Donor−Acceptor (Electronic) Coupling in the Precursor Complex to Organic Electron Transfer:  Intermolecular and Intramolecular Self-Exchange between Phenothiazine Redox Centers

• Published in: Journal of the American Chemical Society 2004-02, Vol.126 (5), p.1388-1401
• Main Authors: Sun, Duoli, Rosokha, Sergiy V, Kochi, Jay K
• Format: Article
• Language: English
• Subjects: Chemistry; Condensed matter: structure, mechanical and thermal properties; Crystal binding; cohesive energy; Crystalline state (including molecular motions in solids); Exact sciences and technology; General and physical chemistry; Heterocyclic compounds; Organic compounds; Physics; Solution properties; Solutions; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja038746v

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 λ derived from the intervalence transitions, yield activation barriers ΔG ET ⧧ 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.