Tetraaryl biphenyl diamine hole transport materials: a structural study utilizing both single crystal and high resolution powder diffraction

The use of synchrotron based instruments has allowed the crystal structures of the triarylamine based hole transport materials N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine (TPD) and N,N-diphenyl-N,N-bis(4-methylphenyl)-1,1-biphenyl-4,4-diamine (TPD-4) to be determined for the first time. The structure of TPD, based on a single crystal experiment using a microcrystal, has profound implications for work on elucidating the hole transport mechanism of these materials as it is shown to contain two disordered but distinct molecular conformations. Neither conformation corresponds to previous predictions from density functional theory. Further complicating the system is the presence of a second polymorph detected in bulk TPD. The crystal structure of TPD-4, derived from high resolution powder diffraction techniques, is also presented and is discussed with reference to that of TPD.