Electron–vibration interactions in triphenylamine cation: Why are triphenylamine-based molecules good hole-transport materials?

To investigate the hole-transport properties of triphenylamine (TPA) and its derivatives, such as TPD, the relative order of vibronic coupling constants (VCCs) for TPA cation is analyzed using vibronic coupling density analysis. Vibronic coupling density analysis reveals that strong localization of electron-density difference in N atoms reduces reorganization energies for TPA and TPD cations. For TPD, conformational change in not only central biphenyl unit but also tolyl and phenyl groups contribute to the reorganization energy. Introducing a heteroatom into a carbon backbone is expected to reduce VCCs and reorganization energies, and hence improve hole mobility.