) 

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