Charge separation zinc porphyrins

Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

Fullerenes such as C60 are excellent electron acceptors. In a fullerene-porphyrin-based dyad, the photoexcited state of the Qo accepts an electron from the linked zinc porphyrin group to give a charge-separated state. 

FIGURE 2. Zinc(II) porphyrin-o-quinone dyads and two typical reactions (a) photoinduced charge separation, (b) chelation of in situ formed semiquinone anions. PQ = porphyrin quinine M = metal... 

Ultrafast charge separation and radiationless relaxation processes from S2 excited electronic states of directly linked Zinc-porphyrin-acceptor dyads... 

The first detailed research on the mechanism of electron phototransfer with the participation of MP in vitreous matrices appears to be ref. 55, which deals with the processes of charge separation in vitreous solutions of zinc and magnesium porphyrins in ethanol containing some addition of CC14 at 77 K. Illumination of the solutions in the Soret band or in long-wave bands of MP absorption results in a one-quantum ionization of MP and formation of MPf cation radicals and CC14 anion radicals recorded according to their characteristic EPR and optical spectra [56,57]... 

Aida et al. synthesised a dendritic zinc-porphyrin heptamer (7Pz -C60) with a fullerene terminal entity as an electron-transfer system, which harvests visible light for electron transfer to the fullerene group (Section 6.3.3.5). Electron transfer from the porphyrin to the fullerene gives rise to a charge-separated state (Pzn-Qo)> whose lifetime was the subject of investigation. Comparative studies... 

Porphyrin and its derivatives are well-known model compounds for photosynthetic processes that involve charge separation [87], A zinc porphyrin, a copper porphyrin, and a zinc phthalocyanine (Pc) are chosen and incorporated into PPV backbones. Four polymers are synthesized via the Heck polycondensation [88], The incorporation of these metal complex moieties in polymers XI to XIII is manifested by the appearance of Q bands from metalloporphyrin and zinc Pc... 

A relative of 12 has been prepared by Sanders, van der Plas, and coworkers [64], Triad 13 features an N,A-dimethylaniline-type donor and an anthraquinone acceptor. These moieties are linked to the ortho positions of the porphyrin aryl groups, and this leads to a folded conformation for the molecule, as determined from NMR studies. Both the free base and zinc derivatives of 13 were prepared. The folded conformation might be expected to facilitate electron transfer among the components of the triad, and while this could enhance the quantum yield of the initial charge separated state, it might unfavorably affect the yield of the final D+-P-QT state and its lifetime. Unfortunately, photochemical or spectroscopic studies were not reported. 

An interesting variation on this theme has been reported by Lehn and coworkers. In 1986, they reported the synthesis of macrocycle 33, which consists of a zinc porphyrin bearing two linked cyclic binding subunits [87]. It was later found that addition of silver triflate to a solution of 33 in methanol resulted in the incorporation of a silver ion in each of the binding subunits [88], Thus, the complex may be represented as Ag+-P-Ag+. The porphyrin fluorescence of the silver complex was quenched, and transient absorption studies demonstrated that the porphyrin singlet state was quenched with a rate constant of 5.0 x 109 s 1 to yield a charge separated state Ag°-P+-Ag+. Some quenching of the porphyrin triplet... 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

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