School of Chemistry - Research Publications
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ItemMolecularly isolated perylene diimides enable both strong exciton-photon coupling and high photoluminescence quantum yield(Royal Society of Chemistry, 2019-03-14)Strong coupling in organic media holds the promise of efficient room temperature polariton lasing with solution-processed materials. Currently, however, only five pure-organic materials have been shown to demonstrate polariton lasing. A major challenge is to achieve high exciton–photon coupling while maintaining high photoluminescence quantum yield. Here, we utilize a series of diimide perylene materials that possess sterically hindered substituents, dispersed within a polymer matrix. The rigid structures prevent aggregation and allow high photoluminescence quantum yield (PLQY) at large dye loadings. We demonstrate that these systems can exhibit substantial Rabi splittings at dye loadings that yield film PLQYs of up to 85%, making these perylene derivatives promising materials for polariton lasers.
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ItemHighly Efficient Luminescent Solar Concentrators by Selective Alignment of Donor-Emitter Fluorophores(AMER CHEMICAL SOC, 2019-04-23)Vertically aligning fluorophores to the surface of a waveguide is known to be an effective approach to improve the optical quantum efficiency (OQE) of luminescent solar concentrators (LSCs). While the chromophore alignment assists waveguiding of the emitted photons to the LSC edges, it also significantly reduces the light-harvesting properties of the LSC. We report here a fluorophore pair consisting of a sphere-shaped energy donor and a rod-shaped emitter that was incorporated in LSCs to provide selective fluorophore alignment to address the reduced incident-light absorption issue. A liquid-crystal polymer matrix was used to perpendicularly align the rod-shaped acceptors to a favorable orientation for light guiding, while the sphere-shaped donor was randomly oriented to maintain its light-absorbing properties. The OQE of LSC devices with this selectively aligned donor-acceptor fluorophore system is 78% without significant loss of light-harvesting capability.
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ItemTetrabenzo[5.7]fulvalene: a forgotten aggregation induced-emission luminogen(ROYAL SOC CHEMISTRY, 2019-10-04)Tetrabenzo[5.7]fulvalene, one of the first recognized stable members of mixed fulvalenes, has attracted widespread interest for its remarkable structure. However, little has been known about its photoactivity, most likely owing to its very weak luminescence in the solution state. Here we show for the first time that this compound exhibits aggregation-induced emission (AIE) properties. Its photoluminescence and X-ray crystal structure reveal an interesting mechanism of the AIE phenomenon.
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ItemIntramolecular Versus Intermolecular Triplet Fusion in Multichromophoric Photochemical Upconversion(AMER CHEMICAL SOC, 2019-08-22)Photon upconversion is a process that creates high-energy photons under low photon energy excitation. The effect of molecular geometry on the triplet fusion upconversion process has been investigated in this work through the design and synthesis of four new 9,10-diphenylanthracene (DPA) derivatives by employing platinum octaethylporphyrin as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core exhibit high fluorescence quantum yields. Interestingly, large differences in the triplet fusion upconversion performance were observed between the derivatives with the meta-substituted dimer showing the closest performance to the DPA reference. The differences are discussed in terms of the statistical probability for obtaining a high-energy singlet excited state from triplet fusion, f, for both inter- and intramolecular processes and the effect of magnetic field on the upconversion efficiency. These results demonstrate the challenges to be overcome in improving triplet fusion upconversion efficiency based on multichromophoric emitter systems.
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ItemContinuous Flow Synthesis of Conjugated Polymers and Carbon Materials(WILEY-V C H VERLAG GMBH, 2017)The basic equipment associated with the synthesis of chemicals in the laboratory has remained essentially unchanged since the establishment of chemistry as a scientific discipline. Most techniques and apparatus are optimized for batch-by-batch processes. These traditional batch techniques have the benefit of familiar and time-tested methodology and low up-front cost, and therefore the shift to more modern apparatus requires some additional motivating factors. Continuous flow processing is a technique which involves a nontrivial initial investment of both time and finances, but which offers significant benefits to synthetic chemists. Continuous flow processing has been increasing in prominence in the research laboratory setting in the last decade, and a number of research groups now specialize in the development of methods for flow processing. This progress is buoyed by the commercial availability of the associated equipment, which can now be purchased from several vendors.
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ItemAggregation-induced emitters in light harvesting(Springer, 2019)Light harvesting is an integral part of energy conversion of sunlight into chemicals and electricity. In this chapter, the application of materials with aggregation-induced emission properties in artificial photosynthesis and photon refining technologies is summarized and discussed. In artificial photosynthesis, aggregation-induced emitters enable efficient energy transfer in self-assembled arrays. Thin film luminescent solar concentrators have also been made possible by aggregation-induced emitters as high concentrations of these chromophores can be used in such devices. Aggregates are also important in photon upconversion where proximity of chromophores enables efficient triplet energy transfer and triplet-triplet annihilation processes.
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ItemTetraphenylethene 9,10-Diphenylanthracene Derivatives - Synthesis and Photophysical Properties(WILEY-V C H VERLAG GMBH, 2019-06)A series of tetraphenylethene 9,10-diphenylanthracene (TPE-DPA) derivatives have been synthesized, and their photophysical properties studied. Photoluminescence measurements in PMMA, neat films and nanoparticle dispersions reveal that different aggregation states are formed, which leads to different photophysical behavior. The triplet excited state properties were studied using Pt(II) octaethylporphyrin (PtOEP) as triplet sensitizer. Upconverted emission from the DPA moiety is observed in nanoparticle dispersions of each derivative. A higher upconverted emission intensity is observed in aerated (compared to deaerated) solutions of the derivatives following irradiation, which is attributed to oxidation of the TPE moiety. These results provide valuable insight for the design of AIE luminogens for triplet-triplet annihilation upconversion (TTA-UC).
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ItemA Maleimide-functionalized Tetraphenylethene for Measuring and Imaging Unfolded Proteins in Cells(WILEY-V C H VERLAG GMBH, 2019-03-15)Collapse of the protein homeostasis (proteostasis) can lead to accumulation and aggregation of unfolded proteins, which has been found to associate with a number of disease conditions including neurodegenerative diseases, diabetes and inflammation. Here we report a maleimide-functionalized tetraphenylethene (TPE)-derivatized fluorescent dye, TPE-NMI, which shows fluorescence turn-on property upon reacting with unfolded proteins in vitro and in live cells under proteostatic stress conditions. The level of unfolded proteins can be measured by flow cytometry and visualized with confocal microscopy.
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ItemControlled synthesis of poly(3-hexylthiophene) in continuous flow(BEILSTEIN-INSTITUT, 2013-07-25)There is an increasing demand for organic semiconducting materials with the emergence of organic electronic devices. In particular, large-area devices such as organic thin-film photovoltaics will require significant quantities of materials for device optimization, lifetime testing and commercialization. Sourcing large quantities of materials required for the optimization of large area devices is costly and often impossible to achieve. Continuous-flow synthesis enables straight-forward scale-up of materials compared to conventional batch reactions. In this study, poly(3-hexylthiophene), P3HT, was synthesized in a bench-top continuous-flow reactor. Precise control of the molecular weight was demonstrated for the first time in flow for conjugated polymers by accurate addition of catalyst to the monomer solution. The P3HT samples synthesized in flow showed comparable performance to commercial P3HT samples in bulk heterojunction solar cell devices.
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ItemBulk Heterojunction Nanomorphology of Fluorenyl Hexa-peri-hexabenzocoronene-Fullerene Blend Films(AMER CHEMICAL SOC, 2013-11-27)In this study, the nanomorphology of fluorenyl hexa-peri-hexabenzocoronene:[6,6]-phenyl C61-butyric acid methyl ester (FHBC:PC61BM) absorber layers of organic solar cells was investigated. Different electron microscopical techniques, atomic force microscopy, and grazing incidence wide-angle X-ray scattering were applied for a comprehensive nanomorphology analysis. The development of the nanomorphology upon sample annealing and the associated change of the device performance were investigated. It was shown that the annealing process enhances the phase separation and therefore the bulk heterojunction structure. Due to π-π stacking, the FHBC molecules assemble into columnar stacks, which are already present before annealing. While the nonannealed sample consists of a mixture of homogeneously distributed PC61BM molecules and FHBC stacks with a preferential in-plane stack orientation, crystalline FHBC precipitates occur in the annealed samples. These crystals, which consist of hexagonal arranged FHBC stacks, grow with increased annealing time. They are distributed homogeneously over the whole volume of the absorber layer as revealed by electron tomography. The FHBC stacks, whether in the two phase mixture or in the pure crystalline precipitates, exhibit an edge-on orientation, according to results from grazing incidence wide-angle X-ray scattering (GIWAXS), dark-field transmission electron microscopy (DF TEM) imaging and selective area electron diffraction (SAED). The best solar cell efficiencies were obtained after 20 or 40 s sample annealing. These annealing times induce an optimized degree of phase separation between donor and acceptor material.