Spin-Forbidden Ru Sensitizers Enable 1 μm Excitation for Solid-State Triplet-Triplet Annihilation Photon Upconversion

Solid-state near-infrared (NIR)-to-visible triplet-triplet annihilation upconversion (TTA-UC) at the 1 μm edge is attractive for deep-tissue photonics and NIR energy harvesting but remains limited by sensitizer losses and restricted triplet transport in condensed media. Here we demonstrate porous poly(vinyl alcohol) (PVA)/rubrene films sensitized by spin-forbidden Ru complexes (DX1m-DX3m) with appreciable NIR absorption. Photon-flux-normalized action spectra show sensitizer-dependent red-edge response across the series, and DX3m affords quantifiable upconversion under 1000 nm femtosecond and 980 nm continuous-wave excitation, with detectable spectra to 1030 nm. Because spin-forbidden Ru sensitizers offer molecular tunability yet face threshold limitations from red-edge absorption and short triplet lifetimes, we examined what governs the operating thresholds in porous films. Transient kinetics indicate that excitation range is sensitizer-controlled, whereas thresholds are governed by triplet survival and encounter kinetics in porous domains rather than sensitizer-to-annihilator triplet-triplet energy transfer alone. These results establish a Ru-based route to 1-μm-class solid-state TTA-UC in polymer films.