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Two chromophores derived from heptazine, HAP-3MF and HAP-3TPA, weresynthesized and tested as emitters in light-emitting diodes (OLEDs) by Adachiand coworkers. Both emitters were shown to exhibit quantum efficiencies whichexceed the theoretical maximum of conventional fluorescent OLEDs. The enhancedemission efficiency was explained by the mechanism of thermally activateddelayed fluorescence (TADF). In the present work, the electronic excitationenergies and essential features of the topography of the excited-statepotential-energy surfaces of HAP-3MF and HAP-3TPA have been investigated with awave-function based ab initio method (ADC(2)). It is found that HAP-3MF is aninverted singlet-triplet (IST) system, that is, the energies of the S1andT1states are robustly inverted in violation of Hund's multiplicity rule.Notably, HAP-3MF presumably is the first IST emitter which was implemented inan OLED device. In HAP-3TPA, on the other hand, the vertical excitationenergies of the S1 and T1 states are essentially degenerate. The excited statesexhibit vibrational stabilization energies of similar magnitude along differentrelaxation coordinates, resulting in adiabatic excitation energies which alsoare nearly degenerate. HAP-3TPA is found to be a chromophore at the borderlineof TADF and IST systems. The spectroscopic data reported by Adachi andcoworkers forHAP-3MF and HAP-3TPA are analysed in the light of thesecomputational results.

the journal of physical chemistry letters

Are Heptazine-Based Organic Light-Emitting Diode (OLED) Chromophores  Thermally Activated Delayed Fluorescence (TADF) or Inverted Singlet-Triplet  (IST) Systems?