Organic Polymer Hosts for Triplet-Triplet Annihilation Upconversion Systems

Abstract

Triplet-triplet annihilation upconversion (TTA-UC) is a process by which a lower energy photon can be upconverted to a higher energy state. The incorporation of TTA-UC materials into solid-state hosts has enabled advances in solar energy and many other applications. The choice of host system is, however, far from trivial and often calls for a careful compromise between characteristics such as high molecular mobility, low oxygen diffusion, and high material stability, factors that often contradict one another. Here, we evaluate these challenges in the context of the state-of-the-art of primarily polymer hosts and the advantages they hold in terms of material selection and tunability of their diffusion or mechanical or thermal properties. We encourage more collaborative research between polymer scientists and photophysicists in order to further optimize the current systems and outline our thoughts for the future direction of the field.

Figure 1

Energy level diagram showing the steps of the radiative and nonradiative processes involved in triplet–triplet annihilation upconversion. (1) The absorption of photons by the sensitizers (glowing pink arrows) with vibrational nonradiative relaxation (squiggly purple arrows). (2) ISC (blue dashed arrow) over to the excited triplet state. (3) TTET (green dashed arrow) then occurs between the emitter and sensitizer in both pairs, and once the two emitters collide, (4) TTA occurs (orange lines), exciting one molecule to a higher energy singlet state with the second relaxing to the ground state. (5) The excited singlet relaxes radiatively via fluorescence (glowing purple line). The pale blue line represents potential energy losses during the process of TTA-UC.

Figure 2

Structures of common sensitizer–emitter pairs. The sensitizers are shown on the right-hand side, labeled with their name and absorption wavelength maximum. The emitters are shown on the left-hand side, labeled with their name and emission wavelength maximum.

Figure 3

Typical mechanism of triplet energy migration-based TTA-UC demonstrated by an example where nanocrystals of the sensitizer are doped into a metal–organic framework (MOF) of the emitter. The nanocrystals are excited to produce triplet excitons, which migrate rapidly to the emitter causing TTA. The upconverted photon is emitted out of the system. Adapted with permission from ref (68). Copyright Royal Society of Chemistry 2018.

Figure 4

Applications of solid-state TTA-UC systems using commercially available ClearFlex poly(urethane) hosts. (a) An upconversion-based luminescent back reflector is used to assist photoelectrochemical water-splitting at Mo:BiVO₄ photoelectrodes. Adapted with permission from ref (79). Copyright John Wiley and Sons 2019. (b–d) Images showing an upconversion-based photocatalytic microreactor demonstrating (b) upconversion enrichment of blue photon flux in diffuse sunlight; (c) a cross-section of the reactor surface; (d) a dye-doped reactor upconverting a 532 nm (green) laser. Adapted with permission from ref (80) under Creative Commons license CC-BY-3.0.

Figure 5

Poly(acrylate) host system doped with DPA and PdOEP for TTA-UC. (a) Scheme showing the structures of the poly(acrylate) dye-loaded system and an outline of the physical processes occurring during TTA-UC. The red circles represent the sensitizer PtOEP and the blue circles, the emitter DPA. A DPA:PtOEP (10^–2 M:10 ^–4 M) doped poly(octyl acrylate) sample (b) under daylight and under (c) low intensity (0.01 suns) and (d) high intensity (10 suns) laser excitation at 532 nm. Adapted from ref (63). Copyright American Chemical Society 2016.

Figure 6

Stress-sensing system based on TTA-UC. The scheme depicts the Diels–Alder linkage on the DPA derivative covalently attached to the poly(hexyl methacrylate) host. The linkage can be cleaved under force, resulting in the DPA derivative being free to act as an emitter to enable TTA-UC to occur within the system, producing a fluorescence response to sense stress. Adapted from ref (8) under Creative Commons license CC-BY-3.0.

Figure 7

Hydrogel-based TTA-UC host system. The figure demonstrates the coassembly of gelatin and the surfactant Triton X-100, incorporating PtOEP and DPAS as the TTA-UC chromophore pair. Gelatin forms a strong hydrogen-bonded layer into which the PEO tail of Triton X-100 inserts itself, allowing the PtOEP and DPAS to accumulate in the corresponding internal nonpolar domain. The tight hydrogen-bonded gelatin-Triton network provides an effective oxygen barrier layer, allowing for efficient TTA-UC to occur within the hydrophobic domains. Adapted from ref (85). Copyright American Chemical Society 2018.

Figure 8

Computational studies demonstrate that the distribution of the sensitizer and emitter affect the TTA-UC efficiency. Two possible distributions of sensitizer and emitter molecules throughout an upconversion matrix are shown. (a) Randomly placed molecules, emitters in blue and sensitizers in red. Some emitters are very close to each other, whereas others are surrounded by sensitizers and have no partner to perform upconversion. (b) Correlated placement of molecules: A circle represents a cluster of sensitizers, whereas outside the circles only emitters are embedded in the host material. Reproduced with permission from reference (87). Copyright AIP Publishing 2014.

Figure 9

Chemical structures of the sensitizer Ru(dmb)₃ and the combined host and emitter (DPA)₃₀-polymer, respectively. In this study, the DPA emitter is covalently grafted to the polymeric host to achieve rapid intramolecular TTA, yielding a 5% UC efficiency. Redrawn from ref (89).

Figure 10

TTA-UC system design with the DPA emitter covalently grafted to the polymer host. (a) Structures of the PdOEP sensitizer, DPAMA monomer and emitter, and poly(DPAMA-co-MMA) copolymer matrix. (b) Schematic representation of thin film assembly via drop casting solution at 105 °C onto a glass slide and compression molding at 160 °C and light pressure. Reproduced with permission from ref (36). Copyright Royal Society of Chemistry 2014.

Figure 11

Chemical structures of DPA and the bulky substituted DPA derivatives bDPA-1 and bDPA-2. These bulky emitters were used to yield nanocrystals with a diameter of >200 nm to be mixed with sensitizer PtOEP to proceed via the aggregation mechanism of TTA-UC. Redrawn from ref (56).

Figure 12

Chemical structures of two pairs of lumophores used in TTA-UC systems: sensitizer lipophilic osmium complex D1 with the emitter rubrene and sensitizer Os(bptpy)₂²⁺ with the emitter 2,5,8,11-tetra-tert-butylperylene (TTPT). Both pairs are able to facilitate direct S₀-T₁ excitation in a PVA matrix, which can potentially sidestep the energy losses in the intersystem crossing step of TTA-UC. Redrawn from ref (57).