Internal reorganization energy in return electron transfer within geminate radical ion pairs

The internal reorganization energies lambda(v) for return electron transfer (ET) reactions within geminate radical ion pairs were studied using the extended Nelsen method. In the ET systems studied, the common acceptor was 9,10-dicyanoanthracene (DCA). The donors were methylsubstituted compounds of benzene, biphenyl, naphthalene and phenanthrene. The calculated results indicated that the lambda(v) values were associated mainly with the carbon atoms of the aromatic rings and the atoms linked directly to the aromatic rings. Systems with similar substituted conditions are expected to have similar internal reorganization energies. For systems in which the two aromatic rings of the donor can rotate relative to each other, the calculated lambda(v) values include a contribution from the change in torsional angle in the ET process. Compared with the system in which the donor is a fluorene molecule, the contributions of the torsional angles (low-frequency vibration) to lambda(v) were estimated. (C) 1997 Elsevier Science S.A.