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, , and the remaining 75% are in a triplet state, . In thermally activated delayed fluorescence (TADF) chromophores the transition from the nonradiative state to the radiative state can be thermally activated, which improves the efficiency of OLEDs. Chromophores with inverted energy ordering of and states, < , are superior to TADF chromophores, thanks to the absence of an energy barrier for the transition from to . 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 and 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.