Theoretical investigations of triphenylamine derivatives as hole transporting materials in OLEDs: Correlation of the Hammett parameter of the substituent to ionization potential, and reorganization energy level

Recently, triphenylamine (TPA) and its derivatives become widely used in the organic light emitting diode (OLED) devices as a hole transporting material (HTM). The calculated ionization potential (Ip) and reorganization energy for the hole transport (λ+) for a series of triphenylamine (TPA) derivatives are related to their Hammett parameters (σ and σ+) theoretically. In this study, the semiempirical AM1 method has been used to optimize thirty-three TPA derivatives. The electronic structures of these compounds in the neutral and the radical-cation states were obtained based on the optimized geometrical structure. The Ip and λ+ values were generated by means of the calculated heat of formation (or total energy) for both the neutral and the radical-cation states. In particular, the calculated Ip’s for these derivatives were consistent with experimental data. The substitution effect for the mono-substituted TPA derivatives is displayed in that the TPA derivatives with the electron-donating substituent have lower Ip and those with electron-withdrawing substituent have higher Ip, comparing to that of non-substituted TPA derivative. The substituent effect in di- and tri-substituted TPA derivatives is more pronounced than that of the mono-substituted TPA derivatives. According to the calculated results, the calculated Ip shows an excellent agreement with the experimental oxidation potentials (Ep/2) of these TPA derivatives. Furthermore, these calculated results can be employed to predict the electro-luminescence properties for new and improved HTM.