Conformationally Switchable Copper(I) Complexes Amplify BODIPY Triplet Photosensitization.

With its potential to surpass the Shockley-Queisser limit, triplet-triplet annihilation upconversion (TTA-UC) has been heralded as a powerful solution in direct solar-to-fuel energy conversion. TTA-UC combines low-energy photons to generate high-energy states for photocatalysis. TTA-UC requires a triplet photosensitizer (PS), commonly a precious-metal complex, whose structurally rigid MLCT state facilitates intersystem crossing (ISC). However, these metals are costly, toxic, and have low extinction coefficients relative to organic PSs, while organic PSs have limited ISC efficiencies. Taking a bioinspired approach, we combine a light-harvesting organic chromophore with a structurally switchable earth-abundant transition metal. Characterizing the ground and excited state properties of this construct, its metal-free analog, and a version incapable of conformational switching, we show that both the metal ion and its conformational switching are crucial for enhanced photostability, ISC, and energy transfer during TTA-UC. Ultimately, we increase upconversion quantum yields from 6.6% to 12.1% at low threshold power density (0.037 W/cm2), and double photocatalysis yields relative to the chromophore alone. These results unveil a new approach in PS design using conformational gating to amplify photocatalysis.