Transient structures and kinetics of the ferrioxalate redox reaction studied by time-resolved EXAFS, optical spectroscopy, and DFT

The photoredox reaction transients of ferrioxalate in water have been studied by means of time-resolved EXAFS and ultrafast optical transient spectroscopy. The transient spectra and kinetics have been measured from the femtosecond to millisecond range, and the Fe-O bond lengths of the ferrioxalate redox reaction transients have been determined with 2 ps time resolution and 0.04 A accuracy. These data in conjunction with quantum-chemistry DFT and UHF calculations were used to formulate a mechanism for the Fe(III) to Fe(II) redox reaction where dissociation precedes electron transfer. In addition, radical scavenging experiments support the mechanism proposed.     

Photodissociation of acryloyl chloride in the gas phase

The 193 nm photodissociation dynamics of CH2 CHCOCl in the gas phase has been examined with the technique of time-resolved Fourier transform infrared emission (TR-FTIR) spectroscopy.Vibrationally excited photofragments of CO (≤ 5),HCl (≤ 6),and C2H2 were observed and two photodissociation channels,the C-Cl fission channel and the HCl elimination channel have been identified.The vibrational and rotational state distributions of the photofragments CO and HCl have been acquired by analyzing their fully rotationally resolved v→v-1 rovibrational progressions in the emission spectra,from which it has been firmly established that the mechanism involves production of HCl via the four-center molecular elimination of CH2 CHCOCl after its internal conversion from the S1 state to the S0 state.In addition to the dominant C-Cl bond fission along the excited S1 state,the S1→S0 internal conversion has also been found to play an important role in the gas phase photolysis of CH2 CHCOCl as manifested by the considerable yield of HCl.     

Production of atomic oxygen following flash photolysis of ClONO2

The photodissociation of ClONO₂ using a broad-band ultraviolet photolysis source has been investigated using time-resolved atomic absorption spectroscopy in the vacuum ultraviolet. The predominant atomic photolysis product is O(³PJ), the quantum yield for Cl(²PJ) production being less than 4%.     

Chain processes in the photochemistry of PtIV halide complexes in aqueous solutions

The mechanisms of the photoaquation of PtCl₆ ^2? and PtBr₆ ^2? complexes were compared by the experimental results on stationary photolysis, nanosecond laser flash photolysis, and ultrafast pump-probe spectroscopy. The formation of the photoaquation product of the bromide complex, viz., PtBr₅(H₂O)^?, was shown to proceed via the mechanism of heterolytic cleavage of the Pt-Br bond, and the platinum cation remained tetravalent in the course of the whole process. For the chloride complex, the Pt-Cl bond cleavage was found to be homolytic, and precursors of the photoaquation product, viz., PtCl₅(H₂O)^? complex, are intermediates of trivalent platinum sequentially transforming into each other. The reactions of these intermediates determine the chain character of the photoaquation process.     

Electron transfer mechanism and photochemistry of ferrioxalate induced by excitation in the charge transfer band

The photoredox reaction of ferrioxalate after 266/267 nm excitation in the charge transfer band has been studied by means of ultrafast extended X-ray absorption fine structure (EXAFS) analysis, optical transient spectroscopy, and quantum chemistry calculations. The Fe-O bond length changes combined with the transient spectra and kinetics have been measured and in combination with ultrahigh frequency density functional theory (UHF/DFT) calculations are used to determine the photochemical mechanism for the Fe(III) to Fe(II) redox reaction. The present data and the results obtained with 266/267 nm excitations strongly suggest that the primary reaction is the dissociation of the Fe-O bond before intramolecular electron transfer occurs. Low quantum yield electron photodetachment from ferrioxalate has also been observed.     

Electron Transfer in Metal‐Organic Molecules. A Time Resolved EXAFS and Optical Spectroscopy Study

We have measured, by means of ultrafast x‐ray absorption and optical spectroscopy, the M‐O (M=Fe, Co) and Co‐N metal to ligand bond length change as a function of time and the formation and decay of the excited states and intermediate species, after excitation with a 267 nm femtosecond pulse. These experimental data combined with DFT calculations allowed us to determine the mechanism of electron transfer operating in the redox reaction of two metal‐ligand complexes, [M(III)(C₂O₄)₃]^3‐ and [Co(III)(NH₃)₆ ]³⁺. Based on the data we find that, even though both molecules are excited into their charge transfer band, the redox reaction of [M(III)(C₂O₄)₃]^3‐ proceeds via intermolecular electron transfer while [Co(III)(NH₃)₆ ]³⁺ electron transfer mechanism is intramolecular.     

Observation of singlet cycloreversion of thymine oxetanes by direct photolysis

An ultrafast broadband transient absorption spectroscopic study of the direct photolysis of oxetane DMT-BP [which is the oxetane adduct of 1,3-dimethylthymine (DMT) with benzophenone (BP)] is presented. Previous nanosecond time-resolved absorption studies by other researchers observed that direct photolysis of such oxetanes results in a rare, adiabatic photochemical reaction to produce a triplet excited-state carbonyl species. However, the mechanism for this adiabatic photochemical reaction remained unclear for the reaction sequence of the bond scission and the intersystem crossing (ISC) because of the time resolution for the experiments, and this prompted us to further study its mechanism with ultrafast time-resolution. The ultrafast time-resolved spectra presented here indicate that the cycloreversion reaction occurs in a stepwise manner on a singlet excited-state, and then intersystem crossing (ISC) occurs to produce the triplet carbonyl product observed in the previously reported nanosecond time-resolved experiments.     

Quantum chemical studies of Lindqvist-type polyoxometalates containing late 3d transition metals ([(py)MIIW5O18]4? (M?=?Fe, Co, Ni)): MII�CN bonding and second-order nonlinear optical properties

The electronic and nonlinear optical (NLO) properties of Lindqvist-type tungstate containing late 3d transition metals [(py)MW₅O₁₈]^4? (M?=?Fe, Co, Ni) have been systematically investigated using density functional theory (DFT) method. The character of M?CN bond is analyzed using natural bond orbital methods. The first hyperpolarizabilities of [(py)MW₅O₁₈]^4? anions have been investigated by Coulomb-attenuating method (CAM-B3LYP). The NLO properties of [(py)MW₅O₁₈]^4? with different spin states are also studied. The results show that the static second-order polarizability (?? ₀) of [(py)⁵FeW₅O₁₈]^4? (Fe?=?quintet state) is 525.10?×?10^?30 esu, which is larger than those of [(py)⁴CoW₅O₁₈]^4? (?? ₀?=?120.72?×?10^?30 esu) and [(py)³NiW₅O₁₈]^4? (?? ₀?=?30.45?×?10^?30 esu) anions. Time-dependent DFT results reveal that the substituted transition metals-to-pyridine charge transfer may be responsible for the NLO properties of this kind of polyoxometalates.     

Redox behaviors of Fe(II/III) and U(IV/VI) studied in synthetic water and KURT groundwater by potentiometry and spectroscopy

The redox behaviors of Fe(II/III) and U(IV/VI) in both synthetic samples and natural groundwater were investigated with potentiometry, UV/VIS absorption spectroscopy, and time-resolved laser fluorescence spectroscopy. Total dissolved Fe(II/III) concentration along with presence of mixed redox couples of Fe²⁺/Fe³⁺ and Fe²⁺/Fe₂O₃(s) were revealed to be the major factors influencing on the redox potentials. Considerable discrepancies between redox potentials obtained with quantitative analysis and chemical speciation of Fe(II/III) and U(IV/VI) ions were identified in the KAERI Underground Research Tunnel groundwater. Chemical speciation of U(IV) in natural groundwater without considering relevant complexation reaction might cause relatively large uncertainties in redox potential calculations.     

Insight into the photoexcitation effect on the catalytic activation of H2 and C-H bonds on TiO2(110) surface

Semiconductor photocatalysis holds great promise for breaking the inert chemical bonds under mild condition; however, the photoexcitation-induced modulation mechanism has not been well understood at the atomic level. Herein, by performing the DFT+U calculations, we quantitatively compare H₂ activation on rutile TiO₂(110) under thermo- versus photo-catalytic condition. It is found that H₂ dissociation prefers to occur via the heterolytic cleavage mode in thermocatalysis, but changes to the homolytic cleavage mode and gets evidently promoted in the presence of photoexcited hole (h⁺). The origin can be ascribed to the generation of highly oxidative lattice O-radical (O_br^??) with a localized unoccupied O-2p state. More importantly, we identify that this photo-induced promotion effect can be practicable to another kind of important chemical bond, i.e., C–H bond in light hydrocarbons including alkane, alkene and aromatics; an exception is the C(sp¹)-H in alkyne (HCCH), which encounters inhibition effect from photoexcitation. By quantitative analysis, the origins behind these results are attributed to the interplay between two factors: C-H bond energy (E_bond) and the acidity. Owing to the relatively high E_bond and acidity, it favors the C(sp¹)-H bond to proceed with the heterolytic cleavage mode in both thermo- and photo-catalysis, and the photoexcited O_br^?? is adverse to receiving the transferred proton. By contrast, for the other hydrocarbons with moderate/low E_bond, the O_br^?? would enable to change their activation mode to a more favored homolytic one and evidently decrease the C–H activation barrier. This work may provide a general picture for understanding the photocatalytic R–H (R = H, C) bond activation over the semiconductor catalyst.     

Ruby laser-induced two-photon photolysis of potassium ferrioxalate

The ruby laser-induced photolysis of potassium ferrioxalate is shown to follow a two-photon excitation mechanism. The two-photon absorption cross section is found to be δ = (1.5 ± 1.0) × 10^?50 cm⁴ s/photon molecule.     

Primary photophysical and photochemical processes for Pt(SCN)<Subscript>6</Subscript><Superscript>2–</Superscript> complex

Photosolvation of a Pt^IV hexathiocyanate complex Pt(SCN)₆ ^2– in water and ethanol was studied by steady-state photolysis, nanosecond laser flash photolysis, and ultrafast kinetic spectroscopy. Complexes Pt(SCN)₅(H₂O)^– and Pt(SCN)₅(C₂H₅OH)^– were found to be the only reaction products. The quantum yields of photosolvation are independent of the excitation wavelength, being equal to 0.25 and 0.5 for the solutions of the complex in water and ethanol, respectively. Photosolvation proceeds by the mechanism of heterolytic metal—ligand bond dissociation without involvement of redox processes. The characteristic time of formation of the end products for both solvents is about 10 ps. Three successive intermediates detected on the picosecond time scale were interpreted as Pt^IV complexes. The nature of the intermediates and possible mechanisms of photosolvation are discussed.     

Outer-sphere effects on ligand-field excited-state dynamics: solvent dependence of high-spin to low-spin conversion in [Fe(bpy)3]2+

In condensed phase chemistry, the solvent can have a significant impact on everything from yield to product distribution to mechanism. With regard to photo-induced processes, solvent effects have been well-documented for charge-transfer states wherein the redistribution of charge subsequent to light absorption couples intramolecular dynamics to the local environment of the chromophore. Ligand-field excited states are expected to be largely insensitive to such perturbations given that their electronic rearrangements are localized on the metal center and are therefore insulated from so-called outer-sphere effects by the ligands themselves. In contrast to this expectation, we document herein a nearly two-fold variation in the time constant associated with the ⁵T₂ → ¹A₁ high-spin to low-spin relaxation process of tris(2,2′-bipyridine)iron(ii) ([Fe(bpy)₃]²⁺) across a range of different solvents. Likely origins for this solvent dependence, including relevant solvent properties, ion pairing, and changes in solvation energy, were considered and assessed by studying [Fe(bpy)₃]²⁺ and related derivatives via ultrafast time-resolved absorption spectroscopy and computational analyses. It was concluded that the effect is most likely associated with the volume change of the chromophore arising from the interconfigurational nature of the ⁵T₂ → ¹A₁ relaxation process, resulting in changes to the solvent–solvent and/or solvent–solute interactions of the primary solvation shell sufficient to alter the overall reorganization energy of the system and influencing the kinetics of ground-state recovery.

Time-resolved spectroscopic measurements of ground-state recovery for [Fe(bpy)₃]²⁺ reveal that the solvent can induce an outer-sphere reorganization energy effect on excited-state dynamics involving metal-centered ligand-field electronic states.     

Synthesis, structure and properties of a two-dimensional iron(II) metal-organic framework

Reaction of FeSO₄·7H₂O with 5-(isonicotinamido)isophthalic acid (H₂INAIP) in the presence of NaOH results in the formation of a two-dimensional network [Fe(INAIP)(H₂O)]_n·2nH₂O. In the complex, each INAIP^2? ligand links three Fe(II) atoms to give a double-chain structure using its two carboxylate groups in ?? ₁?C?? ¹:?? ¹ and ?? ₂?C?? ¹:?? ¹ coordination modes. The chains are interlinked to form a layer structure through Fe?CN interactions, which it is extended to a three-dimensional supramolecular structure by H-bonding interactions. The thermal and magnetic properties of the complex were investigated, and antiferromagnetic interactions were observed. Moreover, the adsorption behavior shows that the title complex has obviously selective adsorption of CO₂ over N₂ after the removal of the solvent molecules within the pores.     

Local structure analysis of Bi2(Ga1-xFex)4O9 mullite-type solid solutions synthesized via combination of ball milling and thermal treatment

Mullite-type Bi₂(Ga_1-xFe_x)₄O₉ solid solutions, with 0.1 ≤ x ≤ 0.9, have been synthesized by a combination of mechanical and thermal treatments of a Bi₂O₃/Ga₂O₃/α-Fe₂O₃ stoichiometric mixture. The microstructure of the as-prepared materials on the long-range and local atomic scales was investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy, respectively. The XRD data analysis revealed in all cases linear dependence of the lattice parameters related on x. Due to the ability of the applied Mössbauer spectroscopy to probe the local environment of Fe cations, the local structural disorder in investigated solid solutions is provided. It was shown that the presence of Fe³⁺ cations in octahedral sites of the orthorhombic structure causes a local distortion of polyhedra in the material. The preferential occupation of Fe in octahedral site was revealed. Detailed quantitative information on both the cation distribution and the bond lengths provided is discussed in relation to the derived hyperfine parameters.     

Cover Picture: Journal of the Chinese Chemical Society 4/2011

By means of ultrafast x‐ray absorption, optical spectroscopy and DFT theoretical calculation, we find that even though both molecules are excited into their charge transfer band, the redox reaction of [M(III)(C₂O₄)₃]^3‐ (M=Fe, Co) proceeds via intermolecular electron transfer while [Co(III)(NH₃)₆]³⁺ electron transfer mechanism is intramolecular. This article entitled “Electron Transfer in Metal‐Organic Molecules. A Time Resolved EXAFS and Optical Spectroscopy Study” was contributed by Prof. Peter M. Rentzepis who was invited as a Visiting Lecturer of the Chemistry Research Promotion Center, Taiwan, R.O.C. (November 12, –November 20, 2010). For full text, please see pp. 415–427 in this issue.     

Ultrafast Dynamics Through Conical Intersections in 2,6-dimethylpyridine Studied with Time-resolved Photoelectron Imaging

The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been stud-ied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S₂ state with a ππ^* character from S₀state. The time evolution of the parent ion sig-nals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo-electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S₂ state. In brief, the ultrafast component is a population transfer from S₂ to S₁ through the S₂/S₁ conical intersections, the slow component is attributed to simultaneous IC from the S₂ state and the higher vibrational levels of S₁ state to S₀ state, which involves the coupling of S₂/S₀ and S₁/S₀ conical intersections. Additionally, the observed ultrafast S₂→S₁ transition occurs only with an 18% branching ratio.     

非血红素铁(III)活化氧分子反应的自旋轨道耦合和零场分裂

采用密度泛函理论对原儿茶酚3,4-双加氧酶(3,4-PCD)活化O₂分子的反应机理进行了探讨. 初始复合物, 六重态⁶1的超快形成主要归因于电子交换诱导系间穿越(EISC), Fe d_z:O₂ π^*(z)是主要的交换通道, 在Fe―O键长为0.2487 nm处, 交换重叠积分S_ij=ád_z α|π^*(z) β>=0.3758. 从六重态⁶1 形成四重态中间体⁴1, 有两种效应共存, 即电子交换耦合作用和自旋轨道耦合(SOC)作用, 且相互竞争. 计算结果表明, 自旋轨道耦合(SOC)作用起主导因素(SOC=353.16 cm^-1). 至于O―O键的解离主要取决于儿茶酚(PCA)最高占据分子轨道(HOMO)的电子转移, 非血红素酶的铁中心仅承担PCA向O₂电子转移的缓冲作用.²²     

The absorption cross section of the HCO radical at 614.59 nm and the rate constant for HCO+HCO → H2CO+CO

Using a combination of XeCl exciplex laser flash photolysis of gas-phase glyoxal and formaldehyde and time-resolved cw dye laser absorption at 614.59 nm, we have determined the ratio k₁/σ for the reaction HCO+HCO → H₂CO+CO (1) at 295 ±2 K. Similar studies involving the 308 nm photolysis of a variety of aldehydes combined with a determination of the absolute yields of the resulting hydrocarbon products have allowed us to deduce the initial yields of HCO radicals and hence the absorption cross section for HCO at the monitoring wavelength. We find σ=(2.3±0.6) × 10⁻¹⁸ cm², giving k₁=(7.5±2.9)× 10⁻¹¹cm³ molecule⁻¹ s⁻¹. Our values are compared with previous results.     

Photo-switchable spin-crossover iron(III) compound based on intermolecular interactions

Iron(III) spin-crossover compounds, [Fe(qnal)₂]CF₃SO₃·MeOH (1·MeOH) and [Fe(qnal)₂]CF₃SO₃·acetone (1·acetone) were prepared and their spin transition properties were characterized by magnetic susceptibility measurement, Mössbauer spectroscopy and single crystal analysis. Two iron(III) compounds exhibited abrupt spin transition with thermal hysteresis loop (T _1/2?? = 115 K and T _1/2?? = 104 K for 1·MeOH, and T _1/2?? = 133 K and T _1/2?? = 130 K for 1·acetone). Single crystal analysis revealed both of the structures in high-spin (HS) and low-spin (LS) states for 1·acetone. The difference of bond length between the HS and LS states for 1·acetone was ~0.10 Å, which was corresponding to that of typical iron(III) SCO compounds. Specially, it showed strong intermolecular interactions by ???C?? stacking formed between the neighbor complexes leading to 2-D sheet. Both 1·MeOH and 1·acetone exhibited LIESST effect when it was illuminated at 1000 nm. We also confirmed that the introduction of strong intermolecular interactions, such as ???C?? stacking, can play an important role in LIESST effect.