Simulation of the Electron Paramagnetic Resonance Spectrum of the Triplet Ground State Dimer Di-μ-(Pyridine N-Oxide)Bis[Bisnitrato(Pyridine N-Oxide)Copper(II)]

The EPR spectrum of the triplet ground state dimer di-μ-(pyridine N-oxide)bis[bisnitrato(pyridine N-oxide)copper(II)] has been reported recently¹. Of the very few triplet ground state copper(II) dimers with resolved metal hyperfine structure^2,3, the EPR spectrum of this complex is most complete. Previously, the analysis of the spectra of triplet ground state copper(II) complexes, in order to extract magnetic parameters, has been made using the equations reported by Wasserman et al.⁴ The best magnetic parameters should be obtained from a simulation of the experimental spectrum. We wish to report here the computer simulation of the EPR spectrum of a powdered sample of [Cu(II) (PYO)₂ (NO₃)₂]₂.     

Kr-KCl exciplex lamp

A new source of UV radiation (excilamp) based on Kr-KCl vapor-gas mixture excited by a longitudinal high-voltage pulsed-periodic discharge is created. The emission spectrum and time characteristic of the discharge radiation are studied. The B → X transition in the KrCl* excimer molecule (λ = 222 nm) is found to dominate in the radiation spectrum of the source. The dependence of the pulse output power of the lamp on various factors is investigated, and the mechanism of B state formation in the excimer molecule is discussed.     

乙醇-水团簇分子形成激基缔合物及荧光发射机理研究

紫外光照射具有特殊结构的长链式乙醇-水团簇分子时,处于激发态和基态的分子形成了分子间激基缔合物,并发射荧光.根据实验结果分析和能量转移理论可知,激发态单分子和激基缔合物间形成了电子迁移洛合物并发生了能量转移.根据Mulliken理论对电子迁移洛合物进行量子力学处理,得出了团簇分子在基态和激发态能量E^b_N和E^b_E以及由于电子迁移而引起的静电相互作用能E^s;根 关键词： 荧光光谱 激基缔合物 电子迁移 乙醇-水团簇     

Isotope selective ionization of tellurium dimers Te2 formed by excimer laser photodissociation of an organometallic compound: CH3TeTeCH3

Usingone standard commercial dye laser, tellurium dimers Te₂ — formed by excimer laser induced photoelimination from CH₃TeTeCH₃ at room temperature — are ionized isotope selectively.     

Fluorescence Study on Translational Mobility in Polystyrene–Polyethyleneglycol Microbeads

Polyethyleneglycol (PEG) chain mobility in gelatineous microbeads is investigated by means of dynamic excimer formation. Pyrenebutyric acid (PYB) is covalently linked to the chain ends as probe molecule. Excimer formation is monitored by steady-state and time-resolved fluorescence spectroscopy in the presence of a series of liquid phases and in the dry state. PYB concentration in the beads is varied over three orders of magnitude up to c = 6·10^–2 M. The concentration is derived from absorption measurements in stirred bead suspensions, considering the deviations from Lambert–Beer's law in heterogeneous systems. Excimer formation is found to be a dynamic process in the presence of liquid phases which solvate both the polymer and the fluorophore. The collisional rate constant, k _DM, is of the order of k _DM-values of PYB in homogeneous solutions, indicating a high translational mobility. Excimer-to-monomer intensity ratios are in general accordance with the solvation capacity of the liquid phase. In the dry state excimer formation is found only at high PYB concentrations, c 3·10^–2 M. It is concluded that this excimer emission arises from aggregated PYB, since corresponding fluorescence response curves show no rise time.     

磺化金属萘酞菁聚集倾向的光谱研究

在磺化金属萘酞菁的荧光光谱和电子吸收光谱的Q谱带上观察到激基缔合物的荧光光谱峰和基态聚集物的吸收光谱峰。荧光光谱和吸收光谱实验说明 ,溶液中聚体分子和单体分子同时存在 ;聚体放出或吸收光子后都解离为单体 ;在荧光光谱中金属萘酞菁的激基缔合物荧光峰相对于单体荧光峰发生红移 ,在吸收光谱中金属萘酞菁基态聚集体的吸收峰相对于单体吸收峰发生蓝移 ,聚集体的荧光光谱和吸收光谱有镜像对称关系。磺化萘酞菁钴浓度较大时 ,其荧光光谱的猝灭较大。     

E.S.R. study of copper and silver N,N-dialkyldiselenocarbamates

Single-crystal E.S.R. studies of Cu(II) diethyldiselenocarbamate and Cu(II) diethyldithiocarbamate, diluted in the corresponding Zn(II) complexes, are reported. The pure copper and zinc crystals consist of dimeric units. When copper is built into the zinc lattice, dimers are formed with one copper and one zinc atom. The g-tensor, the metal hyperfine and quadrupole coupling tensors and also the hyperfine coupling tensors of the ligand selenium atoms have been obtained. Especially, from the measured selenium hyperfine couplings, it could be concluded that the guest molecules do not accept the structure of the host crystal, but have a structure similar to that of the pure copper compound. This is in contrast to the situation in systems where monomeric host crystals (e.g. nickel (II) complexes) are used and where it is found that the guest molecules do accept the structure of the host. As in the monomeric diselenocarbamate systems, in these dimers the principal axes of the g and metal hyperfine coupling tensor do not coincide. In the dimers a rotation of the hyperfine principal axes has taken place in the plane of the molecule, while a rotation of the g-tensor axes in a plane perpendicular to the molecule is typical for the monomeric systems.     

RVB theory of the Hubbard model on the triangular lattice

We propose a Green's function technique, to investigate finite-temperature properties of the Hubbard model on the triangular lattice. The lattices are covered by dimers. The method is exact in two limits:U=0 or decoupled dimers. We apply this approximate method to calculate the ground state energy, the specific heat and the single-particle spectral weight for the 1/2-filled case. The largest lattice considered has 16×16 sites. The approximate ground state energy as a function of the on-site interactionU oscillates around the exact energyin the 1/2-filled case. We find two peaks in the specific heat. ForU5t the single-particle spectral weight splits into upper and lower Hubbard bandasymmetrically. Thus in the 1/2-filled case the chemical potential is placed in the upper band leading to a metallic state. The approximate technique yields a finite zero-point entropy for mediumU. All the investigations signal a RVB state in the range of mediumU as formerly proposed by Callaway.     

Generation of excimer emission in dielectric barrier discharges

Dielectric barrier discharges (silent discharges) are used to excite a large number of excimers radiating in the VUV, UV or visible spectral range. The excited species include rare-gas dimers, halogen dimers as well as rare-gas halogen excimers and mercury halogen excimers. In many cases narrow-band UV radiation of typically 1–17 nm halfwidth and remarkable efficiency (1–10%) could be generated. Thus, dielectric barrier discharges provide a simple, versatile arrangement to study the basic reaction kinetics of excimer formation and also bear a substantial potential for large-scale industrial UV processes.     

Immobilization of pyrene via diethylenetriamine on quartz plate surface for recognition of dicarboxylic acids

A novel photo-luminescence film has been prepared by immobilizing pyrene on a quartz plate surface via diethylenetriamine. Imino structure was intentionally introduced into the long flexible spacer due to their hydrogen bond forming abilities with carboxylic acids. It has been found that the film shows combined monomer and excimer emission of pyrene both in wet and dry states. Steady-state and time-resolved fluorescence emission measurements demonstrated that the excimer emission mainly came from direct excitation of ground state dimers, and/or monomers in aggregated state. The structures of the excimers formed during the excitation are mixtures of “standard excimers” and “distorted excimers” of the fluorophore moieties. Fluorescence lifetime measurements showed that the decay of the film is complex, and a four exponential fit is necessary for getting a satisfied result. The photophysical behavior of the present film could be rationalized by employing the model proposed before. Sensing property studies showed that the emission of the film is sensitive to the presence of dicarboxylic acids, including ethanedioic acid, malonic acid, etc. In contrast, presence of monocarboxylic acids, such as formic acid and acetic acid, had little effect upon the fluorescence emission of the film. Compared with the films taking ethylenediamine or 1,3-diaminopropane as their spacer component, introduction of diethylenetriamine into the spacer improved the performance of the film greatly. In addition, the sensing process is reversible, and the film is stable within a number of months provided it is properly preserved. These characteristics showed that the film is worthwhile for further exploration.     

Monomers,dimers, and tetramers of 1- and 2-naphthoic acids

The absorption and emission spectra of 1- and 2-naphthoic acids and a number of their derivatives have been investigated. The S₁, T₁, and S₀ energy levels of solvent H-bonded monomers and internally H-bonded dimers are affected by the type of H-bonding, by periinteractions, by steric factors provided by substituent groups, and by solvent effects. The excimer fluorescence of 1-naphthoic acid has been detected and is the sole fluorescece of the crystal; it is attributed to a tetrameric sandwich complex of two dimers. The 1-naphthoic acid crystal also exhibits delayed excimer flurescence of a triplet-triplet annihilative type.     

Kinetics of the fluorescence of poly(N-vinylcarbazole) and poly(N-vinylcarbazole) with 2,4,7-trinitrofluorenone in the solid state

The fluorescence decay time measurements by the stroboscopic method have been made on films of amorphous poly(N-vinylcarbazole) (PVCA) and PVCA doped with 2,4,7-trinitrofluorenone (TNF). The obtained lifetimes of two excimer species have at 77 K and at 300 K values of about 43 ns and 9 ns, which are similar to other measurements. The kinetics of excimer fluorescence and energy transfer in PVCA films are described by a simple model of the Stern-Volmer type. It assumes the fast migration of localized singlet excitons and their trapping in two species of excimer traps.We propose a modified kinetic model for films of PVCA with TNF. In this model we assume besides the exciton migration and trapping on excimer and impurity traps also the direct energy transfer from two excimer species to charge transfer complexes of PVCA with TNF by the Förster dipole-dipole mechanism. The estimated critical distance of this transfer are about 10 Å.     

Laser-based techniques for living cell pattern formation

In the production of biosensors or artificial tissues a basic step is the immobilization of living cells along the required pattern. In this paper the ability of some promising laser-based methods to influence the interaction between cells and various surfaces is presented. In the first set of experiments laser-induced patterned photochemical modification of polymer foils was used to achieve guided adherence and growth of cells to the modified areas: (a) Polytetrafluoroethylene was irradiated with ArF excimer laser (λ=193 nm, FWHM=20 ns, F=9 mJ/cm²) in presence of triethylene–tetramine liquid photoreagent; (b) a thin carbon layer was produced by KrF excimer laser (λ=248 nm, FWHM=30 ns, F=35 mJ/cm²) irradiation on polyimide surface to influence the cell adherence. It was found that the incorporation of amine groups in the PTFE polymer chain instead of the fluorine atoms can both promote and prevent the adherence of living cells (depending on the applied cell types) on the treated surfaces, while the laser generated carbon layer on polyimide surface did not effectively improve adherence. Our attempts to influence the cell adherence by morphological modifications created by ArF laser irradiation onto polyethylene–terephtalate surface showed a surface–roughness dependence. This method was effective only when the Ra roughness parameter of the developed structure did not exceed the 0.1 micrometer value. Pulsed laser deposition with femtosecond KrF excimer lasers (F=2.2 J/cm²) was effectively used to deposit structured thin films from biomaterials (endothelial cell growth supplement and collagen embedded in starch matrix) to promote the adherence and growth of cells. These results present evidence that some surface can be successfully altered to induce guided cell growth.     

Structural, magnetic and redox properties of a new cathode material for Li-ion batteries: the iron-based metal organic framework

The iron-based metal organic framework (MOF) presently studied is the first example of MOF showing a reversible electrochemical Li insertion with a very good cycling life. Its potential application as a cathode material in Li-ion battery is nevertheless curbed by a rather poor capacity of 70 mAh/g. To figure out the origin of this limited insertion, first-principles density functional theory (DFT)+U calculations were performed. The results show that Fe^III{OH(BDC)} is a weak anti-ferromagnetic charge transfer insulator at T = 0 K with iron in the high-spin S = 5/2 state. In agreement with the absence of electronic de-localisation along the inorganic chains, lithium insertion leads to the stabilisation of a Fe^II/Fe^III mixed-valence state of class I or II in the Robin–Day classification, whatever the Li sites considered in the calculations. Among these Li sites, the most probable site I (OH-Li) and site II (O=CO-Li) are shown to induce incompatible structural changes on the reduced Li_0.5Fe{OH(BDC)} form that could be at the origin of the small capacity measured for this compound. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Investigation of Abnormal Long‐Wavelength Fluorescence Emissions Occurring in Binary Organic Nanoparticle Films

In some fluorophores with planar groups, a long‐wavelength emission band different from their native one can sometimes be observed. The main cause of this long‐wavelength band is excimer formation. It is generally accepted that once condensed in the solid state, whether fluorophores can exhibit excimer emission or not depends only on their molecular structure and packing. However, here it is shown that there are exceptions when fluorophores are present in nanoparticles (NPs), where excimer emission can be affected by the environment surrounding the NPs, even in the solid state. It is found that in some binary NP films consisting of fluorophore NPs and other NPs, unusual long‐wavelength bands ascribed to excimer emission can be activated, even though these bands are absent from the photoluminescence spectra of the pure fluorophore NP films. This finding is beneficial to better understand and control excimer emissions. In addition, such a binary NP system provides an ideal platform to investigate the interplay between two fluorophores, because it keeps them effectively separated while maintaining suitable spatial distances for exciton migration and dipole–dipole interactions. This work also provides evidence for the long‐debated origin of the green emission band (g‐band) of fluorene‐based fluorophores.     

Self-assembling effects and mechanisms of interchromophore interactions in porphyrin pentads

The directional self-assembly of nanosized, structurally organized pentads that include five tetrapyrrole macrocycles and are based on the two-point coordination interaction of two covalently bound dimers of Zn porphyrins (homo-and heterodimers) with molecules of either the free base or the Cu complex of tetrametapyridyl-substituted porphyrin extra ligand is implemented in methylcyclohexane at 295 K. Using the method of the density functional theory (DFT) in the B3LYP/6-31g(d) approximation, the geometry of the pentad is fully optimized and the main factors that determine its redox properties are determined. The energies of the lowest excited states of the pentad are calculated by the ZINDO/S method, and it is shown that the occurrence of identical molecules in the system facilitates the formation of excitonic states with different contributions from the charge-transfer component. The directional energy transfer and the photoinduced electron transfer, which leads to the formation of a low-lying charge-transfer state (CT state), are studied and the rate constants of these two processes are determined (k _ET ∼ 10¹¹ s⁻¹ and k _PET ≈ (1.8−6.0) × 10⁹ s⁻¹, respectively). The strong effect of the temperature of the solvent on the efficiency of relaxation processes in pentad complexes under study is revealed and studied. Roles played by the low-lying CT state and d-π exchange effects (the Cu-contained pentad) in fluorescence quenching of pentad complexes are determined.     

Two-center molecular states in9B,9Be,10Be,and10B

The molecular states in the mass 9 and 10 nuclei, which consist of two α-particles plus one or two valence nucleons (protons or neutrons) are discussed. Arguments for the existence of two-center dimers as excited states in¹⁰Be and corresponding resonances (p+⁹Be) in¹⁰B are given. The latter states are observed as anomalous (non statistical) population in the final state interactions in thep+⁹Be channel in various heavy ion collisions. With the establishment of two-center states (dimers) based on the αα-potential and a localized binding via two nucleons in¹⁰Be, the existence of more extended structures (multimers) by adding (α2n) structures to¹⁰Be* is postulated. Generally clustering intoα-particles and nucleons in terms of molecular states is expected to occur at excitation energies close to the threshold for these substructures in analogy to the clustering rules of Ikeda forα-particle nuclei. Consequences to clustering properties of neutron rich nuclei are discussed.     

Single-crystal structural study of the pressure-temperature-induced dimerization of C<Subscript>60</Subscript>

We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C₆₀ single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C₆₀ dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C₆₀-C₆₀ dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.Received: 3 October 2003, Published online: 19 February 2004PACS: 61.48. + c Fullerenes and fullerene-related materials - 61.43.Bn Structural modeling: serial-addition models, computer simulation - 78.30.Na Fullerenes and related materialsT. Wågberg: Present address: Groupe de Dynamique des Phases Condensées, Université Montpellier II, Place E. Bataillon, CC026, 34095 Montpellier, France     

Dimer structures formed in smectic films by inclusions with parallel and antiparallel topological dipole moments

Self-organization of cholesteric and nematic droplets in smectic free-standing films is investigated. Strong tangential anchoring at the droplet boundary leads to nucleation of a topological defect and formation of a topological dipole from the droplet and the defect. The interaction between droplets results in assembly of the droplets in dimers and line chains. Topological dipoles of the droplet-defect pairs can be oriented in dimers with polar (ferroelectric) and nonpolar (antiferroelectric) ordering. In this paper we found novel dimer structures formed by droplets with different handedness of the c-director field near the droplets. Depending on the relative magnitude of the droplet dipoles the resulting structure may be dipolar or quadrupolar. Formation of antiferroelectric dimers is discussed on the basis of electromagnetic analogy.     

Detailed mapping of the local Ir4+ dimers through the metal-insulator transitions of CuIr2S4 thiospinel by X-ray atomic pair distribution function measurements

The evolution of the short-range structural signature of the Ir4+ dimer state in CuIr2S4 thiospinel has been studied across the metal-insulator phase transitions as the metallic state is induced by temperature, Cr doping, and x-ray fluence. An atomic pair distribution function (PDF) approach reveals that there are no local dimers that survive into the metallic phase when this is invoked by temperature and doping. The PDF shows Ir4+ dimers when they exist, regardless of whether or not they are long-range ordered. At 100 K, exposure to a 98 keV x-ray beam melts the long-range dimer order within a few seconds, though the local dimers remain intact. This shows that the metallic state accessed on warming and doping is qualitatively different from the state obtained under x-ray irradiation.