Phenanthro[9,10-d]imidazole having electroactive derivatives as potential host materials for third generation organic light emitting diodes

Abstract

The new bipolar phenanthro[9,10-d]imidazole based compounds were synthesized from corresponding key starting aldehydes in reactions with phenanthrene-9,10-dione, ammonium acetate and aniline. Thermo-gravimetric analysis and differential scanning calorimetry confirmed that the new derivatives are highly thermally stable compounds and can form amorphous materials having very high glass transition temperatures exceeding 150 °C in some cases, which is big a advantage for the compounds with potential application in optoelectronic devices. Electron photoemission spectra of thin layers of the materials were used to determine ionization potentials of the materials, which are found to be in the range from 5.27 eV to 5.59 eV. LUMO energy levels of the materials were determined from their optical band gaps and values of the ionization potentials, and are in the range of 1.86–2.26 eV. Positive charge drift mobility in thin layers of some of the derivatives exceed 2.8 × 10⁻⁵ cm²V⁻¹s⁻¹ at high electric field at room temperature. Some of the developed materials were preliminary tested as hosts in red organic light-emitting diodes for commercial TADF based emitter: 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP). The results indicated that device with 9-(2-ethylhexyl)-3-(1-phenylphenanthro[9,10-d]imidazol-2-yl)carbazole exhibited good performance with rather low turn-on voltage of 3.5 V, maximum brightness of about 1000 cd/m² and peak efficiency exceeding 5.6%. The properties are better than those of a similar device using the well-known commercial host material: 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP).