Aggregation-induced photon upconversion (iPUC) based on control of the triplet energy landscape is demonstrated for the first time. When a triplet state of a cyano-substituted 1,4-distyrylbenzene derivative is sensitized in solution, no upconverted emission based on triplet-triplet annihilation (TTA) was observed. In stark contrast, crystalline solids obtained by drying the solution revealed clear upconverted emission. Theoretical studies unveiled an underlying switching mechanism: the excited triplets in solution immediately decay back to the ground state through conformational twisting around a C-C bond and photoisomerization, whereas this deactivation path is effectively inhibited in the solid state. The finding of iPUC phenomena highlights the importance of controlling excited energy landscapes in condensed molecular systems. Affairs of state: Whereas a cyano-substituted 1,4-distyrylbenzene derivative shows no upconverted (UC) emission in solution, a clear upconverted emission is observed for solution-cast crystals (see picture). The difference is attributed to immediate decay of excited triplets through conformational twisting and isomerization being possible in solution but not in the solid state.
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