Temperature Effects on Electron Transfer within Intervalence Bis(Hydrazine) Radical Cations

Abstract

Analyses of the shape of intervalence charge-transfer bands at various temperatures (255−325 K in most cases) in acetonitrile are reported for the radical cations of bis(2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl) hydrazines that are bridged by 2,5-xylene-1,4-diyl (2^+), durene-1,4-diyl (3^+), naphthalene-1,4-diyl (4^+), biphenyl-4,4‘-diyl (5^+), and 9,9-dimethylfluorene-2,7-diyl (6^+) aromatic rings. Electron-transfer (ET) rate constants measured by ESR as a function of temperature are reported for 4^+−6^+. Despite the fact that the ET barriers for these compounds are dominated by vibrational reorganization, explicit inclusion of vibronic coupling effects is not necessary for the prediction of their ET rate constants from the optical spectra. Rate constants in excellent agreement with the measured ones are predicted by a classical analysis of charge-transfer band shape, if the diabatic surfaces are changed from the usual assumption that they are parabolas to ones that fit the shape of the charge-transfer bands.