Donor-acceptor (electronic) coupling in the precursor complex to organic electron transfer: Intermolecular and intramolecular self-exchange between phenothiazine redox centers

Intermolecular electron transfer (ET) between the free phenothiazine donor (PH) and its cation radical (PH.+) proceeds via the [1:1] precursor complex (PH)(2)(.+) which is transiently observed for the first time by its diagnostic (charge-resonance) absorption band in the near-IR region. Similar intervalence (optical) transitions are also observed in mixed-valence cation radicals with the generic representation: P(br)P.+, in which two phenothiazine redox centers are interlinked by p-phenylene, o-xylylene, and o-phenylene (br) bridges. Mulliken-Hush analysis of the intervalence (charge-resonance) bands afford reliable values of the electronic coupling element H-IV based on the separation parameters for (P/P.+) centers estimated from some X-ray structures of the intermolecular (PH)(2)(.+) and the intramolecular P(br)P.+ systems. The values of H-IV, together with the reorganization energies lambda derived from the intervalence transitions, yield activation barriers DeltaG(ET)(double dagger) and first-order rate constants k(ET) for electron-transfer based on the Marcus-Hush (two-state) formalism. Such theoretically based values of the intrinsic barrier and ET rate constants agree with the experimental activation barrier (E-a) and the self-exchange rate constant (k(SE)) independently determined by ESR line broadening measurements. This convergence validates the use of the two-state model to adequately evaluate the critical electronic coupling elements between (P/P.+) redox centers in both (a) intermolecular ET via the precursor complex and (b) intramolecular ET within bridged mixed-valence cation radicals. Important to intermolecular ET mechanism is the intervention of the strongly coupled precursor complex since it leads to electron-transfer rates of self-exchange that are 2 orders of magnitude faster (and activation barrier that is substantially lower) than otherwise predicted solely on the basis of Marcus reorganization energy.