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 T-1 -> S-0 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 T-2 state by spatially separating T-2 and T-1 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 S-1 to T-2 than that to T-1, owing to the small energy-gap Delta ES1-T2 and large spin-orbital coupling xi(S1-T2). On the other hand, T-2 -> T-1 internal conversion is inhibited owing to spatial separation, i.e., T-2 on CbDBT and T-1 on Cz, respectively. Also, combination of very fast radiative decay from T-2 to S-0 owing to large xi(T2-S0), the efficient solution-phase ORTP emission from the T-2 state was finally achieved.