Inverted Lowest Singlet and Triplet Excitation Energy Ordering of Graphitic Carbon Nitride Flakes

In organic light-emitting diodes (OLEDs), only 25% of electrically generated excitons are in a singlet state, S₁, and the remaining 75% are in a triplet state, T₁. In thermally activated delayed fluorescence (TADF) chromophores the transition from the nonradiative T₁ state to the radiative S₁ state can be thermally activated, which improves the efficiency of OLEDs. Chromophores with inverted energy ordering of S₁ and T₁ states, S₁ < T₁, are superior to TADF chromophores, thanks to the absence of an energy barrier for the transition from T₁ to S₁. We benchmark the performance of time-dependent density functional theory using different exchange-correlation functionals and find that scaled long-range corrected double-hybrid functionals correctly predict the inverted singlet-triplet gaps of N-substituted phenalene derivatives. We then show that the inverted energy ordering of S₁ and T₁ is an intrinsic property of graphitic carbon nitride flakes. A design strategy of new chromophores with inverted singlet-triplet gaps is proposed. The color of emitted light can be fine-tuned through flake size and amine substitution on flake vertices.