Intramolecular electron-transfer rates in mixed-valence triarylamines: Measurement by variable-temperature ESR spectroscopy and comparison with optical data

The electron spin resonance spectra of the radical cations of 4,4-bis[di(4-methoxyphenyl)ami- no]tolane, E-4,4 -bis[di(4-methoxyphenyl)amino] stilbene, and E,E-1,4-bis{4-[di(4-methoxyphenyl)amino]styryl}- benzene in dichloromethane exhibit five lines over a wide temperature range due to equivalent coupling to two ¹⁴N nuclei, indicating either delocalization between both nitrogen atoms or rapid intramolecular electron transfer on the electron spin resonance time scale. In contrast, those of the radical cations of 1,4-bis{4- [di(4-methoxyphenyl)amino]phenylethynyl}benzene and E,E-1,4-bis{4-[di(4-n-butoxyphenyl)amino]styryl}- 2,5-dicyanobenzene exhibit line shapes that vary strongly with temperature, displaying five lines at room temperature and only three lines at ca. 190 K, indicative of slow electron transfer on the electron spin resonance time scale at low temperatures. The rates of intramolecular electron transfer in the latter compounds were obtained by simulation of the electron spin resonance spectra and display an Arrhenius temperature dependence. The activation barriers obtained from Arrhenius plots are significantly less than anticipated from Hush analyses of the intervalence bands when the diabatic electron-transfer distance, R, is equated to the N-N distance. Comparison of optical and electron spin resonance data suggests that R is in fact only ca. 40% of the N-N distance, while the Arrhenius prefactor indicates that the electron transfer falls in the adiabatic regime.