Temperature dependence of the reactions of singlet and triplet diphenylcarbene. : Evidence for reversible ylide formation in the reaction with alcohols

The temperature dependences of the reaction of singlet diphenylcarbene are consistent with reversible intermediate formation.     

Excimer formation in crystalline and nanostructured coordination polymers

A new anthracene-based ligand shows emission shift and decrease of excimer band upon coordination to silver ions when crystallized from methanol. Changing the solvent leads to the formation of amorphous nanowires exhibiting large emission bands before transforming into a second crystalline compound with excimer emission similar in intensity to the ligand alone.     

Excimer formation dynamics in the isolated tetracene dimer

The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state ¹(TT) is of high significance for the development of efficient organic electronics. Here, we study the photoinduced dynamics of the tetracene dimer in the gas phase by time-resolved photoionisation and photoion imaging experiments as well as nonadiabatic dynamics simulations in order to obtain mechanistic insight into the excimer formation dynamics. The experiments are performed using a picosecond laser system for excitation into the S₂ state and reveal a biexponential time dependence. The time constants, obtained as a function of excess energy, lie in the range between ≈10 ps and 100 ps and are assigned to the relaxation of the excimer on the S₁ surface and to its deactivation to the ground state. Simulations of the quantum-classical photodynamics are carried out in the frame of the semi-empirical CISD and TD-lc-DFTB methods. Both theoretical approaches reveal a dominating relaxation pathway that is characterised by the formation of a perfectly stacked excimer. TD-lc-DFTB simulations have also uncovered a second relaxation channel into a less stable dimer conformation in the S₁ state. Both methods have consistently shown that the electronic and geometric relaxation to the excimer state is completed in less than 10 ps. The inclusion of doubly excited states in the CISD dynamics and their diabatisation further allowed to observe a transient population of the ¹(TT) state, which, however, gets depopulated on a timescale of 8 ps, leading finally to the trapping in the excimer minimum.

The understanding of excimer formation and its interplay with the singlet-correlated triplet pair state ¹(TT) is of high significance for the development of efficient organic electronics.     

Excimer formation and charge carrier photogeneration in solid polymers

Data are presented on excitonic photogeneration processes in solid polymers similar to those observed in molecular-crystal photoconductivity. It is shown that excimeric states responsible for the intrinsic photogeneration are preferably of intramolecular origin.     

Excimer formation in the interlayer region of arene-derivatized zirconium phosphonates

Photophysical investigations are reported for two arene-derivatized zirconium phosphonates (layered solids). The chromophore groups that are attached within the interlayer region of these materials are seen to form excimer pairs. Whereas the naphthalene-containing system exhibits both monomer and excimer fluorescence, the potentially greater overlap of chromophores for the system containing anthracene-pendant groups causes that compound to only show excimer fluorescence.     

Comment: Excimer formation and low-temperature fluorescence in 9,10-dichloroanthracene crystals

Low-temperature monomer emission detected from β-9,10-dichloroanthracene crystals can be greatly reduced by annealing. Thus excimer formation in this system is not a thermally activated process in contrast to the case of perylene.     

Excimer formation mechanism in gaseous krypton and Kr/N2 mixtures

A new approach for the understanding of the energy relaxation dynamics of excited atoms involving a long-lived molecular precursor is presented here for krypton. Excitation of the gas close to the 5s[3/2]₂ metastable atomic level (E _at. ?E _exc.<kT) is achieved with an intense VUV laser source (I ≈ 10¹² photon/pulse) realized by resonantly enhanced 4-wave mixing (2ω₁ + ω₂) in room temperature mercury vapor (N _Hg ≈ 10¹³ at./cm³). The decay of the II. continuum luminescence (145 nm) is studied. In the pressure range 200–500 mbar, decay rates depend linearly on pressure but have a negative zero-pressure intersect. We show here that this result can be understood as an effect of the exchange of energy between two different “reservoirs” of atomic (5s[3/2]₂) and molecular (1_g) nature, and can be an inherent peculiarity of the recombination kinetics of excited atoms with several product channels. The efficiency of the model is checked for the Kr/N₂ system. Rate constants for relaxation processes are determined in pure krypton and in Kr/N₂ mixtures.     

A quasichemical model for comb-like aggregation of monohydric alcohols: supramolecular structure and macroscopic properties

A new quasichemical model of the supramolecular structure of the liquid consisting of chain-like and comb-like hydrogen-bonded aggregates with branches of unit length has been developed. Analytical expressions for structural characteristics (size and structure distributions of aggregates), dielectric (permittivity, dipole correlation factor), optic (mean molecular anisotropy in liquid, optic correlation factor, anisotropic Rayleigh light scattering ratio) and thermodynamic (energy of intermolecular interactions in liquid, vaporisation enthalpy) properties of the liquid as functions of structural and thermodynamic aggregation parameters have been derived. The analytical model developed creates the foundation for studying the role of branched aggregates in the supramolecular structure of liquids and various macroscopic properties determined by different molecular parameters. The model is applied to pure methanol at ambient conditions. The dependence of structural characteristics of liquid, dipole correlation factor, permittivity, mean molecular anisotropy in liquid, vaporisation enthalpy on the equilibrium constants of chain-like and branched aggregation is studied. The results are compared with experimental data and computer simulation. The influence of branching degree of aggregates on different physicochemical properties of liquid is revealed and discussed.     

Phospholipid-induced aggregation and anthracene excimer formation

The zinc complex of anthryl bis(dipicolylamine) (1) aggregates upon binding with long-chain aliphatic phosphates and displays anthracene excimer fluorescence, which provides a new strategy toward detection of the biologically important lysophosphatidic acid in aqueous solution.     

Mesoscopic structure formation in colloidal aggregation and gelation

Recent small-angle light scattering experiments have revealed that diffusively aggregating spherical particles develop structure on a mesoscopic length scale (∼ tens of particles). The mesoscopic structural length scale persists even when the aggregation proceeds to the formation of a space-spanning network (a gel). We review the technique of small-angle light scattering, survey the experimental evidence for mesoscopic structure formation, discuss attempts at understanding these experimental observations by computer simulation of irreversible and reversible diffusion-limited cluster aggregation (DLCA), and propose a coherent picture for the understanding of non-equilibrium aggregation in the context of phase transitions.     

Spectroscopic anatomy of molecular-imprinting of cyclodextrin. Evidence for preferential formation of ordered cyclodextrin assemblies.

The processes of molecular-imprinting of beta-cyclodextrin (beta-CyD) with cholesterol and stigmasterol (cross-linking agent = diisocyanate) have been analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy. These templates enormously promote the formation of dimers and trimers of beta-CyD, which are only inefficiently formed in their absence. These ordered assemblies are the guest-binding sites, in which two or three beta-CyD molecules cooperate to bind large steroids. Ordered assemblies are also formed when 2,6-di-O-methyl-beta-cyclodextrin is used in place of beta-CyD. Direct spectroscopic evidence for molecular-imprinting effect has been obtained. Molecular imprinting of CyDs is potent for tailor-made preparation of synthetic receptors for nanometer-scaled guests.     

Complex formation of halogenated alcohols with various electron donors

The results are reported of a study, by the near i.r. spectroscopic method, of the hydrogen bonding between 2-bromoethanol, 2-chloroethanol and 2-fluoroethanol, and hexamethylphosphortriamide, dimethyl sulfoxide, dibenzyl sulfoxide, di-p-tolyl sulfoxide and diphenyl sulfoxide in carbon tetrachloride at 288.15, 298.15, 308.15 and 318.15 K.The K₁₁, ΔH°, ΔG° and ΔS° values for the complex formation show that the complex formation ability of the 2-halogen substituted ethanols is weak towards the selected electron donors and much weaker than found earlier for the corresponding 2,2,2-trihalogen substituted ethanols. The basicity order of the electron donors decreases in the order hexamethylphosphortriamide ⪢ dimethyl sulfoxide ⪢ dibenzyl sulfoxide ⪢ di-p-tolyl sulfoxide ⪢ diphenyl sulfoxide, while the proton donating ability of the halogenated alcohols is in the order 2-bromoethanol ⪢ 2-chloroethanol ⪢ 2-fluoroethanol. The pK_a values of the alcohols determined in water are in the same order. Therefore the order of the complex formation ability is opposite to the acidity order in water.     

Rhenium complex-catalyzed carbon-carbon formation of alcohols and organosilicon compounds

The coupling reactions of allylic and benzylic alcohols and allyltrimethylsilane are efficiently catalyzed by a rhenium complex to give the corresponding 1,5-dienes and alkenes in moderate to good yields. Similarly, alcohols were coupled with ketene silyl acetals to form the corresponding esters.     

Borrowing hydrogen: iridium-catalysed reactions for the formation of C-C bonds from alcohols

Alcohols have been employed as substrates for C-C bond-forming reactions which involve initial activation by the temporary removal of hydrogen to form an aldehyde. The intermediate aldehyde is converted into an alkene via a Horner-Wadsworth-Emmons reaction, nitroaldol and aldol reactions. The 'borrowed hydrogen' is then returned to the alkene to form a C-C bond.     

Intrinsic properties of active sites for hydrogen production from alcohols without coke formation

The detailed reaction pathway and coke formation mechanism over Pt/metal oxide nanoparticles during the steam reforming of ethanol（SRE） at 300℃were studied.The catalysts were prepared by incipient wetness impregnation method and were characterized with CO pulse chemisorption, BET surface measurement,oxygen adsorption,ethanol-TPD,NH₃-TPD,and TPO.The SRE activity of the catalysts with steam/ethanol molar ratio of 3/1 was tested using a continuous fixed-bed reactor.Strong interaction between Pt and supports causes lower H₂ production temperatures and no C₂H₄ formation,while weak interaction leads to C₂H₄ formation and strong bonded CO on Pt particles during ethanol-TPD. H₂ production over Pt-based catalysts is mainly resulted from the decomposition and dehydrogenation of ethanol,and decarbonylation of acetaldehyde.Meanwhile,coke can be formed from acetaldehyde,acetone,C₂H₄ and CO.However,when the interaction between Pt and supports is weak,more coke is formed especially from acetone,C₂H₄ and CO.When the interaction is strong,no coke formation is observed due to high oxygen storage capacity of the catalyst.     

Ether formation from allylic alcohols catalyzed by samarium trichloride

The behaviour of various allylic alcohols in the presence of catalytic amounts of SmCl₃ (by heating in 1,2-dichloroethane) has been studied. Diallyl ethers are obtained in many cases in good yields. Mixed allyl alkyl ethers are also prepared if 2–5 equivalents of an aliphatic alcohol is present. The reactions are interpreted as proceeding through a pseudo allylic carbonium intermediate initiated by a preliminary complexation of the allyl hydroxyl to the samarium ion.