The impact of conformation and substitution on the geometric and electronic properties of N,N'-bis-(m-tolyl)-N,N'diphenyl- 1, 1-biphenyl-4,4'-diamine (TPD) derivatives is evaluated from the results of quantum-mechanical calculations performed at the Hartree-Fock semiempirical and density functional theory levels. Special attention is paid to the evolution of the dipole moments and ionization potentials, which are key parameters that can affect the transport properties in these materials. (C) 2002 Elsevier Science B.V. All rights reserved.