Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer.

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 (ΔG ∼ -0.19 eV) featured very large Stern-Volmer quenching constants (K SV) approaching or achieving 105 M-1 with bimolecular rate constants between 2 and 3 × 108 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.