Multistep photoinduced electron-transfer

II. CONTROL OF MULTISTEP PHOTOINDUCED ELECTRON-TRANSFER SYSTEMS... 

Sc heme 1 Multistep photoinduced electron transfers in a natural photosynthetic system. 

There is a special type of photoelectrochemical devices based on donor/acceptor dyads and triads self-assembled on the metal (gold) or metal oxide (TTO and FTO) electrodes. A multistep photoinduced electron transfer occurring in such devices resembles in some way a natural photosynthetic process. The power conversion efficiencies of such devices are negligibly low because the monolayer of the photoactive material does not absorb much light. However, internal quantum efficiencies (IQEs) of such devices approach 80-90% in many cases. 

Solvated electrons do not inevitably interfere in photoinduced electron transfer. Their observations are often made under laser irradiations in order to detect these transients efficiently. Under these conditions processes may occur in a multistep and biphotonic way [68], the triplet state being one of the possible intermedites [69], The two photon process of electron ejection may dominate under pulsed laser conditions of high excitation energy while a monophotonic process prevails under continuous laser intensity conditions. These differences may explain the quantum yields observed for instance for the electron photoejection from excited phenolate in water under different irradiation conditions (0.23 [70], 0.17 [71], 0.37 [72]). When using conventional light sources, a relatively low yield of solvated electron is to be expected [69, 72]. 

A similar incremental effect of porphyrin-quinone separation was observed with the systems shown in Scheme 36 which were prepared by Wittig condensation of the meso-substituted porphyrin 116 (as the nickel complex) with the phosphorus ylide 117 Demethylation, reduction of the double bonds and then oxidation furnished the free base porphyrins 118 and 119a, b. The rate of photoinduced electron transfer in such systems showed an inverse exponential dependence on the length of the chain In order to demonstrate a multistep electron transfer the bis-quinone porphyrin 120 was prepared in which the pair of quinone rings provide a redox potential gradient and may thus stabilize charge separation. Comparison with the mono-quinone etioporphyrin 119a... 

Importantly, all photoinduced processes share some common features. A photochemical reaction starts with the ground state structure, proceeds to an excited state structure and ends in the ground state structure. Thus, photochemical mechanisms are inherently multistep and involve intermediates between reactants and products. In the course of a photoinduced charge transfer reaction the molecule passes through several energy states with different activation barriers. This renders the electron transfer pathway quite complex. 

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