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

• Published in: Chemical science (Cambridge) 2021-07, Vol.12 (26), p.969-977
• Main Authors: Yang, Mo, Sheykhi, Sara, Zhang, Yu, Milsmann, Carsten, Castellano, Felix N
• Format: Article
• Language: English
• Subjects: Absorption spectra; Carbazoles; Charge transfer; Chemistry; Chromophores; Continuous radiation; Coordination compounds; Energy transfer; Excitation; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Mathematical analysis; NMR; Nuclear magnetic resonance; Photoluminescence; Quenching; Rate constants; Solar flux; Spectrum analysis; Upconversion; Zirconium
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d1sc01662h

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. The LMCT photosensitizer Zr( Mes PDP Ph ) 2 paired with DPA-based acceptors enabled low power threshold photochemical upconversion with record-setting quantum efficiencies.