Oscillating Free Induction Decay in Polymer Systems: Theoretical Analysis

It is shown that during the Anderson-Weiss transformation derivation, as applied to a reptating polymer chain, the spin system can be divided into two sub-systems: an ergodic fluctuating one and a nonergodic quasi-static one. Consequently, the free induction decay (FID) expression is factorized. The factor with a fluctuating dipole-dipole interaction for an arbitrary correlation time is transformed into the generalized Anderson-Weiss exponential, and the factor with a quasi-static dipole-dipole interaction in an isotropic polymer melt is transformed into the oscillating Fourier image of the Pake doublet. The final expression permits both describing the FID shape of a polymer melt as a function of the molecular mass and temperature below the temperature of the primitive segment quasi-static state and forecasting the temperature range in which oscillating FIDs have been observed for poly(isoprene) melt. The new approach is applicable to qualitatively describe the oscillating FID in semi-crystalline poly(ethylene).     

胶束介质中萘敏化双乙酰的室温磷光

研究了ＳＤＳ胶束介质中萘敏化双乙酰有室温磷光，认为ＳＤＳ增加了萘与乙酰的能量转移效率，敏化磷光强度与Ｐｏｉｓｓｏｎ分布有关，并由胶团中含较少给（受）体的Ｐｉ３ｓａ＾Ａ×Ｐｌｅｓｓ＾Ｄ决定。选择了最佳实验条件，该法简便、快速、萘的检出限达６．５×１０＾－８ｍｏｌ／Ｌ。     

Kinetics of Heterogeneous Isotopic Exchange Reaction in Finite Bath

A general method to find the rate constant and particle self-diffusion coefficient is suggested for a heterogeneous isotopic exchange reaction which is controlled by surface mass reaction or controlled by a combination of surface mass reaction and intraparticle diffusion. The values of the kinetic parameter, ξ₁ (ratio of the forward surface mass reaction rate to the intraparticle diffusion rate), particle self-diffusion coefficient D and rate constant k are obtained by the proposed method for the isotopic exchange reaction systems CaCO₃(s)/Ca²⁺(aq) and CaC₂O₄(s)/Ca²⁺(aq).     

Low-Temperature Phosphorescence Analysis of Mebendazole and Related Imidazoles

Abstract

Native low-temperature phosphorescence of mebendazole and flubendazole in ethanol is used for the determination of these imidazoles in anthelmintic preparations with wavelength maxima and detection limits of λ_EXC = 322 nm, λ_EM = 454 nm; 10 ng ml^?1 and λ_EXC = 325 nm, = λ_EM = 455 nm; 5 ng ml^?1, respectively, with linear response up to 8 μg ml^?1 and 9 μg ml^?1, respectively. The structural basis of these phenomena is discussed for both compounds and for related imidazoles and benzimidazoles. Apart from good sensitivity and excellent specificity offered by the technique, the use of cryogenic equipment (liquid nitrogen, special cuvettes, expensive dewar cells) implies some disadvantages for routine analyses.     

Kinetics of Photochemical Systems with Modulated Optical Excitation

The kinetic equations of a photochemical system are discussed in which the transitions from the ground to the first excited state are produced by a sinusoidally modulated photon source. The system includes irreversible reactions starting from the first excited singlet state S₁ and the first triplet state T₁ (cf. Fig. 1). Explicit solutions are given for a linear approximation of the equations of motion for the parametrically modulated system. The solutions apply only when there is weak optical pumping, low quantum yields of the photochemical end-products and for certain conditions, of the modulation frequency. The concentrations of all particles occurring are shown to consist of modulated and unmodulated components where the amplitudes of the modulated components are slowly decaying exponentials. Thc phases of the modulated components are shown to be non-decreasing along a sequence of irreversible steps. Formulae are given which permit determination of the photochemical system rate constants and location of a particle in a sequence of irreversible reactions.     

铜在硫胺燐光分析中的作用机制

硫胺在碱中加入硫酸铜可促进硫胺转化为燐光体。提高反应温度或剧烈振荡及延长反应时间均可提高硫胺燐光，加入Ｎａ２ＳＯ２Ｏ４则燐光下降。从加铜后燐光光谱不变，活化能下降，说明铜是催化硫胺氧化。氧化的硫胺有强的燐光，再加入铜则燐光下降，且不受反应温度、时间的影响，说明燐光下降是铜的猝灭作用。在一定浓度范围内，无论催化氧化与猝灭，铜量与燐光变化呈线性相关。     

Excitation Dynamics in Hetero‐bichromophoric Calixarene Systems

In this work, the dynamics of electronic energy transfer (EET) in bichromophoric donor–acceptor systems, obtained by functionalizing a calix[4]arene scaffold with two dyes, was experimentally and theoretically characterized. The investigated compounds are highly versatile, due to the possibility of linking the dye molecules to the cone or partial cone structure of the calix[4]arene, which directs the two active units to the same or opposite side of the scaffold, respectively. The dynamics and efficiency of the EET process between the donor and acceptor units was investigated and discussed through a combined experimental and theoretical approach, involving ultrafast pump–probe spectroscopy and density functional theory based characterization of the energetic and spectroscopic properties of the system. Our results suggest that the external medium strongly determines the particular conformation adopted by the bichromophores, with a direct effect on the extent of excitonic coupling between the dyes and hence on the dynamics of the EET process itself.     

Selective Reductive Transformations of Organic Nitro Compounds in Heterogeneous Photocatalytic Systems: A Review

Theoretical and Experimental Chemistry - The cutting-edge research on the photocatalytic selective reductive transformations of organic nitro compounds in heterogeneous systems on the basis of...     

Tunable Phosphorescence/Fluorescence Dual Emissions of Organic Isoquinoline-Benzophenone Doped Systems by Alkoxy Engineering

Dual/multi-component organic doped systems with room-temperature phosphorescence (RTP) properties have been developed. However, the unknown luminescence mechanism still greatly limits the development of the doped materials. Herein, a new doped system exhibiting phosphorescence/fluorescence dual emission (Φ_phos=4–24 % and τ_phos=101–343 ms) is successfully constructed through prediction and design. A series of isoquinoline derivatives with different alkoxy chains were selected as the guests. Benzophenone was chosen as the host owing to the characteristics of low melting point and good crystallinity. The alkoxy chain lengths of the guests are first reported to be used to control the fluorescence and phosphorescence intensities of the doped materials, which results in different prompt emission colors. Additionally, the doped ratio of the guest and host can also control the luminous intensities of the materials. In particular, the doped materials still exhibit phosphorescent properties even if the ratio of the guest/host is as low as 1:100 000.     

Analysis of Impurities in Commercial Fluorene by Synchronous Nonprotected Room-Temperature Phosphorescence

Abstract

The applicability of nonprotected room-temperature phosphorescence (NP-RTP) in real samples is demonstrated in this work. In this methodology only two reagents, potassium iodide and sodium sulfite, were used to obtain the phosphorescent signals. Synchronous phosphorimetry was used to enhance sensitivity. Analyses of dibenzofuran, acenaphthene, and fluorene were carried out in a sample of impure fluorene.     

Nanoscale Magnetic Stirring Bars for Heterogeneous Catalysis in Microscopic Systems

Nanometer‐sized magnetic stirring bars containing Pd nanoparticles (denoted as Fe₃O₄‐NC‐PZS‐Pd) for heterogeneous catalysis in microscopic system were prepared through a facile two‐step process. In the hydrogenation of styrene, Fe₃O₄‐NC‐PZS‐Pd showed an activity similar to that of the commercial Pd/C catalyst, but much better stability. In microscopic catalytic systems, Fe₃O₄‐NC‐PZS‐Pd can effectively stir the reaction solution within microdrops to accelerate mass transfer, and displays far better catalytic activity than the commercial Pd/C for the hydrogenation of methylene blue in an array of microdroplets. These results suggested that the Fe₃O₄‐NC‐PZS‐Pd could be used as nanoscale stirring bars in nanoreactors.     

Macrocyclic Polyethers as Enolate Activators in Homogeneous and Heterogeneous Systems

Abstract

The ability of macrocyclic polyethers to activate enolates has been studied in the alkylation of deoxybenzoin (1) with butyl derivatives nBuY (Y = Br, I, OMes) catalyzed by crown ether PHDB18C6 (7) or cryptand [2.2.2, C₁₀] (8) under phase-transfer catalysis (PTC) and homogeneous (chlorobenzene) conditions. The enolate reactivity is mainly determined by the ligand (cryptand>crown ether) and solvent (increasing with the polarity, in the order: toluene<chloroben-zene<1,2dichlorobenzene). Regioselectivity of the reaction is also remarkably affected by ligand and alkylating agent.     

多相催化体系的反应机理和失活模式(英文)

Solving the problem of catalyst deactivation is essential in process design. To do this, various aspects of the kinetics of processes with catalyst deactivation, and their different mechanisms, are discussed. Catalyst deactivation often cannot be avoided, but more knowledge on its mechanism can help to find kinetic means to reduce its harmful consequences. When deactivation is caused by coke, the generation of coke precursors is the determining step in the deactivation kinetics. Different types of deactivation were distinguished that lead to different evolution of the process. The phenomenon of non-uniform coking can be linked to catalyst surface non-uniformity. For the class of catalysts with more than one type of active sites, an explanation was suggested for the observed trends in the deactivation modes. For catalytic proc-esses using catalyst particles of industrial size, the influence of intraparticle diffusion resistance is important. The analysis showed that for a number of processes, the decrease of the reaction rate due to deactivation is less under diffusion control. For certain reaction mechanisms, there exist operation conditions where the rate of the process under diffusion control exceeds the rate in the kinetic control regime. A signifi-cant problem is the change of selectivity in the course of catalyst deactivation. The selectivity may either decrease or increase, and depends on the reaction mechanism during deactivation. The changes are larger when there is no diffusion resistance. The intentional poisoning of catalysts and its influence on catalyst activity and selectivity for the process of ethylene oxide production was discussed.     

Macrocyclic Polyethers as Enolate Activators in Homogeneous and Heterogeneous Systems

Abstract

The ability of macrocyclic polyethers to activate enolates has been studied in the alkylation of de-oxybenzoin (1) with butyl derivatives nBuY (Y = Br, I, OMes) catalyzed by crown ether PHDB18C6 (7) or cryptand [2.2.2,C10] (8) under phase-transfer catalysis (PTC) and homogeneous (chlorobenzene) conditions. The enolate reactivity is mainly determined by the ligand (cryptand > crown ether) and solvent (increasing with the polarity, in the order: toluene < chlorobenzene < 1,2dichlorobenzene). Regioselectivity of the reaction is also remarkably affected by ligand and alkylating agent.     

Excitation Energy Transfer and Exchange-Mediated Quartet State Formation in Porphyrin-Trityl Systems

Photogenerated multi-spin systems hold great promise for a range of technological applications in various fields, including molecular spintronics and artificial photosynthesis. However, the further development of these applications, via targeted design of materials with specific magnetic properties, currently still suffers from a lack of understanding of the factors influencing the underlying excited state dynamics and mechanisms on a molecular level. In particular, systematic studies, making use of different techniques to obtain complementary information, are largely missing. This work investigates the photophysics and magnetic properties of a series of three covalently-linked porphyrin-trityl compounds, bridged by a phenyl spacer. By combining the results from femtosecond transient absorption and electron paramagnetic resonance spectroscopies, we determine the efficiencies of the competing excited state reaction pathways and characterise the magnetic properties of the individual spin states, formed by the interaction between the chromophore triplet and the stable radical. The differences observed for the three investigated compounds are rationalised in the context of available theoretical models and the implications of the results of this study for the design of a molecular system with an improved intersystem crossing efficiency are discussed.     

Decay Kinetics of Benzophenone Triplets and Corresponding Free Radicals in Soft and Rigid Polymers Studied by Laser Flash Photolysis

The kinetics of transients formed under photoexcitation of benzophenone (B) dissolved in three different polymers was studied by ns laser flash photolysis. These polymers were the soft rubbers poly (ethylene‐co‐butylene) (EB), polystyrene block‐poly(ethylene‐ran‐butylene)‐block‐polystyrene (SEBS) and hard polystyrene (PS). We monitored the decay kinetics of triplet state ³B^*and of ketyl radicals BH^●. We observed exponential decay of ³B^* and two‐stage decay kinetics of BH^●in EB. The first stage is a fast cage recombination of a radical pair (BH^●, radical of polymer R^●). The second slow stage of BH^● decay follows the second‐order law with a relatively high rate constant, which corresponds to recombination of BH^● in a homogeneous liquid with a viscosity of only ~0.1 P (about five times of 2‐propanol viscosity). Application of a magnetic field (MF) of 0.2 T leads to deceleration of both stages of BH^● decay in EB by approximately 20%. Decay kinetics of both transients were observed in SEBS. There was no MF effect on BH^● decay in SEBS. We only observed ³B^* in PS. Decay kinetics of ³B^* in this case were described as polychromatic dispersive first‐order kinetics. We discuss the effects of polymer structure on transient kinetics and the MF effect.     

Conversion of Solar Energy to Fuels by Inorganic Heterogeneous Systems

Over the last several years,the need to find clean and renewable energy sources has increased rapidly because current fossil fuels will not only eventually be depleted,but their continuous combustion leads to a dramatic increase in the carbon dioxide amount in atmosphere.Utilisation of the Sun’s radiation can provide a solution to both problems.Hydrogen fuel can be generated by using solar energy to split water,and liquid fuels can be produced via direct CO2 photoreduction.This would create an essentially free carbon or at least carbon neutral energy cycle.In this tutorial review,the current progress in fuels’ generation directly driven by solar energy is summarised.Fundamental mechanisms are discussed with suggestions for future research.     

Automatic Computerized Analysis of Multi-Exponential Decay Curves

Abstract

A prototype method is described which automatically analyzes exponential decay curves by using nonlinear regression analysis and deviation pattern recognition. Lifetimes and preexponential factors are computed for the proper model. The program successfully classified simulated data consisting of one, two, or three exponentials when noise followed a Poisson distribution. However, for real luminescence decay data, systematic instrumental noise of higher frequency tended to obscure low frequency deviations from incorrect models in smoothed deviation plots. This factor complicated classification of multi-exponential data, although the program easily distinguished between single and multiple exponential behavior.     

非均匀体系介电研究的现状与展望*

用介电谱方法讨论化学中的非均匀体系是近十几年发展起来的交叉性新领域, 它在分散体系、高分子膜以及生物细胞体系的研究中占有重要地位。本文从理论和应用两方面综述该领域的发展过程及研究现状, 并结合实例阐述了构造和介电弛豫模式的关系以及该方法的特点。在对遗留问题和最新进展给予评述性讨论的同时, 指出了进一步的研究方向。