An efficient reverse intersystem crossing process exploiting non-bonding states in an inverted singlet–triplet gap system†

Reverse intersystem crossing (rISC) is an essential process in organic light-emitting diodes to populate singlet excited states from non-emissive triplet states. A small or negative singlet–triplet energy gap and a large spin–orbit coupling between low-lying singlet and triplet states are key requirements to enhance the rISC rate. Here, we present a molecular design exploiting the n–π* excited state to maximize the efficacy of the rISC process for efficient light emitters using thermodynamic and kinetic calculations validated with high-level quantum chemical methods. Heptazine-based molecules with carbonyl groups attached are shown to possess a reasonable singlet energy gap for blue-light emission with the energy level of the n–π* triplet state modulated by addition of electron withdrawing or donating groups to achieve the optimal energy level ordering of T(π–π*) > T(n–π*) > S₁, leading to enhanced spin–orbit coupling between the lowest triplet and singlet states with an inverted energy gap.