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 HIV 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 HIV, together with the reorganization energies λ derived from the intervalence transitions, yield activation
barriers ΔGET⧧ and first-order rate constants kET 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 (Ea) and the self-exchange rate constant (kSE) 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.