Intramolecular electron transfer (ET) within the class II/III transition for the mixed-valence state of N,N'-diphenyl-1,4-phenylenediamine (PDA) derivatives which were substituted in the center or the outer phenyl ring and N,N'-diphenylbenzidine (BZ) was examined. Each compound showed two reversible redox couples. The splitting of the redox waves, DeltaE, is related to the interaction intensity between redox sites. The introduction of a substitutent into the central phenylene ring of the PDAs resulted in a decrease in DeltaE. A similar result was noted for the expansion of the distance between the redox centers such as in BZ. In opposition, the DeltaE of N,N'-bis(2,6-dimethylphenyl)-1,4-phenylenediamine (2,6-DMPDA) as a compound with substituents introduced into the outer phenyl rings was spread. The mixed-valence state of these compounds also exhibited an intervalence charge transfer (IV-CT) band in the near-IR region which provided the determination of the Marcus reorganization energy (lambda), the electron coupling (V), the thermal ET barrier (DeltaG*), and the electron-transfer rate (k(th)) using the Marcus-Hush theory. We first confirmed the electron-transfer rate of PDA derivatives in the class II/III transition state by two methods. The v(N-H) stretching vibrational spectra of the mixed-valence states were analyzed by a Bloch-type equation analysis using variable-temperature IR spectra measurements which were to be in good agreement with the those obtained from the Marcus-Hush theory. On the basis of this approach, the electron-transfer rate of PDA was determined to be 8.2 x 10(11) s(-1) at 298 K, yielding DeltaG(double dagger) = 420 cm(-1) (the activation free energy from the Eyring plot) for the underlying process.