Invoking ultralong room temperature phosphorescence of purely organic compounds through H-aggregation engineering

Molecular aggregation plays an important role in the luminescence processes of organic π-conjugated materials. However, the exact relations between aggregated structures and organic emission behaviors are still unclear. Here, we designed a series of CN-substituted phenylcarbazole isomers, which show varied aggregated structures and extraordinary organic ultralong room-temperature phosphorescence (OURTP) with lifetime up to 0.92 s. Systematic experimental and theoretical studies reveal that the OURTP properties are closely related to the splitting energy (Δϵ) of H-aggregation. Specifically, the Δϵ-controlled thermally activated reversed phase transformation from the low-lying dark state to the high-lying transition-allowed emission state of H-aggregation was identified, for the first time, as the key process in OURTP: larger Δϵ leads to longer lifetime, while smaller Δϵ results in short lifetime but higher OURTP efficiency. These findings, which refer to the fundamental luminescence mechanism in aggregated states, should shed new light on the understanding of the emission behaviors of organic solids.