Geometric and electronic structures of a hole-transport material, TPD, and related materials studied by DFT calculations and solid-state NMR

The geometric and electronic structures of organic materials in OLEDs are the underlying basis of the devices. However, the materials are in the amorphous states and the detailed analysis has been difficult so far. In this study, we attempt the detailed analysis of a hole-transport material, N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD), by solid-state NMR measurements. The chemical shifts are found to depend significantly on the molecular structures. The chemical shifts also reflect the electronic states of TPD. With the help of DFT calculations, the geometric and electronic structures of TPD are analyzed. The DFT calculations are also carried out for TPD and the related materials, replacing N atoms of N,N,N',N'-tetraphenylbenzidine to P or B. Not only the calculations of reorganization energies for single molecules but also those of intermolecular interactions between adjacent two molecules provide us information on the performances of carrier transports for respective materials.