Charge-separation model solvent dependence

Table 9.4 Solvent dependent driving forces for charge separation (CS) out of the porphyrin singlet excited state and charge recombination (CR) to the ground state/porphyrin triplet excited state calculated after the dielectric continuum model (dielectric constant e toluene 2.4 THF 7.6 oDCB 9.8, benzonitrile 24.9). The case, where charge recombination to the porphyrin triplet state is prohibited, is assigned as n.p. ...

Figure 8 Dependence of the solvent reorganization energy in the neutral (1, twoi) and charge-separated (2, moi) states on the polarizability of the final state aoi- The solvent response coefficients are estimated from the continuum dielectric model (Eq. [95]). Solute and solvent parameters are moi = 0, woi = 15 D, aoi = 20 A, Rq = 4 A, oo = 2, s = 30. In this and subsequent figures, some of the axes are labeled as the ratios shown in order to make the quantities dimensionless. For example, the ordinate in this plot is in units of electron volts, and the abscissa is in units of cubic angstroms.

Electrochemically, C70 behaves very similarly to C o- Six reduction waves are observed in toluene/acetonitrile, but unlike 50, all six waves can be detected at room temperature (see Fig. 3) [7]. Reduction potentials for C70 obtained under various conditions of solvent and temperature are presented in Table 3. In comparing the corresponding values shown in Tables 1 and 3 for the first and second reduction potentials of Cgo and C70 in acetonitrile/ toluene, one observes that they are nearly identical. However, from the trianion up to and including the hexanion, C70 becomes increasingly easier to reduce than Cgo- A charge separation delocalization model has been evoked to explain this phenomenon [10c]. A noteworthy observation is the fact that the reduction potentials of C70 also appear to be solvent and/or temperature dependent, although no specific studies on the subject have been published. 

The dyads and triads of ligand (66) were used to study charge-separated species as models for the photosynthetic reaction center. It was observed that charge separation depends on the thermodynamic parameters as well as the solvent system. The Zn/Ir/Au triad was shown to yield a fully charge separated species in toluene when excited in the visible region with a lifetime of 450 ns. 

Kosower and co-workers have found the photoinduced, barrierless charge separation processes of substituted polyaromatics to be controlled by solvent relaxation behavior over a large temperature range in alcohol solvents. Heitele and Michel-Beyerle reported on the complex solvent- and temperature-dependent electron transfer fluorescence quenching in some covalently linked aromatic donor-acceptor compounds in viscous solvents. These authors have attempted a critical comparison between current theoretical models and their experimental results, and the limitations of current theoretical models are discussed. 

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