Porphyrin electron-hole recombination

Harima et al. (1989) measured the photogeneration efficiencies of 5,10,15,20-tetraphenylporphyrin (TPP) and its Zn complex (ZnTPP) doped with tetracyano-quinodimethane (TCNQ), o-chloranil (Chi), phenothiazine (Pz), and I. TCNQ, Chi, and I are electron acceptors while Pz is a donor. The porphyrins were selected on the basis of differences in exciton diffusion lengths (Tanimura et al., 1980 Yamashita et al., 1987) and oxidation potentials (Felton, 1978). The photogeneration of I doped ZnTPP was described by a direct ionization process via a singlet state of ZnTPP and ascribed to a reduction of the electron-hole recombination rate in the depletion layer. For TPP doped with Chi, Pz, or I, the results were explained by exciplex dissociation (Loutfy and Menzel, 1980). For... 

For thermodynamic reasons, electron-hole recombination between the porphyrin radical cation and the semiquinone is expected to be slower than recombination involving the quinone rascal anion (vide infra). Retarding electron-hole recombination should lengthen the lifetime of the porphyrin radical cation and thereby enhance the yield of electron donation to it by the attached carotenoid, and thus increase the yield of the final charge-separated state C+-P-Q -H -OOC. 

---