The conformation of NN'-Biphenyl-NN'-di(m-tolyl)benzidine (TPD) is studied by solid-state N-15 NMR and density functional theory (DFT) calculations. The results of the N-15 NMR for amorphous TPD agree with those of the DFT calculations, confirming that the DFT-optimized TPD single molecule reflects the structure in the condensed amorphous state. Various stable conformers are considered to exist in the amorphous state. Torsion angles change the shape of molecules and the state of the electron clouds around the nitrogens, and therefore significantly affect the intermolecular electron coupling. It suggests that the torsion angle is a crucial factor for the carrier transport properties. (C) 2004 Elsevier B.V. All rights reserved.