Upconverting Green Light to Blue via Solid-State Triplet-Triplet Annihilation in Hydrogen-Bonded Organic Frameworks

The conversion of low energy photons into higher energy photons via triplet-triplet annihilation upconversion (TTA-UC) has important implications for wavelength-shifting technology and is a target for increasing the efficiency of solar energy harvesting. Given that the efficiencies of solid-state TTA-UC are drastically lower than that of solution-phase TTA-UC, strategies are needed to augment the process. Herein, hydrogen-bonded organic frameworks (HOFs) comprised of zinc tetrakis(4-carboxyphenyl)porphyrin (Zn-TCPP) acting as the sensitizer and dipyridyl thiazolothiazole (TzBIPY) acting as the annihilator were synthesized and evaluated for TTA-UC. Modifying the coordination angle of the TzBIPY to the porphyrin, two structures were obtained and characterized via single crystal x-ray diffraction. Using both femtosecond laser excitation of the HOFs as colloidal dispersions in solvent and continuous wave laser excitation on single crystal samples, evidence of superlinear power-dependent emission at 460 nm from TzBIPY upon resonant excitation of Zn-TCPP suggests the occurrence of TTA-UC. This investigation illustrates the potential of HOFs for light harvesting applications and could lead to further development in photocatalysis where the HOF is used as the photosensitizer for encapsulated guests.