Zinc Phthalocyanine as a Promising Photosensitizer for Triplet-Triplet Annihilation Upconversion^†

Zinc phthalocyanines (ZnPc) are widely recognized as efficient triplet photosensitizers in photodynamic therapy and photocatalysis, owing to their intense absorption in the visible range and long triplet-state lifetimes. However, their application in triplet-triplet annihilation (TTA) upconversion is lacking to date. In this study, we synthesized a new ZnPc photosensitizer, 4I-ZnPc, and composed a TTA upconversion system using rubrene as the energy acceptor. Upon photoexcitation at 663 nm, yellow fluorescence from rubrene was observed in deoxygenated dichloromethane, demonstrating TTA upconversion with an anti-Stokes shift of 0.331 eV and a quantum yield of 1.82% (out of the 50% maximum). Using nanosecond transient absorption spectroscopy, we determined the triplet lifetime of 4I-ZnPc, the triplet-triplet energy transfer efficiency, and the fluorescence quantum yield. These measurements provide critical insights into the photophysical processes governing the TTA upconversion system. Our results highlight the potential advantages and limitations of ZnPc as a triplet photosensitizer for TTA upconversion.