School of Chemistry - Research Publications

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    Multi-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states
    Cho, I ; Kendrick, WJ ; Stuart, AN ; Ramkissoon, P ; Ghiggino, KP ; Wong, WWH ; Lakhwani, G (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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    A luminescent solar concentrator ray tracing simulator with a graphical user interface: features and applications
    Zhang, B ; Yang, H ; Warner, T ; Mulvaney, P ; Rosengarten, G ; Wong, WWH ; Ghiggino, KP (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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    Medium effects on the fluorescence of Imide-substituted naphthalene diimides
    Pervin, R ; Manian, A ; Chen, Z ; Christofferson, AJ ; Owyong, TC ; Bradley, SJ ; White, JM ; Ghiggino, KP ; Russo, SP ; Wong, WWH (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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    Limitations of conjugated polymers as emitters in triplet-triplet annihilation upconversion
    O'shea, R ; Gao, C ; Bradley, S ; Owyong, TC ; Wu, N ; White, JM ; Ghiggino, KP ; Wong, WWH (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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    Revealing the influence of steric bulk on the triplet-triplet annihilation upconversion performance of conjugated polymers
    O'shea, R ; Kendrick, WJ ; Gao, C ; Owyong, TC ; White, JM ; Ghiggino, KP ; Wong, WWH (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.
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    Concentrating Aggregation-Induced Fluorescence in Planar Waveguides: A Proof-of-Principle
    Banal, JL ; White, JM ; Ghiggino, KP ; Wong, WWH (NATURE PORTFOLIO, 2014-04-10)
    The photophysical properties of fluorescent dyes are key determinants in the performance of luminescent solar concentrators (LSCs). First-generation dyes--coumarin, perylenes, and rhodamines--used in LSCs suffer from both concentration quenching in the solid-state and small Stokes shifts which limit the current LSC efficiencies to below theoretical limits. Here we show that fluorophores that exhibit aggregation-induced emission (AIE) are promising materials for LSC applications. Experiments and Monte Carlo simulations show that the optical quantum efficiencies of LSCs with AIE fluorophores are at least comparable to those of LSCs with first-generation dyes as the active materials even without the use of any optical accessories to enhance the trapping efficiency of the LSCs. Our results demonstrate that AIE fluorophores can potentially solve some key limiting properties of first-generation LSC dyes.
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    Efficient light harvesting of a luminescent solar concentrator using excitation energy transfer from an aggregation-induced emitter
    Banal, JL ; Ghiggino, KP ; Wong, WWH (ROYAL SOC CHEMISTRY, 2014)
    The compromise between light absorption and reabsorption losses limits the potential light conversion efficiency of luminescent solar concentrators (LSCs). Current approaches do not fully address both issues. By using the excitation energy transfer (EET) strategy with a donor chromophore that exhibits aggregation-induced emission (AIE) behaviour, it is shown that both transmission and reabsorption losses can be minimized in a LSC device achieving high light collection and concentration efficiencies. The light harvesting performance of the LSC developed has been characterized using fluorescence quantum yield measurements and Monte Carlo ray tracing simulations. Comparative incident photon conversion efficiency and short-circuit current data based on the LSC coupled to a silicon solar cell provide additional evidence for improved performance.
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    A Transparent Planar Concentrator Using Aggregates of gem-Pyrene Ethenes
    Banal, JL ; White, JM ; Lam, TW ; Blakers, AW ; Ghiggino, KP ; Wong, WWH (WILEY-V C H VERLAG GMBH, 2015-10-07)
    The luminescence properties of pyrene ethenes, both as monomer and aggregate species, are found to depend on the regioisomer structure. Systematic shifts in absorption, emission, and excitation spectra of the gem‐pyrene ethenes, both in solution and in rigid polymer hosts, are consistent with weakly interacting H‐aggregate formation. This aggregation leads to excimer‐like emission with Stokes shifts greater than 1 eV. Planar concentrators fabricated from gem‐pyrene diphenylethenes show comparable performance to previously reported inorganic phosphors. The UV absorption and emission properties of the planar concentrator devices exhibit potential for transparent solar concentrators or visible–blind photodetector applications. This is the first demonstration of exploiting the unusual photophysics of molecular aggregates in planar concentrators.
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    Energy Migration in Organic Solar Concentrators with a Molecularly Insulated Perylene Diimide
    Banal, JL ; Soleimaninejad, H ; Jradi, FM ; Liu, M ; White, JM ; Blakers, AW ; Cooper, MW ; Jones, DJ ; Ghiggino, KP ; Marder, SR ; Smith, TA ; Wong, WWH (AMER CHEMICAL SOC, 2016-06-23)
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    Emissive Molecular Aggregates and Energy Migration in Luminescent Solar Concentrators
    Banal, JL ; Zhang, B ; Jones, DJ ; Ghiggino, KP ; Wong, WWH (American Chemical Society, 2017-01-17)
    Luminescent solar concentrators (LSCs) are light harvesting devices that are ideally suited to light collection in the urban environment where direct sunlight is often not available. LSCs consist of highly luminescent compounds embedded or coated on a transparent substrate that absorb diffuse or direct solar radiation over a large area. The resulting luminescence is trapped in the waveguide by total internal reflection to the thin edges of the substrate where the concentrated light can be used to improve the performance of photovoltaic devices. The concept of LSCs has been around for several decades, and yet the efficiencies of current devices are still below expectations for commercial viability. There are two primary challenges when designing new chromophores for LSC applications. Reabsorption of dye emission by chromophores within the waveguide is a significant loss mechanism attenuating the light output of LSCs. Concentration quenching, particularly in organic dye systems, restricts the quantity of chromophores that can be incorporated in the waveguide thus limiting the light absorbed by the LSC. Frequently, a compromise between increased light harvesting of the incident light and decreasing emission quantum yield is required for most organic chromophore-based systems due to concentration quenching. The low Stokes shift of common organic dyes used in current LSCs also imposes another optimization problem. Increasing light absorption of LSCs based on organic dyes to achieve efficient light harvesting also enhances reabsorption. Ideally, a design strategy to simultaneously optimize light harvesting, concentration quenching, and reabsorption of LSC chromophores is clearly needed to address the significant losses in LSCs. Over the past few years, research in our group has targeted novel dye structures that address these primary challenges. There is a common perception that dye aggregates are to be avoided in LSCs. It became apparent in our studies that aggregates of chromophores exhibiting aggregation-induced emission (AIE) behavior are attractive candidates for LSC applications. Strategic application of AIE chromophores has led to the development of the first organic-based transparent solar concentrator that harvests UV light as well as the demonstration of reabsorption reduction by taking advantage of energy migration processes between chromophores. Further developments led us to the application of perylene diimides using an energy migration/energy transfer approach. To prevent concentration quenching, a molecularly insulated perylene diimide with bulky substituents attached to the imide positions was designed and synthesized. By combining the insulated perylene diimide with a commercial perylene dye as an energy donor–acceptor emitter pair, detrimental luminescence reabsorption was reduced while achieving a high chromophore concentration for efficient light absorption. This Account reviews and reinspects some of our recent work and the improvements in the field of LSCs.