Charge-transfer mechanism, contribution

The charge transfer mechanism contributes to the triplet quenching process for some cyanine dyes [18], whose scheme is presented below ... 

Bands in this region are ascribed to vCC and vCN aromatic vibrations. The observed relative intensity change observed in the SERS was ascribed (1) to the specific orientation that the molecules adopt on the surface, which is probably plane parallel to the surface, or (2) to a resonant effect which is a consequence of the charge-transfer mechanism contributing to the overall SERS effect. A general trend to equalise the energy of these bands by surface effect in the series of the complexes is also observed. ... 

The Kl and K4 triplet states were produced in isopropanol by triplet-triplet energy transfer from 1,2-benzanthracene. As the result, we obtained estimated values of kq for the triplet states of Kl and K4 in isopropanol, which were 1.5 x 10 and 3 x lO 1 mol" s , respectively. For K4, the kq value is typical for quenching by the mechanism of acceleration of intersystem crossing to the ground state (2) [17]. The higher value of the constant for Kl permits assumption of contribution of the charge transfer mechanism (3) to quenching of its triplet state. 

The relatively high kq values for K2 and the observed dependence of kq on the dielectric permittivity (polarity) of the solvent (Table 2) show that in polar solutions, the charge transfer mechanism (3) becomes crucial in the quenching process. This is probably explained by the redox properties of the dye favorable for the charge transfer mechanism. Correlation of kq with the solvent viscosity is not observed, which indicates the absence of contribution of diffusion of reactants to the quenching process (the quenching occurs in the kinetic regime). 

A list of second-order contributions and corrections in the ab initio calculation of J is available. The spin polarization mechanism of exchange is different from the charge transfer mechanisms discussed so far and is perhaps best illustrated by the exchange between metals ions bridged by the azide ligand (Scheme 3). 

There is very limited information on polymer degradation by charge-transfer mechanisms. However, it has been reported that crosslinking of poly(2-phenylbutadiene) is induced by photoexcitation of the polymeric charge-transfer complex with tetracyanobenzene in polar solvents, and that ionic species such as the radical cation of the polymer contribute to the primary processes of the crosslinking [1159]. 

In a recent paper. Mo and Gao [5] used a sophisticated computational method [block-localized wave function energy decomposition (BLW-ED)] to decompose the total interaction energy between two prototypical ionic systems, acetate and meth-ylammonium ions, and water into permanent electrostatic (including Pauli exclusion), electronic polarization and charge-transfer contributions. Furthermore, the use of quantum mechanics also enabled them to account for the charge flow between the species involved in the interaction. Their calculations (Table 12.2) demonstrated that the permanent electrostatic interaction energy dominates solute-solvent interactions, as expected in the presence of ion species (76.1 and 84.6% for acetate and methylammonium ions, respectively) and showed the active involvement of solvent molecules in the interaction, even with a small but evident flow of electrons (Eig. 12.3). Evidently, by changing the solvent, different results could be obtained. 

Electron and charge transfer reactions play an important role in many chemical and biochemical processes. Dynamic solvation effects, among other factors, can largely contribute to determine the reaction rate of these processes and can be studied either by quantum mechanical or simulation methods. 

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