Breaking Kasha’s Rule as a Mechanism for Solution-Phase Room-Temperature Phosphorescence from High-Lying Triplet Excited State

Organic room-temperature phosphorescence (ORTP) has been demonstrated successfully in solids. In contrast, solution-phase ORTP is rarely achieved, because the T1 → S0 phosphorescence is too slow to compete against nonradiative decay and the oxygen-quenching effect. Here, we reported that suppression of Kasha's rule is a strategy to achieve solution-phase ORTP from the high-lying T2 state by spatially separating T2 and T1 on different parts of the molecule (CzCbDBT) composed of carbonyl (Cb), dibenzothiophene (DBT), and carbazole moiety (Cz). On one hand, intersystem crossing (ISC) is much faster from S1 to T2 than that to T1, owing to the small energy-gap ΔES1–T2 and large spin–orbital coupling ξS1–T2. On the other hand, T2 → T1 internal conversion is inhibited owing to spatial separation, i.e., T2 on CbDBT and T1 on Cz, respectively. Also, combination of very fast radiative decay from T2 to S0 owing to large ξT2–S0, the efficient solution-phase ORTP emission from the T2 state was finally achieved.