Recently, organic diamine compounds have been widely used as hole-transporting materials. In this work, DFT B3LYP method with the 6-31G* basis set was performed to investigate the influence of molecular conformation on the reorganization energy of a series of tetra(aryl)benzidine-based hole-transport materials. The results indicate that there are two types (i.e., ISB and BD/TPD) of geometric differences of the organic diamines with the relaxation processes. The reorganization energy of the ISB type is lower than that of the BD/TPD type. For the ISB type, the terminal phenyl moiety of the molecular framework plays an important role in determining the Marcus-type reorganization energy and the central biphenyl moiety does not. A methyl group attached to a terminal phenyl can be used to tune the reorganization energy. According to the statistical analysis, four geometric parameters could affect the reorganization energy of the BD/TPD type. The conformation of either the central biphenyl or the terminal phenyl moiety of the BD/TPD type determines the Marcus-type reorganization energy associated with the charge transport process at the molecular level. Presumably, this calculation can be employed to predict the electroluminescence (EL) character of the other organic diamines and to improve the design of new hole-transporting materials in organic light-emitting devices (OLEDs).
Journal of Physical Organic Chemistry 20(10), pp.743-753