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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ItemExtended Reichardt's Dye-Synthesis and Solvatochromic Properties.(Wiley, 2024-04-08)Three new pyridinium-phenolate dyes based on the benchmark solvatochromic dye Betaine 30 were synthesised. The dyes contained phenylene spacers between the donor and acceptor groups. Their UV-Vis absorption spectra were measured, with the dyes showing strong negative solvatochromic behaviour comparable to that of Betaine 30. These results stood in contrast to the behaviour of the π-extended dye Betaine 21, originally reported in 1963. This dye was synthesised and found to be significantly more solvatochromic than previously reported but prone to degrade. All π-extended dyes synthesised were found to be unstable in certain solvents. Although the increased distance between donor and acceptor did not enhance solvatochromism to the extent predicted, it was still determined that the reduced planarity caused by a phenylene spacer is not as detrimental as believed.
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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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ItemUnusual Alternating Crystallization-Induced Emission Enhancement Behavior in Nonconjugated ω-Phenylalkyl Tropylium Salts(American Chemical Society, 2021-11-22)The alternating physical properties, especially melting points, of α,ω-disubstituted n-alkanes and their parent n-alkanes had been known since Baeyer’s report in 1877. There is, however, no general and comprehensive explanation for such a phenomenon. Herein, we report the synthesis and examination of a series of novel ω-phenyl n-alkyl tropylium tetrafluoroborates, which also display alternation in their physicochemical characters. Despite being organic salts, the compounds with odd numbers of carbons in the alkyl bridge exist as room temperature ionic liquids. In stark contrast to this, the analogues with even numbers of carbons in the linker are crystalline solids. These solid nonconjugated molecules exhibit curious photoluminescent properties, which can be attributed to their ability to form through-space charge-transfer complexes to cause crystallization-induced emission enhancement. Most notably, the compound with the highest photoluminescent quantum yield in this series showed an unusual arrangement of carbocationic dimer in the solid state. A combination of XRD analysis and ab initio calculations revealed interesting insights into these systems.
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ItemInsights into dynamic properties of water in lipidic cubic phases by 2D nuclear Overhauser effect (NOE) NMR spectroscopy.(Elsevier BV, 2024-07-15)Two-dimensional NOE (nuclear Overhauser effect) NMR spectroscopy was employed to investigate the dynamic properties of water within lyotropic bicontinuous lipidic cubic phases (LCPs) formed by monoolein (MO). Experiments observed categorically different effective residence times of water molecules: (i) in proximity to the glycerol moiety of MO, and (ii) adjacent to the hydrophobic chain towards the hydrocarbon tail of MO, as evidenced by the opposite signs of intermolecular NOE cross peaks between protons of water and those of MO in 2D 1H-1H NOESY spectra. Spectroscopic data delineating the different effective residence times of water molecules within both the gyroid (QIIG) and diamond (QIID) phase groups corresponding to hydration levels of 35 and 40 wt%, respectively, are presented. Additionally, an increase in effective residence time of water molecules in proximity to the glycerol moiety of MO in LCPs was observed upon storage at ambient temperature and in the presence of an additive lipid, cholesterol. Atom-specific NOE build-up curves for protons of water and those of MO are also given. The results presented herein provide new insight into the physicochemical properties and behaviour of water in LCPs, and demonstrate an additional avenue for experimental study of water-lipid interactions and hydration dynamics in model membranes and nanomaterials using 2D NOE NMR spectroscopy.