title: Triplet-Triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design.
abstract: Efficient visible-to-UV photon upconversion via triplet-triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
authors.forename: Xinpeng
authors.initials: X
authors.lastname: Jiang
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
authors.forename: Xinyan
authors.initials: X
authors.lastname: Guo
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
authors.forename: Jiang
authors.initials: J
authors.lastname: Peng
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
authors.forename: Dahui
authors.initials: D
authors.lastname: Zhao
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of the Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
authors.forename: Yuguo
authors.initials: Y
authors.lastname: Ma
keywords: iridium complex; photon upconversion; solid matrix; triplet sensitizer; triplet−triplet annihilation