Indole and phenylenediamine based enamines as amorphous hole-transporting materials

New aromatic enamines were synthesized by condensation of the commercially available N-phenyl-p-phenylenediamine, indole or 2-methylindole with 2,2-diphenylacetaldehyde or 2-phenylpropionaldehyde. Twin derivatives of the 2-methylindole-based enamine were prepared. The materials were examined by various techniques including differential scanning calorimetry, UV spectrometry, electron photoemission and time of flight techniques. The electron photoemission spectra of layers of the amorphous materials showed the ionization potentials of 5.26–5.6 eV. Hole drift mobilities in the layers range from 4.7 × 10−6 to 10−4 cm2/(V s). Some of the materials were tested as hole-transporting layers in light emitting diodes with Alq3 as the electroluminescent/electron transport layer. The device containing indole-based enamine exhibited the best overall performance with a turn-on voltage of 3 V, maximum brightness of about 3500 cd/m2 and luminance efficiency of 1.62 lm/W.