Abstract
Sensitized triplet-triplet annihilation photon upconversion (sTTA-UC) allows blue-shifting non-coherent low-intensity light and is potentially useful in solar-powered devices, bioimaging, 3D printing, and other applications. For technologically viable solar energy harvesting systems, solid materials that capture a large fraction of the solar spectrum and efficiently upconvert the absorbed energy must be developed. Here, it is shown that broadband-to-blue UC is possible in air-tolerant, easy-to-access, nanostructured polymers comprising a rigid hydrophilic matrix and liquid nanodroplets with dimensions on the order of tens of nanometers. The droplets contain 9,10-bis[(triisopropylsilyl)ethynyl] anthracene (TIPS-Ac) as emitter/annihilator and palladium(II) octaethyl porphyrin (PdOEP) and palladium(II) meso-tetraphenyl tetrabenzoporphine (PdTPBP) as sensitizers. The confinement of the three dyes in the liquid domains renders the various bimolecular energy transfer processes that are pivotal for the TIPS-Ac's triplet sensitization highly efficient, and the simultaneous use of multiple light harvesters with triplet energy levels resonant with the emitter/annihilator increases the absorption bandwidth to ca. 150 nm. The UC process at low power densities is most efficient when both sensitizers are simultaneously excited, thanks to their confinement in the nanodroplets, which leads to an increase in the triplet density, and therefore TTA rate and yield, optimizing the use of the harvested energy.