title: Fast and long-range triplet exciton diffusion in metal-organic frameworks for photon upconversion at ultralow excitation power.
abstract: The conversion of low-energy light into photons of higher energy based on sensitized triplet-triplet annihilation upconversion (TTA-UC) has emerged as a promising wavelength-shifting methodology because it permits UC at excitation powers as low as the solar irradiance. However, its application has been significantly hampered by the slow diffusion of excited molecules in solid matrices. Here, we introduce metal-organic frameworks (MOFs) that promote TTA-UC by taking advantage of triplet exciton migration among fluorophores that are regularly aligned with spatially controlled chromophore orientations. We synthesized anthracene-containing MOFs with different molecular orientations, and the analysis of TTA-UC emission kinetics unveiled a high triplet diffusion rate with a micrometre-scale diffusion length. Surface modification of MOF nanocrystals with donor molecules and their encapsulation in glassy poly(methyl methacrylate) (PMMA) allowed the construction of molecular-diffusion-free solid-state upconverters, which lead to an unprecedented maximization of overall UC quantum yield at excitation powers comparable to or well below the solar irradiance.
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, Fukuoka-819-0395, Japan.
authors.forename: Prasenjit
authors.initials: P
authors.lastname: Mahato
authors.affiliation: Dipartimento di Scienza dei Materiali, Università degli Studi Milano-Bicocca, Via Roberto Cozzi 55 20135 Milano, Italy.
authors.forename: Angelo
authors.initials: A
authors.lastname: Monguzzi
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, Fukuoka-819-0395, Japan.|PRESTO, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Yanai
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, Fukuoka-819-0395, Japan.|PRESTO, JST, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Teppei
authors.initials: T
authors.lastname: Yamada
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, Fukuoka-819-0395, Japan.
authors.forename: Nobuo
authors.initials: N
authors.lastname: Kimizuka