School of Chemistry - Research Publications
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ItemTriplet fusion upconversion using sterically protected 9,10-diphenylanthracene as the emitter(ROYAL SOC CHEMISTRY, 2020-03-21)Improving the efficiency of triplet fusion upconversion (TF-UC) in the solid-state is still challenging due to the aggregation and phase separation of chromophores. In this work, two 9,10-diphenylanthracene (DPA) derivatives based on the modification of the 9,10-phenyl rings with bulky isopropyl groups (bDPA-1 and bDPA-2) were used as emitters. By using platinum octaethylporphyrin (PtOEP) as the sensitizer, TF-UC performance was comprehensively investigated in 3 media: toluene solution, polyurethane thin film and nano/micro-crystals in a polyvinyl alcohol matrix. Only a small difference in upconversion efficiency between the bulky DPAs and the DPA reference was observed in toluene solution and polyurethane thin film. However, a large improvement of TF-UC quantum yield was achieved in bDPA-2/PtOEP crystals (ΦUC = (0.92 ± 0.05)%) with a low excitation intensity threshold (52 mW cm-2) compared to that of DPA/PtOEP crystals (ΦUC = (0.09 ± 0.03)%). This difference was largely attributed to improved dispersibility of the PtOEP sensitizer in the bDPA-2 emitter crystals. The bulky DPAs also show excellent stability under UV irradiation with exposure to oxygen compared to DPA. These results provide a strategy for developing efficient solid-state TF-UC systems based on nano/micro-particles of emitter-sensitizer mixtures.
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ItemSimple improvements to Gilch synthesis and molecular weight modulation of MEH-PPV(Royal Society of Chemistry, 2020-04-28)The solvent and temperature used in the Gilch synthesis of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were varied to achieve an improved set of reaction conditions. A range of molecular weights from 20 to 500 kg mol−1 were obtained in moderate to near-quantitative yields. The best conditions for producing low molecular weight MEH-PPV (32 kg mol−1, 97% yield) was to use n-pentane as the reaction solvent at 25 °C. For high molecular weight (397 kg mol−1, 65% yield), toluene at 55 °C gave the best result. The photophysical properties for all MEH-PPV samples were examined and no significant variation was found between samples obtained under different polymerization conditions.
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ItemCompetitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission(AMER CHEMICAL SOC, 2020-05-28)Developing photostable compounds that undergo quantitative singlet fission (SF) is a key challenge. As SF necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to intermolecular coupling in the solid state. We investigate SF in thin films for a series of perylenediimide (PDI) molecules. By adding different substituents at the imide positions, the packing of the molecules in the solid state can be changed. The relationship between SF parameters and the stacked geometry in PDI films is investigated, with two-electron direct coupling found to be the main SF mechanism. Time-resolved emission and transient absorption data show that all of the PDI films undergo SF although with different rates and yields varying from 35 to 200%. The results show that PDI1 and 2, which are stacked PDI pairs twisted out of alignment along the highest occupied molecular orbital to lowest unoccupied molecular orbital transition, exhibit faster and more efficient SF up to 200% yield. We demonstrate that both triplet formation and decay rates are highly sensitive to the ordering of the molecules within a film. The results of this study will assist in the design of optimized structures with a fast SF rate and low recombination rate that are required for useful light harvesting applications.
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ItemFRET-enhanced photoluminescence of perylene diimides by combining molecular aggregation and insulation(Royal Society of Chemistry, 2020-07-14)The photoluminescence quantum yield (ϕPL) of perylene diimide derivatives (PDIs) is often limited by aggregation caused quenching (ACQ) at high concentration or in the neat solid-state. Energy transfer in high dye concentration systems is also a key factor in determining ϕPL as a result of energy funneling to trap sites in the sample. By tuning the substituents, we present two classes of PDIs with aggregation and insulation of the PDI core. By combining these fluorophores in a polymer film, we demonstrate highly emissive samples (85% ϕPL) at high concentration (140 mM or 20% w/w). Experimental and theoretical studies provide insight into why such a combination is necessary to achieve high ϕPL. While insulated fluorophores maintain respectable ϕPL at high concentration, an improved ϕPL can be achieved in the presence of appropriately oriented fluorophore aggregates as emissive traps. The theoretical calculations show that the relative orientation of aggregated monomers can result in energetic separation of localized states from the charge-transfer and bi-excitonic states thereby enabling high ϕPL.
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ItemBilirubin analogues as model compounds for exciton coupling(ROYAL SOC CHEMISTRY, 2020-07-21)A series of phycobilin analogues have been investigated in terms of coupled excitonic systems. These compounds consist of a monomer, a tetrapyrrole structurally similar to bilirubin (bR), and two conjugated bR analogues. Spectroscopic and computational methods have been used to investigate the degree of interchromophore coupling. We find the synthesised bR analogue shows stronger excitonic coupling than bR, owing to a different molecular geometry. The excitonic coupling in the conjugated molecules can be controlled by modifying the bridge side-group. New computed energy levels for bR using the DFT/MRCI method are also presented, which improve on published values and re-assign the character of excited singlet states.
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ItemOrganic polariton lasing with molecularly isolated perylene diimides(AMER INST PHYSICS, 2020-07-27)A dye immersed in a polymer matrix represents a promising material class for organic polariton lasers. While polariton condensation has been established for boron-dipyrromethene blends, it has yet to be extended to other small dye blends. Here, we demonstrate polariton lasing in a molecularly isolated perylene diimide immersed in polystyrene. The emission exhibits threshold behavior, strong directionality, and slight blueshifting of organic polariton systems. We expect that continuous exploration of new dyes will allow improved performance in organic polariton lasing.
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ItemRevealing the Interfacial Photoreduction of MoO3 with P3HT from the Molecular Weight-Dependent "Burn-In" Degradation of P3HT:PC61BM Solar Cells(American Chemical Society, 2020-10-26)“Burn-in” degradation occurs in many polymer solar cells, which dramatically reduces the overall power output of the cells at the early hundred hours. Understanding the “burn-in” degradation mechanism is therefore highly important to improve the lifetime of the cell. In this article, the decay behaviors of P3HT:PC61BM solar cells depending on the molecular weight of P3HT were systematically investigated. Although all of these P3HTs were highly crystalline with regioregularity of 94–97%, the stability of P3HT:PC61BM cells showed a nonmonotonic dependence on P3HT molecular weight. The cells based on P3HT with a weight average molecular weight (Mw) of 20 K showed much faster decadence in open circuit voltage (VOC) and fill factor (FF) during aging, yielding the lowest stability in comparison with that based on P3HT of 10, 25, and 30 K. UV–vis absorption and external quantum efficiency spectra demonstrated that the performance decay is not attributed to the change in the photoactive layer. The recovery of VOC and FF of the aged cells after renewing the MoO3/Al electrode revealed that the performance decay is mainly because of the interfacial degradation of P3HT:PC61BM/MoO3. Electron spin resonance spectroscopy and X-ray photoelectronic spectroscopy confirmed the photon-induced redox reaction between P3HT and MoO3 under light illumination, where P3HT is oxidized to the polaron and Mo(VI) was partially reduced to Mo(V). The photon chemical reduction (PCR) of MoO3 by P3HT is then ascribed as the essential reason for the fast VOC and FF decays of the cells during aging. The surface morphology of the photoactive layer measured by the atomic force microscope revealed the much rougher surface of the P3HT-20 K/PC61BM film. Such a rough surface increases the contact area between P3HT and MoO3, and consequently enhances the PCR of MoO3 and P3HT, which is considered as the main reason for the molecular weight-dependent degradation behaviors. For the first time, the current work clearly demonstrates that the photoreduction of the metal oxide and photoactive layer would lead to fast VOC and FF decays, which could be a very important degradation pathway for polymer solar cells.
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ItemA luminescent solar concentrator ray tracing simulator with a graphical user interface: features and applications(IOP PUBLISHING LTD, 2020-07)A Monte-Carlo ray tracing simulator with a graphical user interface (MCRTS-GUI) has been developed to provide a quantitative description, performance evaluation and photon loss analysis of luminescent solar concentrators (LSCs). The algorithm is applied to several practical LSC device structures including multiple dyes in the same waveguiding layer, and structures where a dye layer is sandwiched between clear substrates. The effect of the host matrix absorption and the influence of the neighboring layers are investigated. Validations demonstrate that the MCRTS-GUI developed provides a reliable and accurate description of LSC performance. Code for the mixed-dye single layer configuration is converted into a ray-tracing package with a user-friendly interface and is made available as open source software.
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ItemNo Preview AvailableIncorporating whey protein aggregates produced with heat and ultrasound treatment into rennet gels and model non-fat cheese systems(Elsevier, 2020-12-01)Native whey proteins (WP) are expulsed from cheese coagulation during syneresis. Although incorporating denatured WP aggregates into cheese gels has been previously proposed to improve the overall cheese yield, the effects of WP aggregate properties on gelation kinetics and protein retention are not yet fully understood. In this study, heat and power ultrasound were used to produce denatured whey protein aggregates with a wide range of sizes. The effects of size and hydrophobicity differences in WP aggregates produced by heat and heat coupled with ultrasound were investigated in relation to the kinetics of rennet gelation and protein retention in model non-fat cheddar cheeses. Rheological measurements showed that sufficiently large, denatured WP aggregates could avoid impairment of rennet gelation caused by native WP, irrespective of changes in the soluble calcium concentration or the surface hydrophobicity of the aggregates. WP aggregates formed by the combined heat and ultrasound treatment were more hydrophobic than the larger heat-treated aggregates and were better retained in the cheese. However, inclusion of sufficiently large aggregates in cheese milk conferred an openness to the cheese microstructure and showed promise in improving the otherwise rigid non-fat cheese.
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ItemIncorporation of Vanadium and Molybdenum into Yttrium-Arsenotungstates Supported by Amino Acid Ligands(CSIRO PUBLISHING, 2020)The preference for incorporation of molybdenum over tungsten into specific sites of a family of yttrium-arsenotungstates with amino acid ligands prompted exploration of the incorporation of other metals, affording three new vanadium-containing (V/W and V/Mo/W) analogues: K2(GlyH)10[As4(V2W2)W44Y4O160(Gly)8(H2O)12]·11Gly (1), (MBAH)9(L-NleH)3[As4(V2W2)W44Y4O160(L-Nle)8(H2O)12] (2), and (MBAH)9(L-NleH)3[As4(V2W2)Mo2W42Y4O160(L-Nle)8(H2O)12] (3) (Gly=glycine and L-Nle=l-norleucine, MBAH=4-methylbenzylammonium). These hybrid polyoxometalates all possess a tetrametallic oxo-bridged {VIV2WVI2} central core surrounded by an amino acid-ligated cyclic metal-oxo framework. X-Ray photoelectron, UV-visible reflectance, and electron paramagnetic resonance spectroscopy, together with metal analysis, confirm the incorporation of vanadium into the polyoxometalates, while single crystal X-ray diffraction analysis supports the location of the vanadium atoms in the central core.