School of Chemistry - Research Publications
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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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ItemMulti-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states(Royal Society of Chemistry, 2023-11-07)Thermally activated delayed fluorescence (TADF) emitters suffer from molecular aggregation that limits their applicability in light emitting devices. Aggregation-induced excimer formation often leads to a larger Stokes shift, broader emission spectrum, and reduced emission quantum yields, limiting emitter dye loading to a few weight percent in organic light emitting devices. Here, we demonstrate suppression of excimer emission by dispersing a synthesised multi-resonance TADF emitter dye (OQAO(mes)2) in a PMMA host matrix and embedding the host–guest photoactive layer into an optical cavity. Rabi splitting up to 0.24 eV is obtained at 35 wt% dye loading. Under the strong coupling regime, prompt and delayed emission through excimer states is suppressed due to efficient energy transfer to the lower polariton (LP) states, demonstrated by the blue shift of the emission spectrum and narrowing of the emission linewidth. We also observe an increase in reverse intersystem crossing (RISC) rate constants up to 33% that we attribute to a decrease in activation energy by ≈2kT. This work highlights that strong light-matter interactions can be exploited to overcome aggregation-induced excimer losses providing a pathway towards efficient organic light-emitting diodes with high colour purity and organic semiconductor polariton lasing.
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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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ItemMedium effects on the fluorescence of Imide-substituted naphthalene diimides(Elsevier, 2023-03-01)Naphthalene diimides (NDIs) are a common class of chromophores used in photon harvesting applications due to their functional malleability through substitution of the NDI core. However, some derivatives with substitution at the imide position of the NDI core only become emissive in electron-rich aromatic solvents. This study examines this phenomenon from both an experimental and theoretical perspective, in order to understand how NDIs interact with each other and the surrounding medium upon photoexcitation. We report the photophysical properties of cyclohexyl and several aromatic imide-substituted NDI derivatives, and show that fluorescence properties are strongly influenced by solvation in more electron-rich aromatic solvents (e.g. toluene, xylene, mesitylene). Theoretical modeling supports strong interactions, including ground state charge-transfer complexation, with aromatic solvents. In solid poly(methyl methacrylate) (PMMA) and poly(styrene) (PS) film media, both aggregation and complexation are shown to contribute to absorption and emission properties. The results also demonstrate that aromatic imide substituents not only act to provide steric bulk to the NDI chromophore but participate in interactions with the surrounding medium that affect the overall photophysical properties.
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ItemLimitations of conjugated polymers as emitters in triplet-triplet annihilation upconversion(ROYAL SOC CHEMISTRY, 2021-11-29)Triplet–triplet annihilation upconversion performances for poly(phenylene-vinylene) emitters were investigated through a series of copolymers with bulky sidechains.
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ItemRevealing the influence of steric bulk on the triplet-triplet annihilation upconversion performance of conjugated polymers(NATURE PORTFOLIO, 2021-10-01)A series of poly(phenylene-vinylene)-based copolymers are synthesized using the Gilch method incorporating monomers with sterically bulky sidechains. The photochemical upconversion performance of these polymers as emitters are investigated using a palladium tetraphenyltetrabenzoporphyrin triplet sensitizer and MEH-PPV as reference. Increased incorporation of sterically bulky monomers leads to a reduction in the upconversion efficiency despite improved photoluminescence quantum yield. A phosphorescence quenching study indicates issues with the energy transfer process between the triplet sensitizer and the copolymers. The best performance with 0.18% upconversion quantum yield is obtained for the copolymer containing 10% monomer with bulky sidechains.