Deactivation Path Research Articles

Variation of Char Reactivity during Catalytic Gasification with Steam: Comparison among Catalytic Gasification by Ion-Exchangeable Na, Ca, and Na/Ca Mixture

The reactivity profile of char during catalytic gasification is crucial for designing and optimizing the gasification process, and it is greatly affected by the types and changes in activity of the catalyst during gasification. The catalytic gasification of Na-Char, Ca-Char, and Na/Ca-Char mixtures with different concentrations of steam was conducted within the temperature range of 700–900 °C using a microfluidized bed reaction analyzer. The results indicate that the reactivity of Na-Char is always higher than that of Ca-Char during the initial gasification stage (0–Xi) and then lower than that of Ca-Char in the later stage. The observations are mainly attributed to the different deactivation paths for the Na and Ca catalysts. The drastic loss of Na during gasification corresponds well to the sharp decrease of Na-Char reactivity within the initial carbon conversion, and the gradual change of the Ca dispersion contributes to the deactivation of the Ca catalyst. It is also demonstrated that the catalysis of...

Experimental and spin-orbit coupled TDDFT predictions of photophysical properties of three-coordinate mononuclear and four-coordinate binuclear copper(I) complexes with thioamidines and bulky triarylphosphanes

Abstract The reactions of copper halides CuX with the N-heterocyclic thioamidine 5-methyl-1,3,4-thiadiazole-2-thione (mtdztH) in the presence of the sterically demanding tri(o-tolyl)phosphane (totp), depending on the halide, affords either the mononuclear compound [CuX(totp)(mtdztH)] (1), when X = I, with the thione acting as a terminal S-bound ligand or the symmetrical, thione-S-bridged binuclear compound [CuX(totp)(μ-S-mtdzt)]2 (2), when X = Cl. The steric effects of the large cone-angle phosphane ligand are further demonstrated by the distorted coordination environments of the copper centers in the solid-state structures of the two complexes. The two compounds are photoluminescent in the solid state at ambient temperature, with their emission maxima located in the blue region of the electromagnetic spectrum and influenced by the type of halide present in each case. Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations were employed in order to elucidate the mechanism of the photoluminescence of the two Cu(I) complexes. The emissive T1 state exhibits a 3MLCT character and minor structural distortions compared to the S0 state. Employing self-consistent spin-orbit coupling TDDFT (SOC-TDDFT) calculations the radiative lifetimes and the zero-field splitting (ZFS) were estimated to be in the ranges 21–375 μs and 1–4 cm−1 respectively. The highest values for the Spin - Orbit matrix elements, calculated with the SOC-TDDFT method, are the and for 1 and the mononuclear counterpart of 2 (2′) indicating a thermal singlet–singlet deactivation path followed by ISC from S1 to T1. In contrast, the highest value SOC matrix element for 2 is the and therefore this complex should exhibit a direct ISC from S1 to T1 without a preceding thermal deactivation. The very small values of the energy difference between S1 and T1 states (ΔE(S1-T1)) calculated to be in the range 1–9 kcal/mol for these complexes, should also favor the ISC process between these excited states.

Theoretical insights into the photo-protective mechanisms of natural biological sunscreens: building blocks of eumelanin and pheomelanin.

Eumelanin (EM) and pheomelanin (PM) are ubiquitous in mammalian skin and hair--protecting against harmful radiation from the sun. Their primary roles are to absorb solar radiation and efficiently dissipate the excess excited state energy in the form of heat without detriment to the polymeric structure. EU and PM exist as polymeric chains consisting of exotic arrangements of functionalised heteroaromatic molecules. Here we have used state-of-the-art electronic structure calculations and on-the-fly surface hopping molecular dynamics simulations to study the intrinsic deactivation paths of various building blocks of EU and PM. Ultrafast excited state decay, via electron-driven proton transfer (in EU and PM) and proton-transfer coupled ring-opening (in PM) reactions, have been identified to proceed along hitherto unknown charge-separated states in EU and PM oligomers. These results shed light on the possible relaxation pathways that dominate the photochemistry of natural skin melanins. Extrapolation of such findings could provide a gateway into engineering more effective molecular constituents in commercial sunscreens--with reduced phototoxicity.

Photoprotection Mechanism of p-Methoxy Methylcinnamate: A CASPT2 Study.

p-Methoxy methylcinnamate (p-MMC) shares the same molecular skeleton with octyl methoxycinnamate sunscreen. It is recently found that adding one water to p-MMC can significantly enhance the photoprotection efficiency. However, the physical origin is elusive. Herein we have employed multireference complete active space self-consistent field (CASSCF) and multistate complete active-space second-order perturbation (MS-CASPT2) methods to scrutinize the photophysical and photochemical mechanism of p-MMC and its one-water complex p-MMC-W. Specifically, we optimize the stationary-point structures on the (1)ππ*, (1)nπ*, and S0 potential energy surfaces to locate the (1)ππ*/S0 and (1)ππ*/(1)nπ* conical intersections and to map (1)ππ* and (1)nπ* excited-state relaxation paths. On the basis of the results, we find that, for the trans p-MMC, the major (1)ππ* deactivation path is decaying to the dark (1)nπ* state via the in-plane (1)ππ*/(1)nπ* crossing point, which only need overcome a small barrier of 2.5 kcal/mol; the minor one is decaying to the S0 state via the (1)ππ*/S0 conical intersection induced by out-of-plane photoisomerization. For the cis p-MMC, these two decay paths are comparable (1)ππ* deactivation paths: one is decaying to the dark (1)nπ* state via the (1)ππ*/(1)nπ* crossing point, and the second is decaying to the ground state via the (1)ππ*/S0 conical intersection. One-water hydration stabilizes the (1)ππ* state and meanwhile destabilizes the (1)nπ* state. As a consequence, the (1)ππ* deactivation path to the dark (1)nπ* state is heavily inhibited. The related barriers are increased to 5.8 and 3.3 kcal/mol for the trans and cis p-MMC-W, respectively. In comparison, the barriers associated with the photoisomerization-induced (1)ππ* decay paths are reduced to 2.5 and 1.3 kcal/mol for the trans and cis p-MMC-W. Therefore, the (1)ππ* decay paths to the S0 state are dominant relaxation channels when adding one water molecule. Finally, the present work contributes a lot of knowledge to understanding the photoprotection mechanism of methylcinnamate derivatives.

Computational Insights into Excited-State Proton-Transfer Reactions in Azo and Azomethine Dyes.

State of the art density functional theory approaches are employed to provide an accurate description of the photophysical properties of azodyes and Schiff bases displaying intramolecular hydrogen-bonding features. These compounds exist as tautomeric mixtures at the ground state and, in the case of Schiff bases, an excited-state intramolecular proton transfer (ESIPT) occurs upon excitation. The experimentally observed photophysical properties are discussed here in light of the theoretical findings. To rationalize the different experimentally observed radiative behavior of the azo and azomethine structures, a nonradiative decay pathway that is possibly active in such systems is determined. The characterization of this deactivation path, tested for two related compounds exhibiting different fluorescence quantum yields, enables us to disentangle the different and contrasting effects governing the excited-state behavior of these molecular systems.

Photophysical properties of betaxanthins: Vulgaxanthin I in aqueous and alcoholic solutions

Betaxanthins are yellow pigments present in Caryophyllales plants and some higher fungi. We characterize photophysical properties of vulgaxanthin I and extracts of Amanita muscaria L. Vulgaxanthin I photoexcitation at λexc=476nm leads to the S1 excited state with the S1→Sn absorption bands at ca. 390 and 920nm in both aqueous and alcoholic solutions. The S1 state lifetimes (2.9 and 37ps in water) imply that vulgaxanthin I exists in two stereoisomeric forms. An increase in the solvent viscosity extends the S1 lifetimes and fluorescence quantum yields, probably due to hindered internal rotations in the dye. Internal conversion is the major S1 state deactivation path, with fluorescence being a minor channel, and S1→T1 intersystem crossing not observed. Betaxanthins extracted from A. muscaria L. have similar properties. Efficient dissipation of the absorbed light energy as heat supports the postulate of photoprotective role of betaxanthins in vivo.

Aggregation-induced photon upconversion through control of the triplet energy landscapes of the solution and solid states.

Aggregation-induced photon upconversion (iPUC) based on control of the triplet energy landscape is demonstrated for the first time. When a triplet state of a cyano-substituted 1,4-distyrylbenzene derivative is sensitized in solution, no upconverted emission based on triplet-triplet annihilation (TTA) was observed. In stark contrast, crystalline solids obtained by drying the solution revealed clear upconverted emission. Theoretical studies unveiled an underlying switching mechanism: the excited triplets in solution immediately decay back to the ground state through conformational twisting around a CC bond and photoisomerization, whereas this deactivation path is effectively inhibited in the solid state. The finding of iPUC phenomena highlights the importance of controlling excited energy landscapes in condensed molecular systems.

Aggregation‐Induced Photon Upconversion through Control of the Triplet Energy Landscapes of the Solution and Solid States

AbstractAggregation‐induced photon upconversion (iPUC) based on control of the triplet energy landscape is demonstrated for the first time. When a triplet state of a cyano‐substituted 1,4‐distyrylbenzene derivative is sensitized in solution, no upconverted emission based on triplet–triplet annihilation (TTA) was observed. In stark contrast, crystalline solids obtained by drying the solution revealed clear upconverted emission. Theoretical studies unveiled an underlying switching mechanism: the excited triplets in solution immediately decay back to the ground state through conformational twisting around a CC bond and photoisomerization, whereas this deactivation path is effectively inhibited in the solid state. The finding of iPUC phenomena highlights the importance of controlling excited energy landscapes in condensed molecular systems.

Fischer–Tropsch synthesis: Effect of ammonia in syngas on the Fischer–Tropsch synthesis performance of a precipitated iron catalyst

The effect of ammonia in syngas on the Fischer–Tropsch synthesis (FTS) reaction over 100Fe/5.1Si/2.0Cu/3.0K catalyst was studied at 220–270°C and 1.3MPa using a 1-L slurry phase reactor. The ammonia added in syngas originated from adding ammonia gas, ammonium hydroxide solution, or ammonium nitrate (AN) solution. A wide range of ammonia concentrations (i.e., 0.1–400ppm) was examined for several hundred hours. The Fe catalysts withdrawn at different times (i.e., after activation by carburization in CO, before and after co-feeding contaminants, and at the end of run) were characterized by ICP-OES, XRD, Mössbauer spectroscopy, and synchrotron methods (e.g., XANES, EXAFS) in order to explore possible changes in the chemical structure and phases of the Fe catalyst with time; in this way, the deactivation mechanism of the Fe catalyst by poisoning could be assessed. Adding up to 200ppmw (wt. NH3/av. Wt. feed) ammonia in syngas did not significantly deactivate the Fe catalyst or alter selectivities toward CH4, C5+, CO2, C4-olefin, and 1-C4 olefin, but increasing the ammonia level (in the AN form) to 400ppm rapidly deactivated the Fe catalyst and simultaneously changed the product selectivities. The results of ICP-OES, XRD, and Mössbauer spectroscopy did not display any evidence for the retention of a nitrogen-containing compound on the used catalyst that could explain the deactivation (e.g., adsorption, site blocking). Instead, Mössbauer spectroscopy results revealed that a significant fraction of iron carbides transformed into iron magnetite during co-feeding high concentrations of AN, suggesting that oxidation of iron carbides occurred and served as a major deactivation path in that case. Oxidation of χ-Fe5C2 to magnetite during co-feeding high concentrations of AN was further confirmed by XRD analysis and by the application of synchrotron methods (e.g., XANES, EXAFS). It is postulated that AN oxidized χ-Fe5C2 during FTS via its thermal dissociation product, HNO3. This conclusion is further supported by reaction tests with co-feeding of similar concentrations of HNO3. Additional oxidation routes of iron carbide to magnetite by HNO3 and/or by its thermal decomposition products are also considered: Fe5C2+NOx (and/or HNO3)→Fe3O4. In this study, ion chromatography detected that 50–80% HNO3 directly added or dissociated from AN eventually converted to ammonia during or after its oxidation of iron carbide, resulting from the reduction of NOx (NOx+H2+CO→NH3+CO2+N2+H2O) by H2 and/or CO.

Ultrafast Excited-State Deactivation of 8-Hydroxy-2'-deoxyguanosine Studied by Femtosecond Fluorescence Spectroscopy and Quantum-Chemical Calculations.

The fluorescence properties of the 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) in aqueous solution at pH 6.5 are studied by steady-state spectroscopy and femtosecond fluorescence up-conversion and compared with those of 2'-deoxyguanine (dG) and 2'-deoxyguanine monophosphate (dGMP). The steady-state fluorescence spectrum of 8-oxo-dG is composed of a broad band that peaks at 356 nm and extends over the entire visible spectral region, and its fluorescence quantum yield is twice that of dG/dGMP. After excitation at 267 nm, the initial fluorescence anisotropy at all wavelengths is lower than 0.1, giving evidence of an ultrafast internal conversion (<100 fs) between the two lowest excited ππ* states (Lb and La). The fluorescence decays of 8-oxo-dG are biexponential with an average lifetime of 0.7 ± 0.1 ps, which is about two times longer than that of dGMP. In contrast with dGMP, only a moderate dynamical shift (∼1400 vs 10,000 cm(-1)) of the fluorescence spectra of 8-oxo-dG is observed on the time scale of a few picoseconds without modification of the spectral shape. PCM/TD-DFT calculations, employing either the PBE0 or the M052X functionals, provide absorption spectra in good agreement with the experimental one and show that the deactivation path is similar to that proposed for dGMP, with a fast motion toward an energy plateau, where the purine ring keeps an almost planar geometry, followed by decay to S0, via out-of-the plane motion of amino substituent.

The thermal deactivation of all-trans and 15-cis beta-carotene-excited states in the ionic liquids without and with methylenoxy group

Photoacoustic spectroscopy was used to study thermal (dark) paths of excited state deactivation of β-carotene in ionic liquids. The dark paths of excited state deactivation of all-trans and 15-cis β-carotene isomers are very important in photosynthesis and medicine. The photoacoustic measurements of β-carotene in traditional solvents are difficult due to fast evaporation of the solvent which leads to concentration fluctuation. Room temperature ionic liquids are good medium for temperature study due to their high thermal stability and low volatility. Dark deactivation of both β-carotene isomers in ionic liquids occurs in a similar way except for an additional path of deactivation of excited state 11A +g (‘cis’ peak). The thermal-excited state deactivation of all-trans and 15-cis β-carotene isomers is not sensitive to the ionic liquid structure, in contrast to the radiative path.

A multireference configuration interaction study of the photodynamics of nitroethylene.

Extended multireference configuration interaction with singles and doubles (MR-CISD) calculations of nitroethylene (H2C=CHNO2) were carried out to investigate the photodynamical deactivation paths to the ground state. The ground (S0) and the first five valence excited electronic states (S1–S5) were investigated. In the first step, vertical excitations and potential energy curves for CH2 and NO2 torsions and CH2 out-of-plane bending starting from the ground state geometry were computed. Afterward, five conical intersections, one between each pair of adjacent states, were located. The vertical calculations mostly confirm the previous assignment of experimental spectrum and theoretical results using lower-level calculations. The conical intersections have as main features the torsion of the CH2 moiety, different distortions of the NO2 group and CC, CN, and NO bond stretchings. In these conical intersections, the NO2 group plays an important role, also seen in excited state investigations of other nitro molecules. Based on the conical intersections found, a photochemical nonradiative deactivation process after a π–π* excitation to the bright S5 state is proposed. In particular, the possibility of NO2 release in the ground state, an important property in nitro explosives, was found to be possible.

Relaxation Mechanisms of 5-Azacytosine.

The photophysics and deactivation pathways of the noncanonical 5-azacytosine nucleobase were studied using the CASPT2//CASSCF protocol. One of the most significant differences with respect to the parent molecule cytosine is the presence of a dark (1)(nNπ*) excited state placed energetically below the bright excited state (1)(ππ*) at the Franck-Condon region. The main photoresponse of the system is a presumably efficient radiationless decay back to the original ground state, mediated by two accessible conical intersections involving a population transfer from the (1)(ππ*) and the (1)(nNπ*) states to the ground state. Therefore, a minor contribution of the triplet states in the photophysics of the system is expected, despite the presence of a deactivation path leading to the lowest (3)(ππ*) triplet state. The global scenario on the photophysics and photochemistry of the 5-azacytosine system gathered on theoretical grounds is consistent with the available experimental data, taking especially into account the low values of the singlet-triplet intersystem crossing and fluorescence quantum yields observed.

A Conical Intersection Controls the Deactivation of the Bacterial Luciferase Fluorophore

AbstractThe photophysics of flavins is highly dependent on their environment. For example, 4a‐hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature‐dependent fluorescent studies on an N(5)‐alkylated 4a‐hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited‐state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out‐of‐plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited‐state barrier imposed by the rigid glass cavity.

A Conical Intersection Controls the Deactivation of the Bacterial Luciferase Fluorophore

The photophysics of flavins is highly dependent on their environment. For example, 4a-hydroxy flavins display weak fluorescence in solution, but exhibit strong fluorescence when bound to a protein. To understand this behavior, we performed temperature-dependent fluorescent studies on an N(5)-alkylated 4a-hydroxy flavin: the putative bacterial luciferase fluorophore. We find an increase in fluorescence quantum yield upon reaching the glass transition temperature of the solvent. We then employ multiconfigurational quantum chemical methods to map the excited-state deactivation path of the system. The result reveals a shallow but barrierless excited state deactivation path that leads to a conical intersection displaying an orthogonal out-of-plane distortion of the terminal pyrimidine ring. The intersection structure readily explains the observed spectroscopic behavior in terms of an excited-state barrier imposed by the rigid glass cavity.

Photoinduced proton transfer and isomerization in a hydrogen-bonded aromatic azo compound: a CASPT2//CASSCF study.

Intramolecularly hydrogen-bonded aromatic azo compound 1-cyclopropyldiazo-2-naphthol (CPDNO) exhibits complicated excited-state behaviors, e.g., wavelength-dependent photoinduced proton transfer and photoproducts. Its photochemistry differs from that of common aromatic azo compounds in which cis-trans photoisomerization is dominant. To rationalize the intriguing photochemistry of CPDNO at the atomic level, we have in this work employed the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods to explore the S0, S1, and S2 potential-energy profiles relevant to enol-keto proton transfer and isomerization reactions. It is found that the proton transfer along the bright diabatic (1)ππ* potential-energy profile is almost barrierless, quickly forming the fluorescent (1)ππ* keto minimum. In this process, the dark (1)nπ* state is populated via a (1)ππ*/(1)nπ* crossing point, but the proton transfer on this dark state is suppressed heavily as a result of a large barrier. In addition, two deactivation paths that decay the S1 enol and keto minima to the S0 state, respectively, were uncovered. For the former, it is exoenergetic and thereby thermodynamically favorable; for the latter, it is a little endothermic (ca. 5 kcal/mol). Both are energetically allowable concerning the available total energy. Finally, on the basis of the present results, the experimentally observed wavelength-dependent photoproducts were explained very well.

Theoretical Study on the Selective Fluorescence of PicoGreen: Binding Models and Photophysical Properties

Abstract PicoGreen (PG) is used as a probe to selectively quantitate double-stranded (ds-) DNA because it shows unique fluorescence enhancement when complexed with DNA. By binding to ds- and single-stranded (ss-) DNA, the quantum yields of PG–DNA complexes become remarkably larger than that of a free molecule. In the present theoretical study, the fluorescence enhancement mechanism of PG–DNA complexes was investigated using molecular docking simulations and ab initio quantum chemical methods. The binding energies between PG and ds-DNA were calculated to be larger than those in the case of PG and ss-DNA owing to the existence of an extra π–π stacking interaction. Nonradiative deactivation paths through conical intersections between the ground and the first excited states were obtained for a free PG molecule, while steric repulsions between PG and DNA hindered such deactivation processes in the case of PG–DNA complexes.

Photodeactivation paths in norbornadiene

The first high level ab initio quantum-chemical calculations of potential energy surfaces (PESs) for low-lying singlet excited states of norbornadiene in the gas phase are presented. The optimization of the stationary points (minima and conical intersections) and the recalculation of the energies were performed using the multireference configuration interaction with singles (MR-CIS) and the multiconfigurational second-order perturbation (CASPT2) methods, respectively. It was shown that the crossing between valence V2 and Rydberg R1 states close to the Franck-Condon (FC) point permits an easy population switch between these states. Also, a new deactivation path in which the doubly excited state with (π3)(2) configuration (DE) has a prominent role in photodeactivation from the R1 state due to the R1/DE and the DE/V1 conical intersections very close to the R1 and DE minima, respectively, was proposed. Subsequent deactivation from the V1 to the ground state goes through an Olivucci-Robb-type conical intersection that adopts a rhombic distorted geometry. The deactivation path has negligible barriers, thereby making ultrafast radiationless decay to the ground state possible.

A theoretical study of the photochemistry of indigo in its neutral and dianionic (leucoindigo) forms

A thorough analysis of the single and double proton transfer and the internal rotations of neutral indigo and its dianionic leucoindigo form has been performed for the ground and first singlet excited electronic states using, respectively, DFT and TDDFT state-of-the-art methods. Our theoretical analysis discloses that the diketo isomer is the most stable one in the ground state of indigo but not in leucoindigo where the dienol minimum is more stable. Single and double proton transfer processes are not energetically favored in the ground electronic state but a single proton transfer gives a more stable tautomer in the excited electronic state of indigo whereas a double proton transfer is energetically favorable in the excited state of leucoindigo. The internal rotations are not thermodynamically allowed except for the keto-enol tautomer where a full rotation of the inter-ring C-C bond leads to another stable keto-enol structure. A preliminary analysis of the plausible conical intersections for both indigo and leucoindigo allows the discussion of the likely deactivation paths that will follow light irradiation. Our results point to a very different photochemistry of the two molecules. For indigo the proton transfer can only take place through tunneling so the main deactivation path would involve a conical intersection accessed directly upon internal rotation of the keto-keto tautomer. For leucoindigo a richer photochemistry is expected as the single and double proton transfer processes are energetically open. The more favorable path involves single proton transfer followed by a trans to cis isomerization and a second proton transfer. This process competes on equal grounds with several non-radiative decays through conical intersections. All these results are in agreement with the experimental facts known to date.

Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.