Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer |
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Authors: | Mo Yang Sara Sheykhi Yu Zhang Carsten Milsmann Felix N Castellano |
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Institution: | Department of Chemistry, North Carolina State University, Raleigh North Carolina 27695-8204 USA.; C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown West Virginia 26506 USA, |
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Abstract: | 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 (KSV) 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 (λex = 514.5 nm) down to ultralow excitation power densities under optimized experimental conditions. Impressive record-setting ηUC 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.The LMCT photosensitizer Zr(MesPDPPh)2 paired with DPA-based acceptors enabled low power threshold photochemical upconversion with record-setting quantum efficiencies. |
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