Substituent effects on the electrochemical and spectral characteristics of N,N,N′,N′-tetraaryl-p-phenylenediamine derivatives

Mixed-valence compounds of N,N,N′,N′-tetraaryl-p-phenylenediamine derivatives (PDs) with various electron-donating and -withdrawing substituents have been synthesized and their electrochemical and spectral characteristics have been investigated. According to the molecular symmetry, PDs are grouped as two series. The two series of PDs follow Hammett relationships both in the first and the second half-wave oxidation potentials. The isomeric cation radical/dication redox couples occur at the same potentials and the cation radicals exhibit similar absorption maxima and band shapes in the NIR regions, implying the highly delocalized electronic structures in the singly oxidized states. The stability of the oxidation products was monitored with UV/vis/NIR spectroelectrochemical investigations. PDs with electron-withdrawing substituents are stable after one electron oxidation, while PDs with electron-donating substituents are stable after one and two electron oxidation.