A new sensitization strategy for achieving organic RTP in aqueous solution: tunable RTP and UC emission in supramolecular TTA-UC systems

Achieving room temperature phosphorescence (RTP) of heavy-atom-free organic molecules with near-infrared (NIR) emission in solutions is fascinating for medical and biological applications but is highly challengable. Herein, we report a new sensitization strategy to achieve RTP of heavy-atom-free fluorophores which served as energy acceptors in triplet-triplet annihilation upconversion (TTA-UC) by carefully designing the accommodated microenvironments through non-covalent interactions with a PAMAM dendrimer matrix. Sensitized RTP of a serial of diphenyl anthracene (DPA) derivatives peaked at 766 nm were observed for the first time in an aqueous solution with a maximal quantum yield of 1.4%. A ternary supramolecular assembling between the UC components and the dendrimers was formed via electrostatic interaction between the–COOH and peripheral amino groups. On one hand, it assured the efficient population of the triplet states of the fluorophore via triplet-triplet energy transfer (TTET) process; on the other hand, it restricted the motions of the fluorophores, and thus inhibited the nonradiative inactivation of the populated triplet states. These two aspects jointly contributed to the sensitized RTP of DPA units. Significantly, the microenvironment in which the annihilators resided could be regulated by adjusting the ratio of carboxyl to amino groups (–COOH/–NH 2 ), when–COOH/–NH 2 < 1, the annihilators were mainly immobilized at the periphery of the dendrimers, leading to the sensitized RTP, and when COOH/–NH 2 > 1, the UC components could be partly driven to the hydrophobic cavities of the dendrimers, resulting an unprecedentedly simultaneous emission of upconverted fluorescence and down-converted phosphorescence of the annihilators.