Tunable Spin‐Crossover Behavior of the Hofmann‐like Network {Fe(bpac)[Pt(CN)4]} through Host–Guest Chemistry

A study of the spin‐crossover (SCO) behavior of the tridimensional porous coordination polymer {Fe(bpac)[Pt(CN)₄]} (bpac=bis(4‐pyridyl)acetylene) on adsorption of different mono‐ and polyhalobenzene guest molecules is presented. The resolution of the crystal structure of {Fe(bpac)[Pt(CN)₄]} ? G (G=1,2,4‐trichlorobenzene) shows preferential guest sites establishing π???π stacking interactions with the host framework. These host–guest interactions may explain the relationship between the modification of the SCO behavior and both the chemical nature of the guest molecule (electronic factors) and the number of adsorbed molecules (steric factors).     

New Synthetic Routes towards Soluble and Dissymmetric Triphenodioxazine Dyes Designed for Dye‐Sensitized Solar Cells

New π‐conjugated structures are constantly the subject of research in dyes and pigments industry and electronic organic field. In this context, the triphenodioxazine (TPDO) core has often been used as efficient photostable pigments and once integrated in air stable n‐type organic field‐effect transistor (OFET). However, little attention has been paid to the TPDO core as soluble materials for optoelectronic devices, possibly due to the harsh synthetic conditions and the insolubility of many compounds. To benefit from the photostability of TPDO in dye‐sensitized solar cells (DSCs), an original synthetic pathway has been established to provide soluble and dissymmetric molecules applied to a suitable design for the sensitizers of DSC. The study has been pursued by the theoretical modeling of opto‐electronic properties, the optical and electronic characterizations of dyes and elaboration of efficient devices. The discovery of new synthetic pathways opens the way to innovative designs of TPDO for materials used in organic electronics.     

Reduction of electroactive polymers via polyelectron transfer processes at solid electrodes

The reduction of an electroactive polymer at an electrode involves the transfer of a large number of electrons to a single molecule. A quantitative study of this process indicates whether the system behaves ideally. The electron transfer process in polyvinylbenzophenone and in polyvinylbenzophenone-co-styrene in N,N-dimethylformamide solution was investigated by voltammetry at a rotating disk electrode. It is concluded that benzophenone (BP) groups attached to the polymer backbone are noninteracting electroactive centers and that every BP group is reducible at the electrode. The relationship between limiting current and molecular weight was examined quantitatively for a series of polymers of low dispersity with molecular weights ranging up to 300,000, and with a controlled, varied content of electroactive groups. The diffusion coefficient was determined as a function of molecular weight from the electrochemical data, and was found to be in good agreement with the theoretical model based on transport properties. The dependence of the diffusion coefficient on concentration of polymer and on temperature were investigated and activation energies determined.     

Density functional theory and post-Hartree-Fock studies on molecular structure and harmonic vibrational spectrum of formaldehyde

Density functional theory (DFT) with the Becke's three-parameter exchange correlation functional and the functional of Lee, Yang and Parr, gradient-corrected functionals of Perdew, and Perdew and Wang [the DFT(B3LYP), DFT(B3P86) and DFT(B3PW91) methods, respectively], and several levels of conventional ab initio post-Hartree-Fock theory (second- and fourth-order perturbation theory M?ller-Plesset MP2 and MP4(SDTQ), coupled cluster with the single and double excitations (CCSD), and CCSD with perturbative triple excitation [CCSD(T)], configuration interaction with the single and double excitations [CISD], and quadratic configuration interaction method [QCISD(T)], using several basis sets [ranging from a simple 6-31G(d,p) basis set to a 6-311+ +G(3df, 2pd) one], were applied to study of the molecular structure (geometrical parameters, rotational constants, dipole moment) and harmonized infrared (IR) spectrum of formaldehyde (CH₂O). High-level ab initio methods CCSD(T) and QCISD(T) with the 6-311+ +G(3df, 2pd) predict correctly molecular parameters, vibrational harmonic wavenumbers and the shifts of the harmonic IR spectrum of ¹²CH₂ ¹⁶O upon isotopic substitution. Received: 30 January 1997 / Accepted: 7 May 1997     

Stable DOPG/Glycyrrhizin Vesicles with a Wide Range of Mixing Ratios: Structure and Stability as Seen by Scattering Experiments and Cryo-TEM

Phosphatidylglycerols represent a large share of the lipids in the plasmamembrane of procaryotes. Therefore, this study investigates the role of charged lipids in the plasma membrane with respect to the interaction of the antiviral saponin glycyrrhizin with such membranes. Glycyrrhizin is a natural triterpenic-based surfactant found in licorice. Vesicles made of 1,2-dioleoyl-sn-glycero-3-phospho-rac-(1’-glycerol) (DOPG)/glycyrrhizin are characterized by small-angle scattering with neutrons and X-rays (SANS and SAXS). Small-angle scattering data are first evaluated by the model-independent modified Kratky–Porod method and afterwards fitted by a model describing the shape of small unilamellar vesicles (SUV) with an internal head-tail contrast. Complete miscibility of DOPG and glycyrrhizin was revealed even at a ratio of lipid:saponin of 1:1. Additional information about the chain-chain correlation distance of the lipid/saponin mixtures in the SUV structures is obtained from wide-angle X-ray scattering (WAXS).     

Pyrromethene–BF2 complexes as laser dyes:1.

Condensations between 3-X-2,4-dimethylpyrroles (X = H, CH₃, C₂H₅, and CO₂C₂H₅) and acyl chlorides gave derivatives of 3,5,3′,5′-tetramethylpyrromethene (isolated as their hydrochloride salts): 6-methyl, 6-ethyl, 4,4′,6-trimethyl, 4,4′-diethyl-6-methyl, and 4,4′-dicarboethoxy-6-ethyl derivatives for conversion on treatment with boron trifluoride to 1,3,5,7-tetramethylpyrromethene–BF₂ complex (TMP–BF₂) and its 8-methyl (PMP–BF₂), 8-ethyl, 2,6,8-trimethyl (HMP–BF₂),2,6,-diethyl-8-methyl (PMDEP–BF₂), and 2,6-dicarboethoxy-8-ethyl derivatives. Chlorosulfonation converted, 1,3,5,7,8-pentamethylpyrromethene–BF₂ complex to its 2,6-disulfonic acid isolated as the lithium, sodium (PMPDS–BF₂), potassium, rubidium, cesium, ammonium, and tetramethylammonium disulfonate salts and the methyl disulfonate ester. Sodium 1,3,5,7-tetramethyl-8-ethylpyrromethene-2,6-disulfonate–BF₂ complex was obtained from the 8-ethyl derivative of TMP–BF₂. Nitration and bromination converted PMP–BF₂ to its 2,6-dinitro-(PMDNP–BF₂) and 2,6-dibromo- derivatives. The time required for loss of fluorescence by irradiation from a sunlamp showed the following order for P–BF₂ compounds (10⁻³ to 10⁻⁴ M) in ethanol: PMPDS–BF₂, 7 weeks; PMP–BF₂, 5 days; PMDNP–BF₂, 72 h; HMP–BF₂, 70 h; and PMDEP–BF₂, 65 h. Under similar irradiation PMPDS–BF₂ in water lost fluorescence after 55 h. The dibromo derivative was inactive, but each of the other pyrromethene–BF₂ complexes under flashlamp excitation showed broadband laser activity in the region λ 530–580 nm. In methanol PMPDS–BF₂ was six times more resistant to degradation by flashlamp pulses than was observed for Rhodamine-6G (R-6G). An improvement (up to 66%) in the laser power efficiency of PMPDS–BF₂ (10⁻⁴ M in methanol) in the presence of caffeine (a filter for light <300 nm) was dependent on flashlamp pulse width (2.0 to 7.0 μsec).