Photophysics of Benzoxazole and Dicyano Functionalised Diketopyrrolopyrrole Derivatives: Insights into Ultrafast Processes and the Triplet State.

Diketopyrrolopyrrole (DPP) functionalised with an electron donating unit acts as a donor-acceptor molecules that have shown potential for application in dyes and photovoltaics. These molecules offer broad absorption/emission properties and structure-dependent dynamics. In this study, we used femtosecond pump-probe spectroscopy to investigate the photo-initiated dynamics of thiophene linked DPP derivatives. The thio-DPPs are further functionalised by different electrons withdrawing terminal groups, namely benzoxazole and thiophene dicyanide. The benzoxazole derivative is strongly emissive and directly relaxes directly to the ground state chloroform solution. Thiophene dicyanide derivative exhibits distinct spectral evolution in the first 10 ps, associated with structural and vibronic process. Later, it crosses over to the triplet state with a yield of 20 %. In the solid-state (thin film), we observed a signal that resembles singlet fission. However, upon careful analysis of temperature-dependent steady state absorbance spectra, we conclude that these features are due to laser-induced thermal artifacts. We describe a simplified excited state evolution in the thin film that does not include any additional excited states. These findings have significant implications for the analysis of triplet formation, which plays a major role in the photophysics of many organic materials.

Compact oblique-incidence nonlinear widefield microscopy with paired-pixel balanced imaging.

Nonlinear (vibrational) microscopy has emerged as a successful tool for the investigation of molecular systems as it combines label-free chemical characterization with spatial resolution on the sub-micron scale. In addition to the molecular recognition, the physics of the nonlinear interactions allows in principle to obtain structural information on the molecular level such as molecular orientations. Due to technical limitations such as the relatively complex imaging geometry with the required oblique sample irradiation and insufficient sensitivity of the instrument this detailed molecular information is typically not accessible using widefield imaging. Here, we present, what we believe to be, a new microscope design that addresses both challenges. We introduce a simplified imaging geometry that enables the measurement of distortion-free widefield images with free space oblique sample irradiation achieving high spatial resolution (∼1 µm). Furthermore, we present a method based on a paired-pixel balanced detection system for sensitivity improvement. With this technique, we demonstrate a substantial enhancement of the signal-to-noise ratio of up to a factor of 10. While both experimental concepts presented in this work are very general and can, in principle, be applied to various microscopy techniques, we demonstrate their performance for the specific case of heterodyned, sum frequency generation (SFG) microscopy.

Density functional theory based indicators to predict the corrosion inhibition potentials of ceramic oxides in harsh corrosive media.

Coating metal surfaces with ceramic oxides is an experimentally established technique to curb the corrosion of metals. Herein, we used periodic spin-polarized density functional theory (DFT) to study the ceramic oxides Al2O3, TiO2, HfO2 and ZrO2 for their corrosion-inhibition potentials under different harsh corrosive conditions. The adsorption of corrosive atoms on ceramic oxide surfaces is analyzed using DFT-computed indicators such as binding energies, Bader charges, projected density of states (pDOS), and geometric considerations. Adsorption is carried out on the energetically most favorable sites on the metal oxide slabs. Our DFT calculations predict the experimentally observed trends of the ceramic oxides reported in the literature in a chlorine-rich (saline) medium, which was ZrO2 ∼ HfO2 > TiO2 > Al2O3. The computational model is then applied to test the performance of the ceramic oxides as protective layers in sulfur-rich and oxidizing harsh environments. Such a comprehensive DFT-based comparative analysis to predict the corrosion-inhibition potential of ceramic oxides is established for the first time to the best of our knowledge. This easy-to-use computational approach can be widely utilized to gain first-hand information on the anti-corrosion potentials of ceramic oxides and alloys without creating different corrosive conditions experimentally.

Excited-State Evolution of Keto-Carotenoids after Excess Energy Excitation in the UV Region.

Carotenoids are molecules with rich photophysics that are in many biological systems involved in photoprotection. Yet, their response to excess energy excitation is only scarcely studied. Here we have explored excited state properties of three keto-carotenoids, echinenone, canthaxanthin and rhodoxanthin after excess energy excitation to a singlet state absorbing in UV. Though the basic spectral features and kinetics of S2 , hot S1 , relaxed S1 states remain unchanged upon UV excitation, the clear increase of the S* signal is observed after excess energy excitation, associated with increased S* lifetime. A multiple origin of the S* signal, originating either from specific conformations in the S1 state or from a non-equilibrated ground state, is confirmed in this work. We propose that the increased amount of energy stored in molecular vibrations, induced by the UV excitation, is the reason for the enhanced S* signal observed after UV excitation. Our data also suggest that a fraction of the UV excited state population may proceed through a non-sequential pathway, bypassing the S2 state.

The Operating Cycle of NO Adsorption and Desorption in Pd-Chabazite for Passive NOx Adsorbers.

Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NOx in the target temperature range for application as a passive NOx adsorber (PNA). The ability of Pd/CHA to trap NOx emissions at low temperatures (<200 °C) is facilitated by the binding of NOx species at various Pd sites available in the CHA framework. Density functional theory (DFT) simulations are performed to understand Pd speciation in CHA and the interaction of NO with Pd/CHA to explain the mechanisms of NO adsorption, oxidation, and desorption processes. The calculations are used to elucidate the important role of Pd1+ cationic species, anchored at 6MR-3NN, in providing a strong (Eb = -272 kJ/mol) NO adsorption site in Pd/CHA. For NO release, the redox transformation of Pd species comes into play and Pd1+ species are suggested to transform into cationic Pd2+, [PdOH]+, or [Pd-O-Pd]2+ species, all of which show significantly reduced NO binding (-116, -153, and -117 kJ/mol, respectively) as compared to Pd1+. This enables NO desorption at the operating temperature of a downstream catalyst for subsequent catalytic reduction.

Photophysics of deinoxanthin, the keto-carotenoid bound to the main S-layer unit of Deinococcus radiodurans.

The keto-carotenoid deinoxanthin, which occurs in the UV-resistant bacterium Deinococcus radiodurans, has been investigated by ultrafast time-resolved spectroscopy techniques. We have explored the excited-state properties of deinoxanthin in solution and bound to the S-layer Deinoxanthin Binding Complex (SDBC), a protein complex important for UV resistance and thermostability of the organism. Binding of deinoxanthin to SDBC shifts the absorption spectrum to longer wavelengths, but excited-state dynamics remain unaffected. The lifetime of the lowest excited state (S1) of isolated deinoxanthin in methanol is 2.1 ps. When bound to SDBC, the S1 lifetime is 2.4 ps, indicating essentially no alteration of the effective conjugation length upon binding. Moreover, our data show that the conformational disorder in both ground and excited states is the same for deinoxanthin in methanol and bound to SDBC. Our results thus suggest a rather loosely bound carotenoid in SDBC, making it very distinct from other carotenoid-binding proteins such as Orange Carotenoid Protein (OCP) or crustacyanin, both of which significantly restrain the carotenoid at the binding site. Three deinoxanthin analogs were found to bind the SDBC, suggesting a non-selective binding site of deinoxanthin in SDBC.

Understanding the Nature of Amino Acid Interactions with Pd(111) or Pd-Au Bimetallic Catalysts in the Aqueous Phase.

The interaction of methionine (Met) with different bimetallic-segregated surfaces comprising a uniform distribution of strips and islands of Au on the Pd(111) surface was examined using molecular dynamics (MD) simulations. Out of all the segregated and uniformly doped surfaces studied, the design of Pd-Au islands showed some reduction in the interaction energy (Eint = -43.7 kJ/mol) as compared to that of the pure Pd(111) surface (Eint = -50 kJ/mol) for a single Met molecule. However, at a higher coverage of 9 Met molecules/simulation cell, none of the Pd-Au alloy surfaces showed any improvement as compared to the Pd(111) surface. In order to develop a comprehensive understanding of the nature of the nonbonded interaction of aqueous biogenic impurities with the Pd catalyst surface, the MD study was extended to include a variety of aliphatic, S-containing, aromatic, and polar amino acids. The potential of mean force (PMF) profiles were observed to be distinct for each class of amino acids with substantial differences among amino acids with acidic and basic side chains. The side chains of all the polar and aromatic amino acids showed direct contact with the surface while aliphatic amino acids had their hydrophobic side chain aligned away from the surface. Interestingly, lysine (Lys) and tyrosine (Tyr) were the only two amino acids which interacted preferentially via the distant backbone nitrogen and backbone oxygen, respectively, despite their side chains being in direct contact with the metal surface. The strength of interaction was correlated with the size of the amino acid; the interaction energies were observed to be the maximum for large molecules such as arginine (Arg, Eint = -87.7 kJ/mol) and tryptophan (Trp, Eint = -73.4 kJ/mol), while it was a minimum for aliphatic amino acids such as alanine (Ala, Eint = -10.9 kJ/mol). The study is focused on examining the sensitivity of the choice of the preferential interaction site, conformational preferences, and interaction energies to the side-chain specificity.

Enhanced fluorescence with nanosecond dynamics in the solid state of metal ion complexes of alkoxy salophens.

Salophen (N,N'-bis-(salicylidene)-o-phenylenediamine) is a Schiff base, which is weakly emissive in solution, but strongly emissive in the solid state. The trend is reversed in its aluminium complex (SalAl+), which is strongly emissive in solution, but feebly emissive in the solid state. Thus, SalAl+ is not attractive for application as a solid state emitter. In the present study, salophen derivatives containing -OCH3 and OC2H5 groups have been studied with the intent to perturb the structure and control the emission properties. The newly synthesized salophen derivatives are feebly emissive with ultrafast lifetime in solution. The mutiexponential nature of their fluorescence decay is ascribed to multiple ground state conformers of the enol and keto tautomers. In the solid phase, the ligands exhibit an order of magnitude increase in emission. Complexation with Al3+ (and Zn2+) does not bring about any significant increase in emissivity in solution, possibly because rotation/vibration of the alkoxy groups (with a contribution from axial water molecules) now provides the major nonradiative deactivation pathway. Such pathways are blocked in the solid state, leading to an increase in emission of up to 50 times in the aluminium complex. Thus, introduction of small functionalities is found to have profound effects on the suitability of a molecule as a solid state emitter.

Gold(I)-Thiolate Coordination Polymers as Multifunctional Materials: The Case of Au(I)-p-Fluorothiophenolate.

Gold-sulfur interaction has vital importance in nanotechnologies and material chemistry to design functional nanoparticles, self-assembled monolayers, or molecular complexes. In this paper, a mixture of only two basic precursors, such as the chloroauric acid (HAu(III)Cl4) and a thiol molecule (p-fluorothiophenol (p-HSPhF)), are used for the synthesis of gold(I)-thiolate coordination polymers. Under different conditions of synthesis and external stimuli, five different functional materials with different states of [Au(I)(p-SPhF)]n can be afforded. These gold-thiolate compounds are (i) red emissive, flexible, and crystalline fibers; (ii) composite materials made of these red emissive fibers and gold nanoparticles; (iii) amorphous phase; (iv) transparent glass; and (v) amorphous-to-crystalline phase-change material associated with an ON/OFF switch of luminescence. The different functionalities of these materials highlight the great versatility of the gold(I) thiolate coordination polymers with easy synthesis and diverse shaping that may have great potential as sustainable phosphors, smart textiles, sensors, and phase change memories.

Improving the Coke Resistance of Ni-Ceria Catalysts for Partial Oxidation of Methane to Syngas: Experimental and Computational Study.

The synthesis of syngas (H2 : CO=2) via catalytic partial oxidation of methane (CPOM) is studied over noble metal doped Ni-CeO2 bimetallic catalysts for CPOM reaction. The catalysts were synthesized via a controlled deposition approach and were characterized using XRD, BET-surface area analysis, H2 -TPR, TEM, Raman and TGA analysis. The catalysts were experimentally and computationally studied for their activity, selectivity, and long-term stability. Although the pure 5Ni/CeO2 catalyst showed high initial activity (∼90%) of CH4 conversion, it rapidly deactivates around 20% of its initial activity within 140 hours of TOS. Doping of Ni/CeO2 catalyst with noble metal was found to be coke resistant with the best-performing Ni-Pt/CeO2 catalyst showed ∼95% methane conversion with >90% selectivity at a temperature of 800 °C, having exceptional stability for about 300 hours of time-on-stream (TOS). DFT studies were performed to calculate the activation barrier for the C-H activation of methane over the Ni, Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces showed nearly equal activation energy over all the studied surfaces. DFT studies showed high coke formation tendency of the pure Ni (111) having a very small C-C coupling activation barrier (14.2 kJ/mol). In contrast, the Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces show appreciably higher C-C coupling activation barrier (∼70 kJ/mol) and hence are more resistant against coke formation as observed in the experiments. The combined experimental and DFT study showed Ni-Pt/CeO2 as a promising CPOM catalyst for producing syngas with high conversion, selectivity and long-term stability suited for future industrial applications.

Boosting the methanol selectivity in CO2 hydrogenation over a MOF-derived CuZn@CN catalyst via Rb incorporation.

The highly selective hydrogenation of CO2 to methanol has been achieved through the simultaneous utilization of alkali metals and Co as promoters over Cu-Zn@CN catalysts derived from MOF. Rb facilitates the dissociation of CO2 in the aqueous phase at relatively mild conditions to yield methanol with a selectivity of 89%.

Highly Efficient ZIF-67-Derived PtCo Alloy-CN Interface for Low-Temperature Aqueous-Phase Fischer-Tropsch Synthesis.

Designing new materials for selective Fischer-Tropsch synthesis (FTS) is an elegant way to enhance local feedstock utilization like biomass and waste. In this approach, we have designed a thermally and chemically stable bimetallic PtCo/NC hybrid nanocomposite catalyst derived from a zeolitic imidazolate framework (ZIF-67, which contains cobalt as a metal center) through carbonization for low-temperature (413-473 K) aqueous-phase Fischer-Tropsch synthesis (AFTS). The selectivity of the desired range of hydrocarbons is adjusted using a highly dispersed PtCo bimetallic alloy, which facilitates extraordinary reduction of a metal oxide to active species by the synergic effect under the AFTS reaction conditions. The ZIF-derived catalyst tested in this study exhibited the highest activity to date for very low temperatures (433 K) in aqueous-phase Fischer-Tropsch synthesis with CO conversion rates between 0.61 and 1.20 molCO·molCo-1·h-1. Insights of the remarkable catalyst activity were examined by in situ X-ray photoelectron spectroscopy (XPS) studies corroborated by density functional theory (DFT) calculation. The bimetallic Co3Pt (111) surface was found to be highly active for the C-C coupling reaction between surface-adsorbed C and CO, forming a CCO intermediate with a very low activation barrier (Ea = 0.37 eV), in comparison to the C-C coupling activation barrier obtained over the Co (111) surface (Ea = 0.87 eV). This unique approach and observations create a new path for developing next-generation advanced catalyst systems and processes for selective low-temperature FTS.

Adsorptive removal of multiple organic dyes from wastewater using regenerative microporous carbon: Decisive role of surface-active sites, charge and size of dye molecules.

Present work addresses the synthesis of microporous activated carbon (SDAC) by a facile thermochemical conversion of teak sawdust powder. The high surface area (1999 m2 g-1), excellent microporosity (average pore size: 2.62 nm), and turbostratic carbon structure with intertwined graphitic domains make SDAC a highly efficient adsorptive material for the removal of organic pollutants. The spectroscopic analyses (FTIR, Raman, and XPS) and adsorption locator calculations revealed multiple interactions between organic dyes and SDAC adsorbent, i.e., electrostatic, π-π, n-π interactions, and hydrogen linkages. The size, chemical functionalities, aromatic rings, electronegative and heteroatoms in dye molecules, along with the surface-active sites, microstructured and textural features of SDAC adsorbent collectively governed the interaction pathways and adsorption efficiency. The calculated adsorption energy using Monte Carlo-based simulation annealing method signified faster and higher adsorption of malachite green than methylene blue dye at surface-active sites (-COOH, CO, C-OH, and π-electron-rich domains) of SDAC adsorbent, corroborating the experimental results. The batch-mode adsorptive separation results showed remarkably high adsorption efficiency (>99%) for industrial wastewater to remove cationic and anionic dyes together. The SDAC displayed significantly high adsorption of methylene blue dye (625 mg.g-1) with excellent recyclability without measurable loss of adsorption efficiency even after ten cycles. The SDAC fixed-bed column showed a dye removal capacity of 594 mg.g-1 at 90% breakthrough in a continuous-mode process signifying its applicability for a real-time industrial run. The excellent conformity between batch mode and fixed bed continuous column adsorption data, along with higher removal capacity and remarkable recyclability, promise the use of SDAC adsorbent for industrial wastewater treatment to remove multiple organic pollutants.

Highly regenerative, fast colorimetric response for organo-toxin and oxo-anions in an aqueous medium using a discrete luminescent Cd(II) complex in a heterogeneous manner with theoretical revelation.

The highly luminescent complex [CdQ2(H2O)2] (1) shows ultra-selectivity and high sensitivity to the explosive organo-toxin trinitrophenol (TNP). This detection is extremely fast with a high quenching constant (5.3 × 104 M-1) and a very low limit of detection (LOD) of 137 nM/59 ppb. This motivated us to detect the lethal carcinogenic arsenical drug roxarsone (ROX), which is reported here for the first time. The quenching constant and LOD for ROX using 1 were found to be 4.9 × 104 M-1 and 86 nM (or 37 ppb), respectively. Moreover, the probe also recognizes three lethal toxic oxo-anions (MnO4-, Cr2O72- and CrO42-) with outstanding quenching constant (2.2 × 104 M-1, 1.4 × 104 M-1 and 1.1 × 104 M-1) and very low LODs (141 nM/61 ppb, 178 nM/78 ppb and 219 nM/95 ppb). Compared to the previously reported homogeneous sensing nature of the discrete complexes, our complex showed the detection of toxic pollutants in a heterogeneous manner, which results in high recyclability and hence multi-cycle sensing capability. Interestingly, 1 shows the possibility for real-time monitoring through naked eye detection by visible colorimetric changes in solid, solution and strip paper methods, i.e., triphasic detection ability. In addition, the sensor also exhibited the cross-sensing ability for these pollutants. The experimental sensing mechanism is strongly supported by the exhaustive theoretical investigation. Based on the fluorescence signal shown by each analyte, an integrated AND-OR logic gate is constructed. Furthermore, the sensing ability of 1 remains intact towards the detection of versatile real field samples including lethal carcinogenic arsenical drug roxarsone in the real food sample.

Unravelling faradaic electrochemical efficiencies over Fe/Co spinel metal oxides using surface spectroscopy and microscopy techniques.

Cobalt and iron metal-based oxide catalysts play a significant role in energy devices. To unravel some interesting parameters, we have synthesized metal oxides of cobalt and iron (i.e. Fe2O3, Co3O4, Co2FeO4 and CoFe2O4), and measured the effect of the valence band structure, morphology, size and defects in the nanoparticles towards the electrocatalytic hydrogen evolution reaction (HER), the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). The compositional variations in the cobalt and iron precursors significantly alter the particle size from 60 to <10 nm and simultaneously the shape of the particles (cubic and spherical). The Tauc plot obtained from the solution phase ultraviolet (UV) spectra of the nanoparticles showed band gaps of 2.2, 2.3, 2.5 and 2.8 eV for Fe2O3, Co3O4, Co2FeO4 and CoFe2O4, respectively. Further, the valence band structure and work function analysis using ultraviolet photoelectron spectroscopy (UPS) and core level X-ray photoelectron spectroscopy (XPS) analyses provided better structural insight into metal oxide catalysts. In the Co3O4 system, the valence band structure favors the HER and Fe2O3 favors the OER. The composites Co2FeO4 and CoFe2O4 show a significant change in their core level (O 1s, Co 2p and Fe 2p spectra) and valence band structure. Co3O4 shows an overpotential of 370 mV against 416 mV for Fe2O3 at a current density of 2 mA cm-2 for the HER. Similarly, Fe2O3 shows an overpotential of 410 mV against the 435 mV for Co3O4 at a current density of 10 mA cm-2 for the OER. However, for the ORR, Co3O4 shows 70 mV improvement in the half-wave potential against Fe2O3. The composites (Co2FeO4 and CoFe2O4) display better performance compared to their respective parent oxide systems (i.e., Co3O4 and Fe2O3, respectively) in terms of the ORR half-wave potential, which can be attributed to the presence of the oxygen vacancies over the surface in these systems. This was further corroborated in density functional theory (DFT) simulations, wherein the oxygen vacancy formation on the surface of CoFe2O4(001) was calculated to be significantly lower (∼50 kJ mol-1) compared to Co3O4 (001). The band diagram of the nanoparticles constructed from the various spectroscopic measurements with work function and band gap provides in-depth understanding of the electrocatalytic process.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent.

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F- and H2PO4- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and 1H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.

Unravelling the reactivity of metastable molybdenum carbide nanoclusters in the C-H bond activation of methane, ethane and ethylene.

C-H bond activation steps in non-oxidative methane dehydroaromatization (MDA), constitute a key functionalization of the reactant and adsorbed species to form aromatics. Previous studies have focused on studying the energetics of these steps at the most stable active sites involving molybdenum carbide species. Herein, a different paradigm is presented via studying the reactivity of a metastable molybdenum carbide (Mo2C6) nanocluster for the C-H bond activation of methane, ethane, and ethylene and comparing it with the reactivity of the lowest energy Mo2C6 nanocluster. Interestingly, the metastable nanocluster is observed to result in a consistent reduction (by half) in the C-H bond activation barrier of the respective alkane and alkene molecules compared to the global minimum isomer. This specific metastable form of the nanocluster is identified from a cascade genetic algorithm search, which facilitated a rigorous scan of the potential energy surface. We attribute this significant lowering of the C-H bond activation barrier to unique co-planar orbital overlap between the reactant molecule and active centers on the metastable nanocluster. Based on geometrical and orbital analysis of the transition states arising during the C-H bond activation of methane, ethane, and ethylene, a proton-coupled electron transfer mechanism is proposed that facilitated C-H bond cleavage. Motivated by the high reactivity for C-H bond activation observed on the metastable species, a contrasting framework to analyze the elementary-step rate contributions is presented. This is based on the statistical ensemble analysis of nanocluster isomers, where the calculated rates on respective isomers are normalized with respect to the Boltzmann probability distribution. From this framework, the metastable isomer is observed to provide significant contributions to the ensemble average representations of the rate constants calculated for C-H bond activation during the MDA reaction.

Effect of substituents and promoters on the Diels-Alder cycloaddition reaction in the biorenewable synthesis of trimellitic acid.

An efficient route to produce oxanorbornene, a precursor for the production of bio-based trimellitic acid (TMLA) via the Diels-Alder (DA) reaction of biomass-derived dienes and dienophiles has been proposed by utilizing density functional theory (DFT) simulations. It has been suggested that DA reaction of dienes such as 5-hydroxymethyl furfural (HMF), 2,5-dimethylfuran (DMF), furan dicarboxylic acid (FDCA) and biomass-derived dienophiles (ethylene derivatives e.g., acrolein, acrylic acid, etc.) leads to the formation of an intermediate product oxanorbornene, a precursor for the production of TMLA. The activation barriers for the DA reaction were correlated to the type of substituent present on the dienes and dienophiles. Among the dienophiles, acrolein was found to be the best candidate showing a low activation energy (<40 kJ mol-1) for the cycloaddition reaction with dienes DMF, HMF and hydroxy methyl furoic acid (HMFA). The FMO gap and (IPdiene + EAdienophile)/2 were both suggested to be suitable descriptors for the DA reaction of electron-rich diene and electron-deficient dienophile. Further solvents did not have a significant effect on the activation barrier for DA reaction. In contrast, the presence of a Lewis acid was seen to lower the activation barrier due to the reduction in the FMO gap.

Excited-State Properties of Canthaxanthin in Cyanobacterial Carotenoid-Binding Proteins HCP2 and HCP3.

Recently a new family of carotenoproteins, homologues of the N-terminal domain of the orange carotenoid protein (NTD-OCP), have been identified in cyanobacteria. These homologues are called helical carotenoid proteins (HCPs) as they are all predicted to maintain the all-helical structure of the NTD-OCP and to bind carotenoids. Here, HCP2 and HCP3 isolated from the cyanobacterium Tolypothrix PCC 7601 were studied by ultrafast transient absorption spectroscopy to explore the excited-state dynamics of the bound carotenoid, canthaxanthin. The lowest excited state, S1, of canthaxanthin in both HCPs yields a lifetime of 3.5 ps; it is thus shorter than for canthaxanthin in solution (4.5 ps). This is because of the longer effective conjugation of canthaxanthin in HCPs, as one of the terminal rings is in an s-trans configuration. Use of two different excitation wavelengths, 470 and 570 nm, revealed excitation wavelength dependent spectroscopic response. Additional excited-state absorption bands are observed after excitation at 470 nm for both HCPs, proving the presence of more than one ground state conformer.

Identifying the Origin of the Limiting Process in a Double Perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells.

Oxygen reduction reaction in a double perovskite material, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), was studied for application as a cathode in a solid oxide fuel cell (SOFC). Electrochemical measurements were performed on a geometrically well-defined dense thin-film (0.8-2 µm thickness) electrode, fabricated as a symmetric cell. In combination with density functional theory (DFT) and molecular dynamics (MD) simulations, experiments provided an insight into the operating mechanism of the SOFC material tested at an open-circuit voltage. The dense thin-film electrode of PBSCF showed a thickness-dependent electrochemical performance, suggesting bulk diffusion limitation. To understand the origin of this diffusion-limiting electrochemical performance, DFT calculations were utilized to calculate the surface (γ) and oxygen vacancy formation (EOV) energies. For example, EOV in the Pr plane (190 kJ/mol) of PBSCF was measured to be lower than that of the BaSr plane (EOV = 297 kJ/mol). In addition, oxygen vacancies were difficult to be created in the BaSr/CoFe terminal surface (EOV = 341.6 kJ/mol) as compared to other terminal surfaces. MD simulations further elaborated on the nature of cation disordering in the surface and subsurface regions, consequently leading to the preferential segregation of the Ba cations to the surface, which is a known phenomenon in such double perovskite materials. Because of cation disordering and segregation of Ba species, the oxygen anion diffusivity (∼10-12 cm2 s-1), calculated from MD, in the near-surface region was observed to be 2 orders of magnitude lesser than that of the bulk (D = 2.98 × 10-10 cm2 s-1) of the material at 973 K. Surface characterization of the thin-film electrode using X-ray photoelectron spectroscopy was indicative of a nonperovskite Ba2+ phase on the electrode surface. The segregation of Ba cations was linked with the transport of oxygen anions, which was limiting the electrochemical performance of the electrode.