Mediator Assisted PET between Porphyrin Molecules

The role of mediator molecules in donor-acceptor electron transfer processes is an item of considerable recent interest [73 — 81]. A lot of research has been done on intermediate acceptors in the electron transfer in photosynthesis and theoretical studies bases on the superexchange interaction have been carried out [76 — 82]. In Refs. [83,84], electron transfer in the presence of ordered mediator molecules with arbitrary energy levels in one-dimensional case [83] as well as electron transfer in the presence of one resonant mediator [84] were considered. 

In a number of works, it was supposed that in biological systems, due to certain structures of the molecules of the medium with vacant electron levels, tunnel reactions are more effective than in non-ordered media. But until recently, no direct experimental evidences supporting this assumption had been presented. 

The first work in which such experimental evidences were presented seems to be [85]. In this work, light-induced two-quanta electron transfer from one porphyrin molecule to another was studied in triethylamine at 77 K. The reaction was shown 

Here P [N(C2H5)3]2 denotes a complex between porphyrin molecule and triethyl-amine. The stability of the long lived radicals P [N(C2H5)3]2 seems to be due to their remoteness from the hole P [N(C2H5)3]2. 

The values of ve = 10 s 1 and ae = 5 A were found from accumulation curves of P [N(C2H5)3]2 anion-radicals. The low value of the frequency factor ve was explained by low stationary concentration of the excited complexes (P [N(C2H5)3]2). As shown in Ref. [85] this concentration is contained as a multiplier in the expression for ve. 

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