Donor–Acceptor–Collector Ternary Crystalline Films for Efficient Solid-State Photon Upconversion

Abstract

It is pivotal to achieve efficient triplet-triplet annihilation based photon upconversion (TTA-UC) in the solid-state for enhancing potentials of renewable energy production devices. However, the UC efficiency of solid materials is largely limited by low fluorescence quantum yields that originate from the aggregation of TTA-UC chromophores and also by severe back energy transfer from the acceptor singlet state to the singlet state of the triplet donor in the condensed state. In this work, to overcome these issues, we introduce a highly fluorescent singlet energy collector as the third component of donor-doped acceptor crystalline films, in which dual energy migration, i.e., triplet energy migration for TTA-UC and succeeding singlet energy migration for transferring energy to a collector, takes place. To demonstrate this scheme, a highly fluorescent singlet energy collector was added as the third component of donor-doped acceptor crystalline films. An anthracene-based acceptor containing alkyl chains and a carboxylic moiety is mixed with the triplet donor Pt(II) octaethylporphyrin (PtOEP) and the energy collector 2,5,8,11-tetra- tert-butylperylene (TTBP) in solution, and simple spin-coating of the mixed solution gives acceptor films of nanofibrous crystals homogeneously doped with PtOEP and TTBP. Interestingly, delocalized singlet excitons in acceptor crystals are found to diffuse effectively over the distance of ∼37 nm. Thanks to this high diffusivity, only 0.5 mol % of doped TTBP can harvest most of the singlet excitons, which successfully doubles the solid-state fluorescent quantum yield of acceptor/TTBP blend films to 76%. Furthermore, since the donor PtOEP and the collector TTBP are separately isolated in the nanofibrous acceptor crystals, the singlet back energy transfer from the collector to the donor is effectively avoided. Such efficient singlet energy collection and inhibited back energy transfer processes result in a large increase of UC efficiency up to 9.0%, offering rational design principles toward ultimately efficient solid-state upconverters.

Annihilation Versus Excimer Formation by the Triplet Pair in Triplet–Triplet Annihilation Photon Upconversion

“…The back energy transfer reduces the overall upconversion quantum yield in TTA-UC systems, and strategies to minimize the effect are discussed in recent publications. 20 , 23 …”

“… 11 − 13 Although the notion of triplet–triplet annihilation dates back more than 60 years, this field has gone through a fast development during the past decade. 14 − 23 …”

Impact of molecular conformation on triplet-fusion induced photon energy up-conversion in the absence of exothermic triplet energy transfer

“…[5][6][7] Following an efficient triplet energy transfer (TET) step a reservoir of triplet excited-state activators is prepared that facilitates TTA reactions between activators and generates TTA-UC delayed luminescence. The established model system of this type of TTA-UC composites is based on the combination of the green-absorbing metallo-organic triplet sensitizers such as (2, 3, 7, 8, 12, 13, 17, 18-octaethyl-porphyrinato) Pt II (PtOEP) or (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) Pd II (PdOEP) mixed with the blue-emitting di-phenyl anthracene (DPA) activator, in which the triplet excited state of the sensitizer is energetically higher than the triplet excited state of DPA. 8,9 Notably, in this class of photon up-converters the DPA activator molecules hold a dual role of annihilation/emitter.…”

“…10 For most of the technologically important applications the development of solid-state TTA-UC schemes is sought, 11 however the fabrication of high performance TTA-UC composite films is difficult to achieve and the TTA-UC photoluminescence quantum yield (PLQY) remains low in the order of 1 -4 %. [12][13][14] The main bottleneck that impedes high efficiency TTA-UC luminescence comes from the high strength of excitonic coupling effects due to enhanced intermolecular interactions of the composite components in the solid state. In particular, the aggregation of the PtOEP triplet sensitizer 15,16 results in the formation of PtOEP dimer states that act as energy-scavengers and funnel energy back to the ground state, thereby limiting the efficiency of the required TET step.…”

Molecular engineering of polymeric supra-amphiphiles

“…This aspect has greatly enriched the supramolecular chemistry of amphiphiles, leading to the emerging development of functional assemblies. Different from conventional amphiphiles, supra-amphiphiles are amphiphiles that are connected through noncovalent interactions, such as electrostatic interactions, [2][3][4] host-guest interactions, [5][6][7] hydrogen bonds, 8 and others 9 . These constructs serve as a bridge between colloid science and supramolecular chemistry.…”