Carbazole-Substituted Heptazines With Hund Order of Excited Singlet/Triplet States for OLEDs

Monomeric heptazines have recently attracted significant attention for applications in photocatalysis, optics, and organic electronics. Many heptazine derivatives are known to have high triplet energies that are close or even above their singlet energy (anti-Hund) that can be useful for applications in high efficiency organic light-emitting diodes (OLEDs). We report a convenient synthetic route to asymmetric bis-diarylamino carbazole-substituted heptazines obtained in high yields via sequential nucleophilic substitutions of trichloroheptazine. These materials were thoroughly characterized by NMR spectroscopy and single-crystal X-ray crystallography. Furthermore, their photophysical properties were systematically examined to determine the energies of excited singlet/triplet states. Photophysical measurements and high-level quantum chemical calculations demonstrate that bis-diarylamino carbazole-substituted heptazines are triplet wide bandgap materials that obey Hund's rule with T₁ lying lower in energy than S₁. Detailed analysis of the electronic structure of the excited states reveals that their nature differs significantly from that of anti-Hund heptazines with substantial localization/contribution of charge transfer character to carbazole and diarylamine moieties, which is responsible for Hund order of excited singlet/triplet states. Finally, application of bis-diarylamino carbazole-substituted heptazine 1 as wide bandgap host material for OLEDs does not interfere with triplet dynamics of emissive phosphor consistently with Hund order of excited singlet/triplet states.