Electron tunneling in molecular layers

The exponential dependence of the efficiency of fluorescence quenching on the distance between a donor and an acceptor may be explained by the tunneling mechanism of electron transfer from a singlet-excited molecule of the donor to the acceptor. Indeed, in case of stationary excitation of donor particles, the value of J is determined by the stationary concentration n of the excited donor particles J = An where A is a constant. The value of n is, in its turn, inversely proportional to the rate constant, k, of deactivation of excited particles nft = nJexc pjk, where Jexc is the intensity of the exciting light, j) is the quantum yield of excited molecules, and n is the concentration of non-excited donor molecules. Thus, J = AnJexc4 lk. Hence, one can easily obtain 

The quenching of the fluorescence of donor particles D and the formation of reduced acceptor particles A in photochemical reactions of the eqn. (1) type followed by a slow decay of particles A have also been observed in a number of other donor-acceptor molecular layers of a similar structure [8, 9], The data obtained have also been explained by electron tunneling from the excited donor particles to the acceptor particles over distances exceeding 20 A. 

This reference reports the appearance of a light-induced potential difference between two electrodes separated by a specially organized molecular multilayer. A barium electrode and a semitransparent aluminium electrode, which have substantially different electronic work functions JAl c/Ba, have been used in these studies. The two electrodes were separated by a multilayer system consisting of a layer of isolating molecules covered by a 

The scheme of the processes occurring in such a multilayer structure can be represented as 

The scheme of reactions (5)-(8) is similar in certain aspects to that of electron transfer processes occurring in the membranes of photosynthesiz-ing organisms, where the light absorption also induces a trans-membrane potential difference. The above scheme of processes in multilayer systems explaining the appearance of the photoelectromotive force requires further refinement. But even the data available at present allow these systems to be regarded firstly as convenient models for analyzing the processes occurring 

Enhanced photovoltage and photocurrent signals were observed by the authors of Refs. [183,184] with linked porphyrin-quinone molecules in planar bilayer lipid membranes (BLM) as compared with preparations containing the non-Iinked components. They interposed BLM between two aqueous compartments containing a secondary electron donor on one side and a secondary acceptor on the other side. The efficiency of PET increased when the P-L-Q molecules were oriented in the membrane. 

PET across a phospholipid bilayer was also shown to occur when triad molecules of the type pictured in Fig. 21a, were brought into BLM [167,185], A steady-state photocurrent across the membrane was observed in these experiments. 

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