Diffusion-controlled rates of formation were found from the temperature dependence of apparent quenching rate constants for exciplexes, when the driving force of excited-state electron transfer -0.1 < deltaG(ET)* < +0.1 eV. This is inconsistent with the conventional mechanism of electron-transfer reactions, involving preliminary reorganisation of the medium and reactants, and provides strong support for the mechanism of medium reorganisation stimulated by strong electronic coupling of locally excited and charge-transfer states.

For the first time, the Holocene history of the Serp-i-Molot tidewater glacier at the eastern side of the Novaya Zemlya archipelago, Kara Sea, has been reconstructed based on AMS 14 C dating and multiproxy analyses (lithology, mineralogy, geochemistry and microfossils) of glaciomarine sediments from the Tsivol'ki Fjord. Three main phases of the Holocene evolution of the fjord environment are identified. During the early phase, c. 10.6 to 4.6 cal. ka BP, the glacier front was located close to the fjord mouth. The middle phase, c. 4.6 to 0.9 cal. ka BP, was characterized by frontal glacier retreat and shallowing of the fjord owing to glacio-isostatic uplift of the North Island of Novaya Zemlya. The late phase, from 0.9AE0.1 cal. ka BP until present, reflects the stabilization of the glacier front in the inner fjord. The absence of major Neoglacial advances of the Serp-i-Molot glacier is explained by the local climate of the Kara Sea and Novaya Zemlya, which prevents the penetration of moist air flow from the North Atlantic.

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The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.

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The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G * et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured

1 More and more data to suggest that many types of electron transfer processes, in particular photoinduced electron transfer between organic molecules in the kinetic region (at a free energy of excited-state electron transfer of ∆ > -0.4 eV), occur via the mechanism of intermediate formation of exciplexes [1][2][3][4][5][6], rather than the generally accepted mechanism of the preceding thermally activated solvent reorganization [7][8][9][10], are appearing in the literature. According to the mechanism of intermediate formation of exciplexes, the apparent rate constant of quenching of excited molecules is equal to k Q = 1/[1/ k 1 + exp ( ∆ / RT )] , where k 1 is the rate constant of exciplex formation, which is close to the diffusion rate constant; = 1/( + + + ) is the exciplex lifetime; ∆ is the Gibbs energy of exciplex formation; and , ,, and are the rate constants of exciplex emission, internal conversion and intersystem crossing of exciplexes, and exciplex dissociation into radical ions, respectively. Competition between different processes of exciplex decay determines not only the quantum yields of primary products of electron transfer but also the magnitude of the rate constants of quenching of excited molecules. Internal conversion and intersystem crossing, which involve partial back charge transfer and lead to either the initial singlet ground state or the triplet state of reactants, are the major processes of the exciplex decay. The back electron transfer in the radical ion pairs has been extensively investigated by many researchers [11][12][13][14][15][16][17][18][19][20][21], especially in view of the fact that it is for these exciplexes that a decrease in the rate of electron transfer with an increase in the reaction exoergicity (free energy of back electron transfer ∆ G BET = electron transfer) predicted by the Marcus theory (inverted region) [11][12][13][14][15][16][17][18][19][20][21] was observed.In order to reveal most important factors governing the rate of internal conversion and intersystem crossing in exciplexes, we will consider two alternative mechanisms, the thermally activated electron transfer and radiationless quantum transition. An analysis of experimental data shows that it is more sound to consider both internal conversion and intersystem crossing (back electron transfer) in exciplexes as radiationless electronic-vibrational diabatic quantum transitions rather than thermally activated electron transfer.In the previous works [22,23], we measured the quantum yields of fluorescence ( ) of exciplexes of 9-cyanophenanthrene (CP) with 1,2,3-and 1,3,5-trimethoxybenzenes (123TMB and 135TMB), the quantum yields of formation from these of triplet states ( ) and of radical ions ( ), and the rate constants of internal conversion and intersystem crossing of these exciplexes in solvents of different polarities. The experimental methods and the procedures of the determination of the quantum yields of formation of triplets, radical ions, and internal conversion were described elsewhere [22,23]. The d...