Supramolecular Assembly-Improved Triplet-Triplet Annihilation Upconversion in Aqueous Solution

Water-soluble 9,10-diphenylanthracene-modified gamma-cyclodextrin derivatives A1 and A2, in which the gamma-cyclodextrin unit serves as a molecular host for a binding sensitizer, and the 9,10-diphenylanthracene moiety plays a role as an emitter/annihilator, were synthesized to investigate the supramolecular triplet-triplet annihilation (TTA) upconversion in aqueous solution. Both A1 and A2 readily aggregate and form nanoscale assemblies in water as a combined result of host-guest complexation and pi-pi stacking among the 9,10-diphenylanthracenes. The aggregation behavior of the supramolecular emitters was fully characterized by using a diversity of methods, including dynamic light scattering (DLS), SEM, NMR, fluorescence, and circular dichroism studies. Fluorescence spectroscopic analysis reveals that the emitters have high fluorescence quantum yields in water (82 and 90 % for A1 and A2, respectively), thus demonstrating that aggregation does not quench the fluorescence. By using a coordinated ruthenium sensitizer, a high TTA upconversion quantum yield of up to 6.9 % was observed for this supramolecular TTA system, which is significantly higher than the value (<0.5 %) obtained with nonassembled emitters in organic solvent and in contrast to the fact that TTA upconversion emission in aqueous solution is usually low or negligible. We ascribe the strong TTA upconversion emission in the present supramolecular assembly system to an efficient TTA process, which is facilitated along the stacked emitters by triplet energy migration and improved triplet-triplet energy transfer through host-guest complexation.