Fate of Electron Excitation Inside Membranes

It is well known that the high quantum efficiency of photosynthesis is provided, first of all, by minimizing a useless dissipation of electron excitation energy in 

Of course, nowadays it is still impossible to utilize in the artificial membrane systems the primary electron excitation with the same efficiency as in natural ones. One of the reasons for this is the lack of sufficient knowledge about the fate of the electron excitation in these systems. The quantitative data about the dissipation of the electron excitation in such systems are quite scares. In the present section we shall discuss these data. 

Synthetic or natural porphyrins are most widely used as photosensitizers of PET across the membranes. It is well known that in homogeneous solutions the electron excited states of porphyrins are efficiently quenched upon the increase of the concentration of the porphyrins [129]. If porphyrins are located in the membranes of the vesicles, these processes are expected to manifest themselves especially strongly due to rather high local concentration of the porphyrins. Note that this concentration may be high enough for the quenching even when only a few molecules of a porphyrin are located in the membrane. 

The non-exponential decay of the triplet excited states of the photosensitizers is observed for Chi-, Phe- and ZnTPP-containing vesicles [132-135], The reason for the non-exponentiality may be, first, a statistical distribution of the concentrations of porphyrin molecules in the membrane, and, second, a simultaneous decay of the triplet excited states via several parallel channels such as spontaneous deactivation, concentration quenching and triplet-triplet annihilation which are known to be characteristic of porphyrins in organic solvents [129]. For ZnTPP and Phe in vesicles, the process of triplet-triplet annihilation is indeed observed [56, 134], while according to [132] this process is surprisingly absent for Chi. 

The kinetics of concentration quenching in vesicles can be described quantitatively in terms of the stochastic approach. This approach takes into account, first, the small (1-100) number of photosensitizer molecules in one vesicle and, second, the statistical character of their distribution in the vesicles. The small number of the photosensitizer molecules in a vesicle leads to a significant influence of the fluctuations of this number on the quenching kinetics. As shown in Phe-containing vesicles [134], under these conditions the stochastic approach describes the 

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