title: Loss channels in triplet-triplet annihilation photon upconversion: importance of annihilator singlet and triplet surface shapes.
abstract: Triplet-triplet annihilation photon upconversion (TTA-UC) can, through a number of energy transfer processes, efficiently combine two low frequency photons into one photon of higher frequency. TTA-UC systems consist of one absorbing species (the sensitizer) and one emitting species (the annihilator). Herein, we show that the structurally similar annihilators, 9,10-diphenylanthracene (DPA, 1), 9-(4-phenylethynyl)-10-phenylanthracene (2) and 9,10-bis(phenylethynyl)anthracene (BPEA, 3) have very different upconversion efficiencies, 15.2 ± 2.8%, 15.9 ± 1.3% and 1.6 ± 0.8%, respectively (of a maximum of 50%). We show that these results can be understood in terms of a loss channel, previously unaccounted for, originating from the difference between the BPEA singlet and triplet surface shapes. The difference between the two surfaces results in a fraction of the triplet state population having geometries not energetically capable of forming the first singlet excited state. This is supported by TD-DFT calculations of the annihilator excited state surfaces as a function of phenyl group rotation. We thereby highlight that the commonly used "spin-statistical factor" should be used with caution when explaining TTA-efficiencies. Furthermore, we show that the precious metal free zinc octaethylporphyrin (ZnOEP) can be used for efficient sensitization and that the upconversion quantum yield is maximized when sensitizer-annihilator spectral overlap is minimized (ZnOEP with 2).
authors.affiliation: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden. abmaria@chalmers.se.
authors.forename: Victor
authors.identifier: http://orcid.org/0000-0001-6583-8654
authors.initials: V
authors.lastname: Gray
authors.affiliation: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden. abmaria@chalmers.se.
authors.forename: Ambra
authors.initials: A
authors.lastname: Dreos
authors.affiliation: Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden.
authors.forename: Paul
authors.identifier: http://orcid.org/0000-0002-2516-6061
authors.initials: P
authors.lastname: Erhart
authors.affiliation: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden. abmaria@chalmers.se.
authors.forename: Bo
authors.identifier: http://orcid.org/0000-0002-5991-7863
authors.initials: B
authors.lastname: Albinsson
authors.affiliation: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden. abmaria@chalmers.se.
authors.forename: Kasper
authors.identifier: http://orcid.org/0000-0003-4018-4927
authors.initials: K
authors.lastname: Moth-Poulsen
authors.affiliation: Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Gothenburg, Sweden. abmaria@chalmers.se.
authors.forename: Maria
authors.identifier: http://orcid.org/0000-0002-6931-1128
authors.initials: M
authors.lastname: Abrahamsson