Planarity of triphenylamine moieties of a typical hole-transport material for OLEDs, N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD), in the amorphous state

N,N′-Diphenyl-N,N′-di(m-tolyl)benzidine (TPD) is a well-known hole-transport material for organic light-emitting diodes (OLEDs). Here, we studied the planarity of triphenylamine moieties of TPD in the amorphous state, by the combined use of solid-state 15N NMR experiments and density functional theory (DFT) calculations. It was shown that the planarity is of crucial importance for hole-transport performance of TPD, because of the strong effects for the shape of HOMO and for the intermolecular repulsion energy. It was also found that the molecular structure around the nitrogen in its amorphous state is not pyramidal, but is instead planar, which is favorable for hole-transport. Natural bond orbital calculation indicates that the nitrogen atom is sp2 hybridized. This is unlike most amine compounds, which have pyramidal structures with sp3 hybridized nitrogens.