title: Preorganized Chromophores Facilitate Triplet Energy Migration, Annihilation and Upconverted Singlet Energy Collection.
abstract: Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has the potential to enhance significantly photovoltaic and photocatalytic efficiencies by harnessing sub-bandgap photons, but the progress of this field is held back by the chemistry problem of how to preorganize multiple chromophores for efficient UC under weak solar irradiance. Recently, the first maximization of UC quantum yield at solar irradiance was achieved using fast triplet energy migration (TEM) in metal-organic frameworks (MOFs) with ordered acceptor arrays, but at the same time, a trade-off between fast TEM and high fluorescence efficiency was also found. Here, we provide a solution for this trade-off issue by developing a new strategy, triplet energy migration, annihilation and upconverted singlet energy collection (TEM-UPCON). The porous structure of acceptor-based MOF crystals allows triplet donor molecules to be accommodated without aggregation. The surface of donor-doped MOF nanocrystals is modified with highly fluorescent energy collectors through coordination bond formation. Thanks to the higher fluorescence quantum yield of surface-bound collectors than parent MOFs, the implementation of the energy collector greatly improves the total UC quantum yield. The UC quantum yield maximization behavior at ultralow excitation intensity was retained because the TTA events take place only in the MOF acceptors. The TEM-UPCON concept may be generalized to collectors with various functions and would lead to quantitative harvesting of upconverted energy, which is difficult to achieve in common molecular diffusion-based systems.
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University , 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Prasenjit
authors.initials: P
authors.lastname: Mahato
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University , 744 Moto-oka, Nishi-ku, 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 Materials Science and Engineering, University of Illinois , Urbana, Illinois 61801, United States.
authors.forename: Melinda
authors.initials: M
authors.lastname: Sindoro
authors.affiliation: Department of Materials Science and Engineering, University of Illinois , Urbana, Illinois 61801, United States.|IBS Center for Soft and Living Matter, UNIST , Ulsan 689-798, South Korea.
authors.forename: Steve
authors.initials: S
authors.lastname: Granick
authors.affiliation: Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University , 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Nobuo
authors.initials: N
authors.lastname: Kimizuka