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The modulus of coacervates made from charged (bio)polymers and small molecules jumps when valency increases from three to four. 

The current investigation demonstrates highly efficient photochemical upconversion (UC) where a long-lived Zr( iv ) ligand-to-metal charge transfer (LMCT) complex serves as a triplet photosensitizer in concert with well-established 9,10-diphenylanthracene (DPA) along with newly conceived DPA–carbazole based acceptors/annihilators in THF solutions. The initial dynamic triplet–triplet energy transfer (TTET) processes (Δ G ∼ −0.19 eV) featured very large Stern–Volmer quenching constants ( K SV ) approaching or achieving 10 5 M −1 with bimolecular rate constants between 2 and 3 × 10 8 M −1 s −1 as ascertained using static and transient spectroscopic techniques. Both the TTET and subsequent triplet–triplet annihilation (TTA) processes were verified and throughly investigated using transient absorption spectroscopy. The Stern–Volmer metrics support 95% quenching of the Zr( iv ) photosensitizer using modest concentrations (0.25 mM) of the various acceptor/annihilators, where no aggregation took place between any of the chromophores in THF. Each of the upconverting formulations operated with continuous-wave linear incident power dependence ( λ ex = 514.5 nm) down to ultralow excitation power densities under optimized experimental conditions. Impressive record-setting η UC values ranging from 31.7% to 42.7% were achieved under excitation conditions (13 mW cm −2 ) below that of solar flux integrated across the Zr( iv ) photosensitizer's absorption band (26.7 mW cm −2 ). This study illustrates the importance of supporting the continued development and discovery of molecular-based triplet photosensitizers based on earth-abundant metals.