Neutral and charged excited states in polar organic films:: Origin of unusual electroluminescence in tri-p-tolylamine-based hole conductors

The photoluminescence (PL) and electroluminescence (EL) of thin films of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) are remarkably different. Similar PL and EL are instead observed in films of the closely related donors tri-p-tolylamine (TTA) and N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD). Such films show a wide range of hole transport that depends on the morphology and on external parameters such as temperature and electric field. Restricted configuration-interaction calculations performed on TTA, TAPC, TPD, and radical ions of TTA indicate that the unusual EL of TAPC films is due to direct recombination from a low-lying charge-transfer (CT) state. The CT state is strongly stabilized by electrostatic interactions with the polar environment. Theory confirms that TAPC can be considered a dimer of TTA. The charge distributions of TTA(+) and TTA(-) indicate charge localization in the anion that rationalizes low electron mobility as well as a strong charge- induced-dipole stabilization of the CT state.