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Photon upconversion based on sensitized triplet–triplet annihilation (sTTA-UC) is a wavelength-shifting technique with potential use in actuators, sensing, and solar technologies. In sTTA-UC, the upconverted photons are the result of radiative recombination of high-energy singlets, which are created through the fusion of metastable triplets of two annihilator/emitter molecules. The emitter triplets are populated via energy transfer (ET) from a low-energy absorbing light-harvester/sensitizer. The process is highly efficient at low powers in solution but becomes relatively ineffective in solid matrices since the limited molecular mobility precludes bimolecular interactions. The realization of efficient solid-state upconverters that exhibit long-term stability and are compatible with industrial fabrication processes is an open challenge. Here, nanophase-separated polymer systems synthesized under ambient conditions that contain the upconverting dyes in liquid nanodomains is reported. The nanostructured polymers show an excellent optical quality, an outstanding upconversion efficiency of up to ≈23%, and excellent stability in air, with only negligible performance losses over a period of three months. Moreover, the dyes’ confinement in nanosized domains

Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self assembling into a lyotropic chiral nematic phase in aqueous solution. This liquid crystalline phase reflects right-handed circular polarized light of a specific color as a function of the HPC weight fraction. Here, we demonstrate that, by introducing a crosslinking agent, it is possible to drastically alter the visual appearance of the HPC mesophase in terms of the reflected color, the scattering distribution and the polarization response, resulting in an exceptional matte appearance in dry solid-state films. By exploiting the interplay between order and disorder, a robust and simple methodology towards the preparation of polarization and angular independent color was developed, which constitutes an important step towards the development of real-world photonic colorants., This work is funded by EPSRC grant EP/R511675/1 to B.F.-P., S.V.; by EPSRC grant EP/L016087/1 to C.A.W.; by BBSRC David Phillips fellowship BB/K014617/1 to G.J., S.V.; by EU’s Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No. 722842 (ITN Plant-inspired Materials and Surfaces – PlaMatSu) to G.T.K., S.V.; by Swiss National Science Foundation #165176 to R.V.; by Croucher Cambridge International Scholarship to C.L.C.C.; by ERC grant ERC 2014 STG H2020 639088 to M.M.B., G.J., S.V.; by ERC grant ERC 2017 POC 790518 to R.M.P., S.V.

We herein report new organogels that permit efficient optical upconversion (UC) by triplet–triplet annihilation. The materials studied consist of a liquid organic phase, composed of a mixture of N,N-dimethylformamide and dimethyl sulfoxide in which the UC chromophore pair Pd(II) mesoporphyrin IX and 9,10-diphenylanthracene was dissolved, and a three-dimensional polymer network formed by covalently cross-linking poly(vinyl alcohol) with hexamethylene diisocyanate. The new gels are highly transparent, shape-persistent, and display efficient green-to-blue upconversion with UC quantum yields of >0.6 and 14% under ambient and oxygen-free conditions, respectively. The design approach presented here permits the fabrication of a hitherto unexplored class of materials with a unique combination of properties. The framework can easily be extended to other materials based on other solvents, polymer networks, and/or chromophore pairs.

Sensitized triplet-triplet-annihilation-based photon upconversion (TTA-UC) permits the conversion of light into radiation of higher energy and involves a sequence of photophysical processes between two dyes. In contrast to other upconversion schemes, TTA-UC allows the frequency shifting of low-intensity light, which makes it particularly suitable for solar-energy harvesting technologies. High upconversion yields can be observed for low viscosity solutions of dyes; but, in solid materials, which are better suited for integration in devices, the process is usually less efficient. Here, it is shown that this problem can be solved by using transparent nanodroplet-containing polymers that consist of a continuous polymer matrix and a dispersed liquid phase containing the upconverting dyes. These materials can be accessed by a simple one-step procedure that involves the free-radical polymerization of a microemulsion of hydrophilic monomers, a lipophilic solvent, the upconverting dyes, and a surfactant. Several glassy and rubbery materials are explored and a range of dyes that enable TTA-UC in different spectral regions are utilized. The materials display upconversion efficiencies of up to ≈15%, approaching the performance of optimized oxygen-free reference solutions. The data suggest that the matrix not only serves as mechanically coherent carrier for the upconverting liquid phase, but also provides good protection from atmospheric oxygen.

Low-power light upconversion by triplet–triplet annihilation (TTA-UC) was only recently achieved in glassy materials. Here, a new strategy based on covalent tethering of diphenylanthracene (DPA) emitters to a polymeric backbone is reported. The design aims to optimize the efficiency of this photophysical process in glassy polymeric materials by increasing the emitter content. To that end, DPA molecules were covalently attached to a methacrylate-type monomer and further copolymerized with methylmethacrylate (MMA). Green-to-blue (543 to 440 nm) upconversion was observed at power densities as low as 32 mW cm−2 in films prepared by solution casting and compression molding (co)polymers containing 8–72 wt% of DPA and palladium octaethyl porphyrin (PdOEP) as a sensitizer (0.03–0.7 wt%). The upconversion intensity was studied as a function of DPA and PdOEP contents and the results suggest that upconversion is optimal for DPA and PdOEP weight fractions of 34 and 0.05 wt% respectively.

Green-to-blue upconverting molecular glasses consisting of a metal octaethylporphyrin (MOEP, M = Pd, Pt) sensitizer and 9-(4-hydroxymethylphenyl)-10-phenyl anthracene (DPA-CH2OH) as an emitter are reported. In these materials, incident light is transformed into higher-energy radiation by way of triplet–triplet annihilation upconversion. The DPA-CH2OH–MOEP mixtures form transparent glasses when cooled from the thermally stable melt, even at rates as low as 1 °C min−1. In a systematic study, the PdOEP concentration was varied from 0.025 to 6.6 mol%. The normalized upconverted light intensity decreased with increasing sensitizer concentration by almost three orders of magnitude, as a result of sensitizer aggregation. The upconverted light intensity also decreased upon deliberate crystallization of the upconverting materials. Beyond demonstrating the first embodiment of upconversion in molecular glasses, the results highlight the importance of morphology control in solid-state upconverting materials.

[EN] Efforts to fabricate low-power up converting solid-state systems have rapidly increased in the past decade because of their possible application in several fields such as bio-imaging, drug delivery, solar harvesting or displays. The synthesis of upconverting cross-linked polyester rubbers with covalently tethered chromophores is presented here. Cross-linked films were prepared by reacting a poly(mannitol- sebacate) pre-polymer with 9,10-bis(4-hydroxymethylphenyl) anthracene (DPA-(CH2OH)2) and palladium mesoporphyrin IX. These chromophores served as emitters and sensitizers, respectively, and through a cascade of photophysical events, resulted in an anti-Stokes shifted emission. Indeed, blue emission (*440 nm) of these solid materials was detected upon excitation at 543 nm with a green laser and the power dependence of integrated unconverted intensity versus excitation was examined. The new materials display upconversion at power densities as low as 32 mW/cm2, and do not display phase de-mixing, which has been identified as an obstacle in rubbery blends comprising untethered chromophores., The authors are thankful for the financial support of the Swiss National Science Foundation (200021_13540/1 and 200020_152968), Spanish Ministry of Economy and Competitiveness (Project MAT2010/21494-C03) and the Adolphe Merkle Foundation. The authors thank Prof. Christoph Weder for his help and support.

We report highly efficient sensitized triplet–triplet annihilation based upconversion in aqueous suspensions of nanoparticles prepared from 9,10-diphenylanthracene-terminated poly(e-caprolactone) and with platinum octaethylporphyrin as the sensitizer. The particles upconversion characteristics are strongly temperature-dependent. This feature gives insights into the mechanisms enabling the process in the nanoparticle environment, and the specific temperature range in which the photophysical parameters change is suitable for live cell and in vivo temperature sensing.