Efficient photochemical electron transfer sensitization of homogeneous organic reactions

Photochemical electron transfer induced reactions have become an interesting tool in organic synthesis since transformations can be easily performed which are difficult or impossible with more conventional organic reactions. In this context, electron transfer sensitized reactions are frequently used since the sensitizer can be considered as a catalyst. Various intermediates such as radical ions with a variety of reaction possibilities are involved. Nevertheless, the reactions have been performed with high yields and high selectivities. Particular attention is paid to the stability of the sensitizer. Reaction steps regenerating the sensitizer from different intermediates are discussed. In photochemical electron donor and acceptor sensitized transformations, these steps are often part of the main reaction course. In other cases, co-sensitizers or mediators significantly enhance the efficiency of the transformations although the number of reactive intermediates is increased.     

A general theoretical model for electron transfer reactions in complex systems

In this paper we present a general theoretical-computational model for treating electron transfer reactions in complex atomic-molecular systems. The underlying idea of the approach, based on unbiased first-principles calculations at the atomistic level, utilizes the definition and the construction of the Diabatic Perturbed states of the involved reactive partners (i.e. the quantum centres in our perturbation approach) as provided by the interaction with their environment, including their mutual interaction. In this way we reconstruct the true Adiabatic states of the reactive partners characterizing the electron transfer process as the fluctuation of the electronic density due to the fluctuating perturbation. Results obtained by using a combination of Molecular Dynamics simulation and the Perturbed Matrix Method on a prototypical intramolecular electron transfer (from 2-(9,9'-dimethyl)fluorene to the 2-naphthalene group separated by a steroidal 5-α-androstane skeleton) well illustrate the accuracy of the method in reproducing both the thermodynamics and the kinetics of the process.     

Solvent effect on electron transfer reactions: Comparison of homogeneous and heterogeneous electron exchange between V3+ and V2+ in water +t-butanol mixtures

The kinetics of homogeneous and heterogeneous electron exchange V³⁺/V²⁺ was investigated in 1 M HClO₄, water +t-butanol, mixed solvent solutions (x_t-but. varied from 0 to 0.65). Non-monotonic heterogeneous and monotonic homogeneous variations of the rate constants were observed. The solvent effect on the work of bringing the reactants from infinite separation to the reaction site was estimated with the Gouy-Chapman and Debye-Hückel approximations respectively. The homogeneous kinetics data were next used to calculate the reorganization energy in the heterogeneous reaction, λ_het. Finally the influence of the solvent composition on the rate of the electrode reaction was separated into surface and bulk effects.     

Solvent adiabaticity effects on ultrafast electron transfer in viologen charge transfer complexes

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.     

离子对内电子转移型染料阴离子-翁盐阳离子光敏体系中的溶剂效应和结构效应

本文研究了溶剂效应和结构效应对染料碘翁盐光物理, 光化学性质的影响。观察到在溶剂中离子对可以各种形式存在, 如紧密离子对、溶剂分隔离子对或溶剂化的自由离子, 溶剂的极性不仅影响各种存在形式的光谱性质, 而且影响它们之间的平衡关系, 进而影响离子对体系的物理化学性质。染料母核和碘翁阳离子的结构均对离子对体系的性质有影响。光诱导电子转移反应的热力学驱动力越大, 反应速度越快。用分子模拟技术(Molecular Modeling)对离子对体系的立体结构进行了研究, 为理解离子对体系的各种物理化学行为提供了重要的参考。     

Solvent effects on electron transfer reactions. The system iron(III)-phenothiazine in water-alcohol mixtures

Summary The effect of solvent composition (aqueous mixtures of methanol, ethanol, propan-1-ol and propan-2-ol) on the rate constants and activation parameters of the electron transfer between iron(III) and phenothiazine has been investigated. The dependence of the kinetic parameters on the solvent composition is discussed with reference to previously investigated systems.     

Solvent isotope effects upon the kinetics of some simple electrode reactions

The effects of replacing H₂O with D₂O solvent upon the electrochemical kinetics of simple transition-metal redox couples containing aquo, ammine or ethylenediamine ligands have been investigated at mercury electrodes as a means of exploring the possible contribution of ligand-aqueous solvent interactions to the activation barrier to outer-sphere electron transfer. The general interpretation of solvent isotope effects upon electrode kinetics is discussed; it is concluded that double-layer corrected isotopic rate ratios (k^H/k^D)^E determined at a constant electrode potential vs. an aqueous reference electrode, as well as those determined at the respective standard potentials in H₂O and D₂O (k_S^H/k_S^D), have particular significance since the solvent liquid-junction potential can be arranged to be essentially zero. For aquo redox couples, values of (k_S^H/k_S^D) were observed that are substantially greater than unity and appear to be at least partly due to a greater solvent-reorganization barrier in D₂O arising from ligand-solvent hydrogen bonding. For ammine and ethylenediamine complexes values of (k^H/k^D)^E substantially greater than, and smaller than, unity were observed upon the separate deuteration of the ligands and the surrounding solvent respectively. Comparison of isotope rate ratios for corresponding electrochemical and homogeneous outer-sphere reactions involving cationic ammine and aquo complexes yields values of (k^H/k^D) for the former processes that are typically markedly larger than those predicted by the Marcus model from the homogeneous rate ratios. These discrepancies appear to arise from differences in the solvent environments in the transition states for electrochemical and homogeneous reactions.     

A comparison of homogeneous and heterogeneous copper catalyzed arylation of amines

Comparison of the homogeneous and heterogeneous copper-catalyzed arylation of model primary amines with (hetero)-aryl iodides in DMSO revealed a comparable efficiency of CuI and commercially available unsupported copper nanoparticles (25 nm size) in the presence of 2-isobutyryl-cyclohexanone or L-proline.     

Cerium-based catalysts for N-oxidation of pyridine-based derivatives: homogeneous and heterogeneous systems

Herein, the catalytic properties of the cerium (IV) salt, cerium (IV)-sandwiched polyoxometalate (POM) and cerium (IV)-sandwiched polyoxometalate intercalated in layered double hydroxides (LDHs) in the H₂O₂-based green oxidation reactions have been evaluated. These cerium (IV)-based systems were applied as homogeneous and heterogeneous catalysts for the oxidation of pyridines. Despite the fact that the cerium (IV)-sandwiched polyoxometalate as a homogeneous reaction system gives good results, there are some disadvantages in recovery and reusability process. To overcome these problems, new nano catalyst was synthesized by intercalation of the Cerium (IV)-sandwiched polyoxometalate into tris(hydroxymethyl) aminomethane-modified layered double hydroxides (Tris-LDH-CO₃). The as-prepared nanocomposite was characterized and used as an effective heterogeneous catalyst for the oxidation of pyridines under mild conditions in the presence of H₂O₂ as an oxidant. The new heterogeneous nanocomposite can be recovered and reused easily from the reaction media at least ten times without significant decrease in catalytic activity.

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Solvent polarity effects on the regioselectivity of photoaddition reactions of arene-ethene systems

Use of polar solvents in the $\text{ortho}$ protocycloaddition of ethenes to anisole greatly increases the regioselectivity of the process and the major adducts reflect 1,2-attack onto the arene.     

Laminar heat and mass transfer with arbitrary order heterogeneous and homogeneous reactions: I: In a rectangular duct

Numerical solutions are presented for the problem of simultaneous convective diffusion of reactants, products and heat with arbitrary order heterogeneo The effects of reaction rate parameters, Lewis number, diffusivity ratio, reaction heat parameters, stoichiometric coefficient ratio and reaction order rectangular duct with small aspect ratios.     

Solvent isotope effects on interfacial protein electron transfer in crystals and electrode films

D(2)O-grown crystals of yeast zinc porphyrin substituted cytochrome c peroxidase (ZnCcP) in complex with yeast iso-1-cytochrome c (yCc) diffract to higher resolution (1.7 A) and pack differently than H(2)O-grown crystals (2.4-3.0 A). Two ZnCcP's bind the same yCc (porphyrin-to-porphyrin separations of 19 and 29 A), with one ZnCcP interacting through the same interface found in the H(2)O crystals. The triplet excited-state of at least one of the two unique ZnCcP's is quenched by electron transfer (ET) to Fe(III)yCc (k(e) = 220 s(-1)). Measurement of thermal recombination ET between Fe(II)yCc and ZnCcP+ in the D(2)O-treated crystals has both slow and fast components that differ by 2 orders of magnitude (k(eb)(1) = 2200 s(-1), k(eb)(2) = 30 s(-1)). Back ET in H(2)O-grown crystals is too fast for observation, but soaking H(2)O-grown crystals in D(2)O for hours generates slower back ET, with kinetics similar to those of the D(2)O-grown crystals (k(eb)(1) = 7000 s(-1), k(eb)(2) = 100 s(-1)). Protein-film voltammetry of yCc adsorbed to mixed alkanethiol monolayers on gold electrodes shows slower ET for D(2)O-grown yCc films than for H(2)O-grown films (k(H) = 800 s(-1); k(D) = 540 s(-1) at 20 degrees C). Soaking H(2)O- or D(2)O-grown films in the counter solvent produces an immediate inverse isotope effect that diminishes over hours until the ET rate reaches that found in the counter solvent. Thus, D(2)O substitution perturbs interactions and ET between yCc and either CcP or electrode films. The effects derive from slow exchanging protons or solvent molecules that in the crystal produce only small structural changes.     

Laminar heat and mass transfer with arbitrary order heterogeneous and homogeneous reactions: II: In a circular tube

Solutions to the coupled heat and mass transfer with arbitrary order heterogeneous and homogeneous reactions for non-newtonian power law fluids in a la of the flow index and the reaction orders on the system performance.     

Absorption spectra related to heterogeneous electron transfer reactions: the perylene TiO2 system

Linear absorption spectra of dye-semiconductor systems (perylene attached to nanostructured TiO2) are studied theoretically and experimentally. The systems show ultrafast photoinduced heterogeneous electron transfer (HET). By applying a time-dependent formulation of the absorbance, the theoretical analysis of the measured data is carried out. The respective electron-vibrational wave packet propagation fully accounts for the electronic coupling to the conduction band continuum of TiO2 and is based on a single-reaction-coordinate model (corresponding to a perylene in-plane C-C stretching vibration with a quantum energy of 1370 cm(-1)). By the insertion of different bridge-anchor groups, the electronic coupling responsible for HET is varied. The dye absorbance in a solvent and the trends in the line broadening of the vibrational progression due to the coupling to the conduction band continuum are reproduced for all investigated types of bridge-anchor groups. HET rates deduced from the calculations on the absorbance displaying line broadenings follow the qualitative trend obtained from transient absorption spectra.     

Silica-supported metallocenes: stereochemical comparison between homogeneous and heterogeneous catalysis

Propylene was polymerized in the presence of the isospecific Et(Ind)₂ZrCl₂ (Et: ethylene, Ind: indenyl) and the aspecific (Ind)₂ZrCl₂ complexes in solution and anchored to SiO₂ and SiO₂/MAO (MAO: methylaluminoxane) supports. From the stereochemical analysis of the polypropene samples obtained it can be deduced that (i) the same active species is formed when a metallocene is in solution and when it is anchored to the SiO₂/MAO support and (ii) a completely different active species is formed when the metallocene is anchored to the silica. The fact that both systems Et(Ind)₂ZrCl₂ SiO₂ and (Ind)₂ZrCl₂ SiO₂ produce the same prevailingly isospecific polymer suggests that only isospecific centers are formed in this case, independently of the metallocene stereochemical structure.     

Solvent effects on competitive intramolecular electron transfer and energy transfer in a covalently linked porphyrin-phthalocyanine heterodimer

Photophysical properties of a porphyrin-phthalocyanine heterodimer covalently linked with a dipentoxy chain have been studied.Absorption spectra show that there is weak exciton coupling between the two chromophores in the ground state.Fluorescence spectra show that intramolecular energy transfer from porphyrin to phthalocyanine moiety occurs in competition with electron transfer.The efficiency of these two processes depends upon the mutual orientation of the two chromophores.The effect of solvent polarity on the intramolecular processes is also discussed.     

Remarkable effects of counter ions on scandium ion-promoted electron transfer reactions

Scandium ion-promoted electron transfer reactions of p-benzoquinone are remarkably accelerated when tetrakis(pentafluorophenyl)borate anion is used instead of trifluoromethanesulfonate anion as the counter anion of scandium ion. Only a catalytic amount of scandium borate salt (Sc[B(C6F5)4]3) accelerates significantly the Diels-Alder reaction of 9,10-dimethylanthracene with p-benzoquinone, which proceeds via Sc(3+)-promoted electron transfer from the anthracene to p-benzoquinone.