Breaking Kasha's Rule as a Mechanism for Solution-Phase Room-Temperature Phosphorescence from High-Lying Triplet...

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 ΔE_S and large spin-orbital coupling ξ_S. 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 ξ_T, the efficient solution-phase ORTP emission from the T₂ state was finally achieved.