Excitation energy transfer in covalently bonded porphyrin heterodimers

We describe the photophysical properties of heterodimers that are formed by the free base 2-(2-carboxyvinyl)-5,10,15,20-tetraphenylporphyrin and the zinc complex of 5-(p-aminophenyl)-10,15,20-triphenylporphyrin and that are covalently bonded by the amide link. These dimers differ in the configuration of the double bond in the spacer group. We determine fluorescence quantum yields of heterodimers and their porphyrin components. The energy transfer rate constants have been estimated from the measured fluorescence lifetimes and fluorescence excitation spectra and, also, they have been calculated from the steady-state absorption and fluorescence spectra according to the Förster theory. We have found that the efficiency of the intramolecular energy transfer in heterodimers is 0.97–0.99, and the energy migration rate constants have been found to be (1.82–4.49) × 10¹⁰ s^?1. The results of our investigation show that synthesized heterodimers can be used as efficient light-harvesting elements in solar energy conversion devices.     

Preparation and luminescence properties of hybrid materials containing lanthanide complexes covalently bonded to a terpyridine-functionalized silica matrix

New kinds of organic-inorganic hybrid materials consisting of lanthanide (Er³⁺, Eu³⁺, and Tb³⁺) complexes covalently bonded to a silica-based network have been obtained by a sol-gel approach. A new versatile compound containing terpyridine has been synthesized by 4′-p-aminophenyl-2,2′:6′,2″-terpyridine and 3-(triethoxysilyl)propyl isocyanate, which is used as the a ligand of lanthanide ions and also the siloxane network precursor. The obtained hybrid materials were characterized by FT-IR, TGA, DSC, near-infrared, and visible spectrofluorometer, as well as decay analysis. For the Hybrid-Er and Hybrid-Eu, excitation at the ligand absorption wavelength resulted in the typical near-IR luminescence (centered at around 1.54 μm) resulting from the ⁴I_13/2-⁴I_15/2 transition of Er³⁺ ions and strong visible region emission of the Eu³⁺ ions (⁵D₀-⁷F_J), which contributed to the efficient energy transfer from the ligands to the lanthanide ions. However, we have not found strong emission for the Hybrid-Tb. This indicated that the energy transfer did not take place in this system. A model of indirect excitation mechanism to explain the phenomenon was also suggested.     

Statistical mechanical models for liquid and amorphous structure in covalently bonded systems

Summary Considerable effort has been given for some years to developing models of interatomic forces aimed at accounting for bond directionality in liquid and amorphous state calculations. Models involving three-body potentials have been especially useful for computer simulation studies of liquid and amorphous states in elemental semiconductors and binary chalcogenides of group-IV elements, starting with the work of Stillinger and Weber on silicon. However, pair potential models that may still account for the main effects of angular dependences of the effective interatomic forces, though at a primitive level, are desirable from the viewpoint of liquid structure theory. Developments in this direction are briefly reviewed, with particular emphasis on bond particle models for the structure of liquid and amorphous germanium. We also discuss the relation between liquid structure in a bond-particle model and crystallization accompanied by electron localization and volume expansion, as observed in elemental and III-V polar semiconductors. Paper presented at the workshop ?Highlights on Simple Liquids?, held in Turin at ISI on 1–3 May, 1989.     

Kinetics of avalanche mixing of granular materials

The problem of the avalanche mixing of two fractions of granular material is solved. Mixing of the fractions takes place in a cylinder that rotates slowly about its longitudinal axis, which is positioned horizontally. The cylinder is not filled completely and at all times mixing only occurs in the surface layer of granules. It is shown that, depending on the relation of the volumes of the fractions and the volume of the empty space, mixing can take place slowly, over a large number of rotations, in a diffusive regime with convection or rapidly, by the time the cylinder has turned through a small angle. The mixing process is described analytically in terms of a purely geometrical approach and can, in a number of situations, be reduced to a sequence of discrete mappings. The characteristic mixing times are determined, including the times over which one or the other of the pure fractions no longer exists in the regions adjacent to the surface of the cylinder. Their dependence on the degree of filling of the cylinder and on the ratio of the volumes of the fractions is found. Zh. éksp. Teor. Fiz. 112, 257–277 (July 1997)     

Strain rate effects on compressive behavior of covalently bonded CNT networks

In this study, strain rate effects on the compressive mechanical properties of randomly structured carbon nanotube (CNT) networks were examined. For this purpose, three-dimensional atomistic models of CNT networks with covalently-bonded junctions were generated. After that, molecular dynamics (MD) simulations of compressive loading were performed at five different strain rates to investigate the basic deformation characteristic mechanisms of CNT networks and determine the effect of strain rate on stress–strain curves. The simulation results showed that the strain rate of compressive loading increases, so that a higher resistance of specimens to deformation is observed. Furthermore, the local deformation characteristics of CNT segments, which are mainly driven by bending and buckling modes, and their prevalence are strongly affected by the deformation rate. It was also observed that CNT networks have superior features to metal foams such as metal matrix syntactic foams (MMSFs) and porous sintered fiber metals (PSFMs) in terms of energy absorbing capabilities.     

Characteristic time of avalanche mixing of granular materials

Fractions of a granular material are mixed in a cylinder rotating slowing about its horizontally oriented longitudinal axis. The cylinder is not completely full, and at any instant, mixing occurs only on the free surface of the material. The complete dependence of the characteristic time of such so-called avalanche mixing on the fill level of the cylinder is constructed within a simple geometric approach. This dependence faithfully describes the actual experimental curve. Near the critical point of half-filling, at which mixing to a homogeneous state does not occur, the reciprocal of the characteristic mixing time is proportional to δ ² ln(|δ|⁻¹), where δ is the deviation of the fill level from half-filling. Zh. éksp. Teor. Fiz. 112, 2114–2123 (December 1997)     

Time of avalanche mixing of granular materials in a half filled drum

The avalanche mixing of granular solids in a slowly rotated 2D upright drum is studied. We demonstrate that the account of the difference δ between the angle of marginal stability and the angle of repose of the granular material leads to a restricted value of the mixing time τ for a half filled drum. The process of mixing is described by a linear discrete difference equation. We show that the mixing looks like linear diffusion of fractions with the diffusion coefficient vanishing when δ is an integer part of π. Introduction of fluctuations of δ suppresses the singularities of τ(δ) and smoothes the dependence τ(δ). Received 27 October 2000 and Received in final form 13 March 2001     

Controlled deposition of covalently bonded tantalum oxide on carbon supports by solvent evaporation sol-gel process

A simple strategy for covalently attaching Ta₂O₅ particles onto functionalized graphitic carbon supports has been developed to fabricate hybrid nanocomposites. In this process, tantalum ethoxide was directly reacted with functional groups on the carbon surface to form covalent bonding, which caused the carbonyl stretches of the carbon supports to be blue-shifted to 50-70 cm⁻¹ after Ta₂O₅ particle deposition. Homogeneously deposited Ta₂O₅ particles on the carbon supports have been studied by X-ray diffraction (XRD), FT-IR spectroscopy, scanning electron microscopy (SEM), and transmission electron microscope (TEM).     

Physicochemical properties of Mg-phthalocyanine molecules covalently bonded to the surface of nanopores in a silicate gel matrix

Molecules of Mg-tetracarboxyphthalocyanine, chemically modified with 3-aminopropyltrimethoxysilane, were immobilized on the surface of the nanopores in a silicate gel matrix by copolymerizing them with tetraethoxysilane monomer. Formation of covalent bonds between the Mg-phthalocyanine molecules and the surface of the nanopores in the silicate xerogel was confirmed by the absence of leaching of the “grafted” pigment from the interior volume of the xerogel to the liquid extraction agent, and also by IR Fourier spectroscopy data. The observed bleaching of the activated silicate xerogel under normal atmospheric conditions is reversible: subsequent heat treatment restores the original absorption spectrum. We hypothesize that two mechanisms are responsible for the observed effect: formation of high order associates of the Mg-phthalocyanine molecules involving water molecules adsorbed from the atmosphere, and/or oxidation of the phthalocyanine macrocycle with disruption of the conjugation chain. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 591–597, September–October, 2007.     

Morphological effects of single-layer graphene oxide in the formation of covalently bonded polypyrrole composites using intermediate diisocyanate chemistry

Single-layer graphene oxide (SLGO) possesses carboxylic and hydroxyl groups suitable for reactions with aliphatic or aromatic diisocyanate molecules. TEM analysis reveals that aliphatic diisocyanate molecules caused SLGO to scroll into star-like formations, whereas aromatic diisocyanate molecules retained SGLO in a flat-sheet morphology. TGA confirms the stabilisation of the formed urea and urethane groups on SLGO, but the onset of sheet pyrolysis occurs at a lower temperature due to isocyanate reactions with anhydride and epoxide groups embedded in the sheet. Pendant isocyanate groups act as bridging units to facilitate the attachment of pyrrole molecules, which are then used as anchor sites for the covalent polymerisation of pyrrole to polypyrrole (PPy). The use of FeCl₃ as the polymerisation catalyst generated both covalent and free PPy, but also iron hydroxide nanoparticles were observed decorating the SLGO surface. When using ammonium persulfate as a catalyst and dodecylbenzenesulfonate as a dopant, free PPy could be removed under treatment with solvents to leave a purely covalent system. Discrete regions of SLGO were observed decorated with nanoparticles of PPy along the edge or across the surface of individual sheets. It was found that the flexibility of the SLGO sheet and the type of diisocyanate used directly affected the electrical resistance of the final composite.     

Synthesis and electron paramagnetic resonance study of a nitroxide free radical covalently bonded on aminopropyl-silica gel

A solid spin-labeled material was obtained starting from 2-chloro-3,5-dinitro-N-(4-(2,2,6,6-tetramethyl-piperidine-1-oxyl)-benzamide) and aminopropyl-silica gel. Stability tests showed that even after several months the spin-labeled material had the same properties as immediately after synthesis. EPR properties of the TEMPO-derivatized silica were studied as a function of solvent polarity and temperature. Rotational correlation times were calculated from EPR spectra and correlated with solvent characteristics and temperature. Polar solvents induce a fast motion of the spin-label, clearly seen in the EPR spectra by the apparition of the well-known TEMPO radical triplet. The solid spin-labeled (dry) sample showed a high interspin interaction, which can be disrupted not only by different (liquid) solvents, but also by absorption of different solids, like cyclodextrins, dendrimers or polyethyleneglycols. Also, changes induced by the temperature were studied in the case of toluene wet sample. From 150 to 370 K, the spectrum is changing from a slow motion spectrum type to a fast motion regime. The preparative procedures to obtain the spin-labeled silica as well as some of its parameters are described.     

卟啉铜接枝SiO2有机-无机复合材料及强的非线性折射率

通过卟啉配合物Meso-四(4-羧基苯基)卟啉铜(简称Cu(Ⅱ)-TCPP)中的羧基与γ-氨基丙基三乙氧基硅烷(NH₂(CH₂)₃Si(OC₂H₅)₃,KH550)中的氨基的相互化学作用,成功地把卟啉配合物接枝到KH550中,随着KH550中乙氧基的水解与聚合反应的进行,卟啉铜连接到固体介质中,从而大幅度提高了卟啉在无机固体介质中的掺杂浓度. 将反应产物与γ-(2,3-环氧丙氧基)丙基三甲氧基硅烷(CH₂CHCH₂O(CH₂)₃Si(OCH₃)₃,KH560) 相杂化,形成物化性能良好、连接卟啉的有机-无机复合材料. 用红外光谱表征了Cu(Ⅱ)-TCPP与KH550的化学反应产物,用紫外—可见吸收光谱研究Cu(Ⅱ)-TCPP的分子状态. 应用Z扫描技术研究不同Cu(Ⅱ)-TCPP掺杂浓度的复合材料的非线性光学性质,其三介非线性折射率n₂达-1.1161×10^-16 m²/W. 关键词： 非线性折射率 有机-无机复合材料 接枝 Meso-四(4-羧基苯基)卟啉铜     

Preparation, characterization, and luminescence properties of novel hybrid material containing europium(III) complex covalently bonded to MCM-41

In this paper, a novel luminescent organic-inorganic hybrid material containing covalently bonded ternary europium complex in mesoporous silica MCM-41 has been successfully prepared by co-condensation of tetrethoxysilane (TEOS) and the modified ligand 2-phenyl-1H-imidazo[4,5-f][1,10]phen-3-(triethoxysilyl)propylcarbamate (PIP-Si) in the presence of cetyltrimethylammonium bromide (CTAB) surfactant as template. PIP-Si containing 1,10-phenanthroline covalently grafted to 3-(triethoxysilyl)propyl isocyanate is used not only as a precursor but also as the second ligand for Eu(TTA)₃·2H₂O (TTA: 2-thenoyltrifluoroacetate) complex to prepare a novel functionalized mesoporous material. The resulted mesoporous composite materials, which demonstrate strong characteristic emission lines of Eu³⁺⁵D₀-⁷F_J (J=0, 1, 2, 3, 4), were characterized by Fourier transform infrared (FT-IR), small-angle X-ray diffraction, excited-state decay analysis. Emission intensity of the Eu(III) complex covalently linked to MCM-41 (Eu-MCM-41) increases with the increasing irradiation time, demonstrating better photostability compared with both pure Eu(III) complex and physically incorporated sample.     

Characterization of alkyl monolayer covalently bonded to Si(111) and soft-landing of vanadium-benzene sandwich clusters onto the alkyl monolayer substrate

A hexadecyl monolayer covalently attached to Si(111) surfaces (C₁₆–Si(111)) was prepared at 200 ^○C from 1-hexadecene. Formation of the monolayer was characterized by water contact angle measurement, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Gas phase synthesized vanadium (V)-benzene (Bz) 1:2 (VBz₂) sandwich clusters were size-selectively deposited onto the C₁₆–Si(111) substrate thus prepared and an oxidized Si substrate. Investigation of the resultant clusters was implemented by thermal desorption spectroscopy (TDS). About 30 K increase in threshold desorption temperature of the landed clusters was observed on going from the oxidized Si to the C₁₆–Si(111) substrate, a result indicating that the clusters are more strongly bound to the C₁₆–Si(111) than to the oxidized Si. This result was explained by the penetration of the landed clusters into the hexadecyl monolayer.     

Silicon diodes in avalanche pulse-sharpening applications

Silicon diodes operated in an avalanche breakdown mode can he used to reduce, or sharpen, the rise times of driving pulses. Proper operation of a diode in this manner requires the application of a driving pulse with sufficient time rate of change of voltage dV/dt. The rapidly changing reverse bias produces an electron-hole plasma of sufficient density that the electric field strength in the n region of a p⁺-n-n⁺ structure is significantly reduced and the plasma is essentially trapped. In effect, the plasma generation causes the device to transition from a high-impedance state to a low-impedance state in a short period of time, and thus acts as a fast closing switch. This paper provides an overview of this mode of operation. A simplified theory of operation is presented. A comparison is made among the results of numerical modeling, the theory of operation of the silicon avalanche shaper (SAS) diode, and the theory of operation of the trapped-plasma avalanche-triggered transit (TRAPATT) mode of operation of a diode. Based on the results of numerical modeling, conclusions are drawn on what factors most greatly affect the performance of avalanche shaper diodes, and one optimized design is provided     

Cascaded homojunction avalanche photodiodes

The structure and the performance of a cascaded avalanche photodiode (APD) based on cascaded home-junctions are discussed. Similar to the hetero-structure superlattice APD, such a device has a lower excess noise and a higher gain at lower bias voltages than conventional APDs. In addition to the ionization ratio, several parameters also affect the device's performance.     

Epitaxial silicon avalanche photodiode

Preparation of silicon avalanche photodiodes by a planar method in epitaxial PP⁺ silicon slices limited charge collection to carriers generated in or close to the depletion region. Reducing the effective size of the zero-field absorption region surrounding the depletion region by this method, resulted in the output signal current of the device reproducing the input light pulse shape, within 15 ns. When illuminated with monochromatic radiation, the shot noise of a detector prepared in epitaxial material was less than that observed with a device fabricated in homogeneous material, due to the reduced signal current.