School of Chemistry - Research Publications
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ItemExcitonic Processes in a Conjugated Polyelectrolyte Complex(CSIRO Publishing, 2020)In aqueous solution, a di-sulfonated phenylenevinylene polymer (DPS-PPV) forms a complex with non-ionic poly(vinyl alcohol) (PVA) leading to absorption spectroscopic shifts and a dramatic (6-fold) increase in DPS-PPV fluorescence intensity. Spectroscopic investigations demonstrate that the complexation with PVA and other neutral polymers results in conformational changes in the DPS-PPV chains that lead to the removal of non-fluorescent energy traps and results in the observed increase in fluorescence in the bulk solution. Single molecule fluorescence measurements of DPS-PPV chains dispersed on glass and in PVA films confirm that efficient exciton energy transfer occurs within each photo-excited DPS-PPV chain and that the observed increase in fluorescence intensity in the PVA film environment is also associated with fewer quenching sites. The results highlight the importance of conjugated polyelectrolyte conformation on exciton relaxation pathways.
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ItemRevealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells(American Chemical Society, 2020-06-10)Layered perovskite films, composed of two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.
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ItemMorphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell(AMER CHEMICAL SOC, 2021-01-14)The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.
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ItemSpectroscopic and Dynamic Properties of Electronically Excited Pendant Porphyrin Polymers with Backbones of Differing Flexibility(American Chemical Society, 2020-12-24)A zinc porphyrin-pendant norbornene polymer with a rigid backbone characterized by a 2:1 E/Z isomeric structure ratio has been synthesized, and its spectroscopic and photophysical properties are examined. Zinc tetraphenylporphyrin, the porphyrin-substituted norbornene monomer, and a previously reported zinc porphyrin-pendant polymer with a flexible polymethylene backbone have been used as comparators. Unlike its flexible counterpart, the rigid norbornene polymer exhibits clear exciton splitting of its Soret band, much more rapid relaxation rates of its excited singlet states, and a very small yield of an unusually short-lived triplet state. Unlike the flexible pendant polymer, which exhibits excimeric S2 fluorescence as a result of chromophore rotation, anti-Kasha emission from the norbornene polymer originates primarily from the unperturbed porphyrin E region. The low triplet yield in the polymer is attributed to greatly increased rates of competing internal conversion within the singlet manifold. Nevertheless, upconverted delayed fluorescence that is quenched by oxygen is observed upon intense steady-state Q-band excitation of degassed polymer solutions, signaling direct triplet involvement. Consistent with the polymer’s rigid structure, this biexcitonic process is assigned to ultrafast singlet exciton migration and triplet–triplet annihilation following absorption of a second photon by the small steady-state concentration of polymer triplets.
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ItemElectronic spectroscopy and photophysics of calix[4]azulene(ELSEVIER SCIENCE SA, 2021-01-15)Calix[4]azulene is a non-alternant aromatic calixarene composed of four azulene chromophores linked by methylene groups. Its photochemical stability, photophysical properties and an analysis of its electronic spectra are reported using monomeric azulene as a known reference standard. The molecule is stable when excited in its visible and near uv absorptions and, unlike azulene, produces no measurable “anti-Kasha” fluorescence when excited to its second excited singlet state, S2. This lack of fluorescence places the lifetime of the initially excited, photochemically stable S2 species at less than 1 picosecond. A significant, bathochromic shift of the S2 absorption band system in the calixarene, and the appearance of an additional weak, broad absorption immediately to the red signals significant intramolecular chromophore interaction. Femtosecond transient absorption spectroscopy using excitation in this red-shifted tail of the S2 band system reveals a very weak transient signal most of which decays within one ps, but with suggestions of a slightly longer-lived underlying component. No longer-lived T1 triplet transient is observed. A complete analysis of the data using monomeric azulene as a reference suggests, following elimination of several alternate mechanisms, that the initially excited S2 species may be relaxing via a novel singlet-singlet fission process.
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ItemUltrafast Nanodrum-on-Chip Pixels(AMER CHEMICAL SOC, 2024-03-15)Environmentally friendly, ultrafast display pixels of micrometer sizes are fabricated with nanometer-thick gold films and Si/SiO2 wafers. The color displayed is due to both the plasmon response of the gold film and the optical interference from the Fabry-Peerot cavity formed by the underlying silicon substrate, the semitransparent gold film and the air gap between them. When an electric potential is applied to the gold film, the electrostatic force induces an attraction between the gold film and the silicon wafer. Due to the flexibility of the film, the size of the air gap changes, resulting in a changing color. By applying different driving signals, we have achieved cyan, magenta, and yellow reflected colors. The maximum switching rate of the pixel is primarily determined by the thickness dependence of the metal drum and its Young's modulus and is typically in the MHz regime.
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ItemNo Preview AvailableFingerprints of Chalcogen Bonding Revealed Through 77Se-NMR.(Wiley, 2024-03-20)77Se-NMR is used to characterise several chalcogen bonded complexes of derivatives of the organoselenium drug ebselen, exploring a range of electron demand. NMR titration experiments support the intuitive understanding that chalcogen bond donors bearing more electron withdrawing substituents give rise stronger chalcogen bonds. The chemical shift of the selenium nucleus is also shown to move upfield as it participates in a chalcogen bond. Solid-state NMR is used to explore chalcogen bonding in co-crystals. Due to the lack of molecular reorientation on the NMR timescale in the solid state, the shape of the chemical shift tensor can be determined using this technique. A range of co-crystals are shown to have extremely large chemical shift anisotropy, which suggests a strongly anisotropic electron density distribution around the selenium atom. A single crystal NMR experiment was conducted using one of the co-crystals, affording the absolute orientation of the chemical shift tensor within the crystal. This showed that the selenium nucleus is strongly shielded in the direction of the chalcogen bond (due to the approach of the lone pair of the Lewis base), and strongly deshielded in the perpendicular direction. The orientation of the deshielded axis is consistent with the presence of a second σ-hole which is not participating in a chalcogen bond, showing the profound effect of electron density anisotropy on the chemical shift.
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ItemNo Preview AvailableDefining the molecular architecture, metal dependence, and distribution of metal-dependent class II sulfofructose-1-phosphate aldolases( 2023-08-08)Sulfoquinovose (SQ or 6-deoxy-6-sulfoglucose) is a sulfosugar that is the anionic head group of plant and cyanobacterial sulfolipids: sulfoquinovosyl diacylglycerols. SQ is produced within photosynthetic tissues, forms a major terrestrial reservoir of biosulfur, and is an important species within the biogeochemical sulfur cycle. A major pathway for the breakdown of SQ is the sulfoglycolytic Embden-Meyerhof-Parnas (sulfo-EMP) pathway, which involves cleavage of the 6-carbon chain of the intermediate sulfofructose-1-phosphate (SFP) into dihydroxyacetone and sulfolactaldehyde, catalyzed by class I or II SFP aldolases. While the molecular basis of catalysis is well studied for class I SFP aldolases, comparatively little is known about class II SFP aldolases. Here, we report the molecular architecture and biochemical basis of catalysis of two metal-dependent class II SFP aldolases from Hafnia paralvei and Yersinia aldovae. 3D X-ray structures in complex with the substrate SFP and product DHAP reveal a dimer-of-dimers (tetrameric) assembly, and identify the sulfonate binding pocket that defines the substrate specificity of these enzymes, two metal binding sites, and flexible loops that are implicated in catalysis. Both enzymes were metal dependent and exhibited high KM values for SFP, consistent with their role in a unidirectional nutrient acquisition pathway. Bioinformatic analysis identified a range of sulfo-EMP gene clusters containing class I/II SFP aldolases. The class I and II SFP aldolases occur exclusively within Actinobacteria and Firmicutes phyla, respectively, while both classes of enzyme occur within Proteobacteria. This work emphasizes the importance of SQ as a nutrient for diverse bacterial phyla and the different chemical strategies they use to harvest carbon from this sulfosugar.
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ItemSpectroscopic study of L-DOPA and dopamine binding on novel gold nanoparticles towards more efficient drug-delivery system for Parkinson's disease(PERGAMON-ELSEVIER SCIENCE LTD, 2022-03-05)Nano-drug delivery systems may potentially overcome current challenges in the treatment of Parkinson's disease (PD) by enabling targeted delivery and more efficient blood-brain penetration ability. This study investigates novel gold nanoparticles (AuNPs) to be used as delivery systems for L-DOPA and dopamine by considering their binding capabilities in the presence and absence of a model protein, bovine serum albumin (BSA). Four different AuNPs were prepared by surface functionalization with polyethylene glycol (PEG), 1-adamantylamine (Ad), 1-adamantylglycine (AdGly), and peptidoglycan monomer (PGM). Fluorescence and UV-Vis measurements demonstrated the strongest binding affinity and L-DOPA/dopamine loading efficiency for PGM-functionalized AuNPs with negligible impact of the serum protein presence. Thermodynamic analysis revealed a spontaneous binding process between L-DOPA or dopamine and AuNPs that predominantly occurred through van der Waals interactions/hydrogen bonds or electrostatic interactions. These results represent PGM-functionalized AuNPs as the most efficient at L-DOPA and dopamine binding with a potential to become a drug-delivery system for neurodegenerative diseases. Detailed investigation of L-DOPA/dopamine interactions with different AuNPs was described here for the first time. Moreover, this study highlights a cost- and time-effective methodology for evaluating drug binding to nanomaterials.
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ItemNo Preview AvailableA simple and ubiquitous device for picric acid detection in latent fingerprints using carbon dots(Royal Society of Chemistry, 2020-05-07)This work addresses the synthetic optimization of carbon dots (CDs) and their application in sensing picric acid from latent fingerprints by exploiting a smartphone-based RGB tool. The optimization of the synthesis of CDs is investigated towards achieving shorter reaction time, better product yield and fluorescence quantum efficiency. Precursors such as citric acid and thiourea were chosen for the synthesis of CDs. Among the various synthetic methodologies, it is found that the pyrolysis method offers ∼50% product yield within 15 min. The morphology and optical properties of the prepared CDs are characterized using the typical microscopic and spectroscopic techniques, respectively. The synthesized CDs exhibit quasi-spherical shape with an average particle size of 1.7 nm. The excitation dependent emissive properties of CDs are investigated by time resolved fluorescence spectroscopy. Furthermore, the excellent fluorescence properties (ϕ = 11%) of CDs are explored as a fluorescent fingerprint powder for the identification of latent fingerprints on various substrates. In addition, the presence of picric acid in latent fingerprints was detected. Furthermore, this study is extended to perform real time detection of fingerprints and harmful contaminants in fingerprints by utilizing a smartphone-based RGB color analysis tool. Based on these investigations, the prepared CDs could be a prospective fluorescent material in the field of forensics.