School of Chemistry - Research Publications

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    Global double hybrids do not work for charge transfer: A comment on "Double hybrids and time-dependent density functional theory: An implementation and benchmark on charge transfer excited states"
    Casanova-Paez, M ; Goerigk, L (Wiley, 2021-03-30)
    We comment on the results published by Ottochian et al. in J. Comput Chem. 2020, 41, 1242. Therein, the authors claim that the second-order, perturbative correlation correction applied to the time-dependent version of the PBE-QIDH global double-hybrid functional approximation (DHDFA) enables the description of charge-transfer (CT) excitations. Herein, we point out some inadvertent oversights related to what had already been previously known and achieved in the field of time-dependent DHDFAs. Exemplified for the same four systems that Ottochian et al. have used to analyze intermolecular CT excitations, we show how a systematic and unacceptably large redshift in global DHDFAs is rectified when using the latest long-range corrected DHDFAs published earlier in J. Chem. Theory Comput. 2019, 15, 4735.
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    Phase Transition Modulation and Defect Suppression in Perovskite Solar Cells Enabled by a Self-Sacrificed Template
    Xiong, Z ; Chen, S ; Zhao, P ; Cho, Y ; Odunmbaku, GO ; Zheng, Y ; Jones, DJ ; Yang, C ; Sun, K (WILEY-V C H VERLAG GMBH, 2021-09)
    Tunable crystal growth offering highly aligned perovskite crystallites with suppressed deep‐level defects is vital for efficient charge transport, which in turn significantly influences the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, a “precursor to perovskite‐like template to perovskite” (PPP) growth strategy is developed, using either MAAc or GuaCl precursor to induce a sacrificial thermal–unstable perovskite‐like template for (FAPbI3)x(MAPbI3)y perovskite growth. The self‐sacrificed intermediate template induces the formation of highly aligned perovskite crystals with greatly enhanced film crystallinity and suppresses deep‐level defect formation. Furthermore, it is proved that MAAc or GuaCl completely evaporates during the high‐temperature annealing process. The reduction in defect densities and nonradiative recombination enhances both carrier lifetime and charge dynamics, yielding impressive PCEs of 22.3% and 22.8% with a high open‐circuit voltage (VOC) of 1.16 V and an incredible fill factor (FF) of 81.5% and 79.4% for MAAc‐ and GuaCl‐based devices, respectively. These results suggest that the formation of the thermal–unstable perovskite‐like sacrificial template is a promising strategy to restrain the deep‐level defects in perovskite films toward the attainment of highly efficient and stable large‐scale PSCs as well as other perovskite‐based electronics.
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    Photophysics and spectroscopy of 1,2-Benzazulene
    Awuku, S ; Bradley, SJ ; Ghiggino, KP ; Steer, RP ; Stevens, AL ; White, JM ; Yeow, C (Elsevier, 2021-12)
    The electronic spectroscopy and photophysics of 1,2-benzazulene (BzAz) have been examined in solution and in thin solid films, with the objective of comparing its intramolecular and intermolecular excited state decay processes with those of azulene. Unlike azulene, the S2 – S0 absorption and fluorescence spectra exhibit a clear mirror image relationship dominated by a single strong Franck-Condon active progression. Picosecond transient absorption spectra and non-linear S2 fluorescence upconversion experiments reveal lifetimes that follow a well-established energy gap law correlation, indicative of a dominant S2 – S1 decay route. Mechanistic interpretations, including the possibility of S2 singlet fission in aggregates, are discussed.
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    A sandwich-like structural model revealed for quasi-2D perovskite films
    Zheng, F ; Hall, CR ; Angmo, D ; Zuo, C ; Rubanov, S ; Wen, Z ; Bradley, SJ ; Hao, X-T ; Gao, M ; Smith, TA ; Ghiggino, KP (Royal Society of Chemistry, 2021-04-28)
    The excellent performance and stability of perovskite solar cells (PSCs) based on quasi-2D Ruddlesden–Popper perovskites (RPPs) holds promise for their commercialization. Further improvement in the performance of 2D PSCs requires a detailed understanding of the microstructure of the quasi-2D perovskite films. Based on scanning transmission electron microscopy (STEM), time-resolved photoluminescence, and transient absorption measurements, a new sandwich-like structural model is proposed to describe the phase distribution of RPPs. In contrast to the conventional gradient distribution, it is found that small-n RPPs are sandwiched between large-n RPP phase layers at the front and back sides owing to crystallization initiated from both interfaces during film formation. This sandwich-like distribution profile facilitates excitons funneling from the film interior to both surfaces for dissociation while free carriers transport via large-n channels that permeate the film to ensure efficient charge collection by the corresponding electrodes, which is favorable for high-performance photovoltaics. This discovery provides a new fundamental understanding of the operating principles of 2D PSCs and has valuable implications for the design and optimization strategies of optoelectronic devices based on quasi-2D RPPs films.
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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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    Incorporating whey protein aggregates produced with heat and ultrasound treatment into rennet gels and model non-fat cheese systems
    Gamlath, CJ ; Leong, TSH ; Ashokkumar, M ; Martin, GJO (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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    Incorporation of Vanadium and Molybdenum into Yttrium-Arsenotungstates Supported by Amino Acid Ligands
    Bagherjeri, FA ; Ritchie, C ; Gable, RW ; Bryant, G ; Boskovic, C (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.
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    Tetrahalocatecholate Rare Earth Complexes: Dinuclear Motifs with Intramolecular RE•••XC(Ar) Interactions
    Rousset, E ; Gable, RW ; Starikova, A ; Boskovic, C (AMER CHEMICAL SOC, 2020-05-06)
    The reaction of yttrium or cerium nitrate and tetrachloro- or tetrabromocatecholate (X4Cat2- with X = Cl or Br) has afforded the compounds (Et3NH)2[{Y(Cl4Cat)(H2O)2}2(μ-Cl4Cat)2]·2MeOH·2H2O (1-Cl), (Et3NH)2[{Y(Br4Cat)(H2O)2}2(μ-Br4Cat)2]·1.5MeCN (1-Br), (Et3NH)4[(Cl4Cat)(H2O)2Y(μ-Cl4Cat)2Y(Cl4Cat)2]·2.5MeOH·3.5H2O (2-Cl), (Et3NH)4[{Y(Cl4Cat)(Cl4CatH)(H2O)}2(μ-Cl4Cat)2]·4H2O (3-Cl), (Et3NH)7[{CeIV(Cl4CatH)(NO3)(μ2-Cl4Cat)3}2CeIII](NO3)2 (4-Cl), and (Et3NH)4[CeIV(X4Cat)4] (5-X with X = Cl, Br). Small variations of the reaction stoichiometry and crystallization methods allow the isolation of dinuclear yttrium complexes with four, five, and six tetrachlorocatecholate ligands in 1-Cl, 2-Cl, and 3-Cl, respectively. Single crystal X-ray diffraction studies of these compounds reveal a conserved tetrachlorocatecholate-bridged dinuclear yttrium core in each case, but with different peripheral ligation. A key feature of the core unit is Y···ClC(Ar) intramolecular interactions with a catecholate chloro substituent ortho to one of the coordinating oxygen atoms. The tetrabromocatecholate analogue 1-Br has also been obtained. Applying similar methods to redox-active cerium, rather than yttrium, instead affords an unusual mixed-valence trinuclear {CeIIICeIV2} complex in 4-Cl, as well as two mononuclear cerium(IV) complexes in 5-Cl and 5-Br. Density functional theory calculations confirm the [CeIV(Cl4Cat)4]4- charge distribution for 5-Cl.
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    Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes
    Gransbury, GK ; Livesay, BN ; Janetzki, JT ; Hay, MA ; Gable, RW ; Shores, MP ; Starikova, A ; Boskovic, C (AMER CHEMICAL SOC, 2020-06-17)
    Valence tautomerism (VT) involves a reversible stimulated intramolecular electron transfer between a redox-active ligand and redox-active metal. Bis(dioxolene)-bridged dinuclear cobalt compounds provide an avenue toward controlled two-step VT interconversions of the form {CoIII-cat-cat-CoIII} ⇌ {CoIII-cat-SQ-CoII}⇌{CoII-SQ-SQ-CoII} (cat2- = catecholate, SQ•- = semiquinonate). Design flexibility for dinuclear VT complexes confers an advantage over two-step spin crossover complexes for future applications in devices or materials. The four dinuclear cobalt complexes in this study are bridged by deprotonated 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol (spiroH4) or 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol (Br4spiroH4) with Mentpa ancillary ligands (tpa = tris(2-pyridylmethyl)amine, n = 0-3 corresponds to methylation of the 6-position of the pyridine rings). Complementary structural, magnetic, spectroscopic, and density functional theory (DFT) computational studies reveal different electronic structures and VT behavior for the four cobalt complexes; one-step one-electron partial VT, two-step VT, incomplete VT, and temperature-invariant {CoIII-cat-cat-CoIII} states are observed. Electrochemistry, DFT calculations, and the study of a mixed-valence {ZnII-cat-SQ-ZnII} analog have allowed elucidation of thermodynamic parameters governing the one- and two-step VT behavior. The VT transition profile is rationalized by (1) the degree of electronic communication within the bis(dioxolene) ligand and (2) the matching of cobalt and dioxolene redox potentials. This work establishes a clear path to the next generation of two-step VT complexes through incorporation of mixed-valence class II and class II-III bis(dioxolene) bridging ligands with sufficiently weak intramolecular coupling.
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    Switching metal complexes via intramolecular electron transfer: connections with solvatochromism
    Nadurata, VL ; Boskovic, C (ROYAL SOC CHEMISTRY, 2021-04-07)
    Metal complexes that can exist in two different charge distributions often exhibit dramatic color changes when switched between them. The underlying spectral changes are fundamentally related to the switchable behavior. In valence tautomeric (VT) systems, the transition is a stimulated, reversible intramolecular electron transfer between a metal center and a ligand, while in charge-transfer-induced-spin-transition (CTIST), also known as electron-transfer-coupled-spin-transition (ETCST), systems, the electron is transferred between two metal centers. We discuss, herein, the relationships between the switchable behavior of these systems and two related optical phenomena: charge transfer and solvatochromism. The insights gained from analyzing these phenomena can illuminate important aspects of VT or CTIST behavior, for example, the energetic relationship between the electromeric forms, or the effects of molecular environment on a VT or CTIST thermal equilibrium. Such insights may assist efforts to employ these compounds as molecular scale components in data storage, sensor and display devices.