A correlation between electrochemical properties and geometrical structure of some triarylamines used as hole transporting materials in organic electroluminescent devices

Two new compounds with four tertiary arylamine moieties connected in a fully para-conjugated system have been synthesised in order to obtain new molecules having low ionisation potentials, as required for hole transporting materials in organic light emitting diodes (OLEDs). Their electrochemical properties have been measured and compared to seven different commercial triarylamines tested in the same experimental conditions. Using the AM I geometries and the statistical average of orbital potential method (SAOP), the redox potentials of the molecules have been estimated and found to be in good agreement with the experimental data. An evident correlation between the molecular geometry and the electrochemical potential of the first oxidation exists and shows that, for equal number of para-conjugated triarylamine moieties, the starburst configuration is more efficient than the linear one in lowering the oxidation potential and that the amine moieties of the inner sphere play a more important role than those of the outer sphere. Besides, amine moieties connected by a biphenyl bridge show generally higher ionisation potentials than those connected via one phenylene.