Bertrand Illien

Synthesis of Oligothiophene-Bridged Bisporphyrins and Study of the Linkage Dependence of the Electronic Coupling

A set of twelve porphyrin dimers has been prepared to give information on how the type of connectivity between a porphyrin core and a bridge can influence the interporphyrin electronic interaction. The new porphyrin systems are substituted directly at the meso position with an oligothiophene chain tethered either with a single C-C sigma bond, a trans ethylenyl group, or a acetylenyl group. The compounds are easily obtained by palladium-catalyzed cross-coupling reactions (Stille, Heck, and Sonogashira) between 5-iodo-10,15,20-(3,5-ditert-butylphenyl)porphyrin and the appropriate oligothiophene derivative. This synthetic approach is straightforward and very effective for preparing oligothiophene-based prophyrin systems. The absorption spectra and the fluorescence properties of the dimers demonstrated the crucial importance of the characteristics of the chemical bond used to connect the bridge to the porphyrin unit. The magnitude of the electronic communication can thus be significantly modulated by altering the type of bond connectivity used to link the chromophore to the bridge. The present work shows that an oligothiophene spacer is a viable class of linker for connecting porphyrins, and that a quaterthiophene appended with ethynyl linkages affords a high electronic interaction over a distance as large as 28 A. A detailed computational study of these dimers has clarified the conditions needed for a conjugated system to behave as a molecular wire. These conditions are full planarity of the molecule and proper energy matching between the frontier orbitals of the bridge and the porphyrin. Intermolecular energy transfer in asymmetrical dyads composed of a zinc porphyrin and a freebase porphyrin has been studied by fluorescence spectroscopy. In all systems, this process is more than 98% efficient, and its rate constant decreases steadily in the order 4ZH > 1ZH > 3ZH approximately 2ZH. Thus, the largest rate (kEnT = 1.2 x 10(11) s-1) was found in the dyad linked with bisethynyl quaterthiophene, which represents the longest bridge within the series. These results clearly demonstrate that strong communication and also efficient photoinduced processes can be promoted over a large distance if the electronic structure of the molecular connector is appropriately chosen.

Can quantum-mechanical calculations yield reasonable estimates of hydrogen-bonding acceptor strength? The case of hydrogen-bonded complexes of methanol.

The thermodynamics and some vibrational properties of hydrogen-bonded complexes of methanol with 23 hydrogen-bond acceptors (HBAs) have been determined in CCl(4) by FTIR spectrometry. The experimental sample contains carbon, nitrogen, oxygen, sulfur, fluorine, and chlorine organic bases and covers an energetic range of 13 kJ mol(-1) in the basicity scale (-ΔG), 22 kJ mol(-1) in the affinity scale (-ΔH), and 400 cm(-1) in the spectroscopic scale (Δν((OH))) (from benzene to trimethylphosphane oxide and amines). The experimental results in CCl(4) are compared to those computed in the gas phase at various levels of theory. Ninety five percent of the variance of the red shift and 89% of the variance of the intensification of the OH stretching upon hydrogen bonding are explained by gas-phase B3LYP/6-31+G(d,p) calculations. However, this level does not satisfactorily explain the thermodynamic properties. Only 68% of the variance of the methanol affinity (-ΔH) is taken into account. MP2/aug-cc-pVTZ//B3LYP/6-31+G(d,p) affinity calculations raise the explanation to 77% for all HBAs and to 93% when three outliers (Me(2)SO, Me(3)PO, and tetrahydrothiophene) are excluded. Discrepancies are analyzed in terms of experimental errors, calculation approximations, and solvation.

The pH dependent adsorption of Coumarin 343 at the water/dichloroethane interface

The surface sensitive technique of optical second harmonic generation (SHG) and quasi-elastic laser scattering (QELS) have been applied to study the effect of pH on the adsorption of Coumarin 343 (C343) dye molecules at the water/1, 2-dichloroethane (DCE) interface. A pH dependent aggregation is observed for C343 molecules adsorbed at the interface. The surface spectrum shows that at pH 8 and lower, C343 adsorb at the interface in J-aggregated protonated form. At pH 9 and pH 10, C343 adsorb in both protonated and deprotonated forms at the interface, whereas, at pH 11, C343 is in H-aggregated-deprotonated form at the interface. The observed large shift in pKa value of C343 at the interface is attributed to the intramolecular hydrogen bonding along with the aggregation of dye molecules. From a light polarization analysis of the SHG signal, the average angle of orientation of C343 at pH 5 was determined to be 30°, and at pH 11, the angle was determined to be 42°. The QELS data show a weak adsorption of C343 at the interface for pH 11 and pH 3 and a strong adsorption at the intermediate pH values, reaching maximum for pH 10, and are consistent with SHG data. The calculated ratio of β values for protonated and deprotonated forms of C343, is in agreement with the experimentally measured ratio of β values. Both, experimental and calculated β values for the deprotonated form is lower than the β values for protonated one and is attributed to the electron-donating effect of the –COO− substituent.

Calculation of static mean polarisability and polarisability anisotropy. Statistical comparison with the results of gases and influence of the geometrical parameters

Abstract The static mean polarisabilities α and polarisability anisotropies Δ α of nine molecules have been computed at the B3LYP level by using 6-311++G(3df,2pd), aug-cc-pVDZ and Sadlej basis sets, experimental r e or optimised geometries. These molecules are either linear (CO 2 , CS 2 , OCS, HCCH) or symmetric top (CH 3 CCH, CH 3 Cl, CHCl 3 , CH 3 CN, C 6 H 6 ). A statistical comparison with accurate experimental data has shown that 6-311++G(3df,2pd) basis set gives slightly better results than the two other sets. More precisely, for α (0), Sadlej basis set has the best statistical indicators to experiment, but a 0.98 scale factor should be applied to calculated values to improve agreement with experimental values, wherefore no scale factor is needed with 6-311++G(3df,2pd) basis set. For Δ α (0), B3LYP/6-311++G(3df,2pd) method clearly performs the best results, however, a 0.96 scale factor is necessary to fit experiment. Also, computed polarisabilities are better when the experimental microwave geometry is used instead of the geometry computed by optimisation.

Ion association of alkali and alkaline earth metal azides in dimethylsulfoxide. Infrared spectrometry and ab initio calculations

The 1:1 species formed in dimethylsulfoxide (DMSO) solution between the azide anion and alkaline and alkaline earth metal cations Mz+ are investigated by infrared spectrometry. The ν3 vibration of N3- at 2000 cm-1 is shifted to higher wavenumbers by ion pairing. Shift values are not significantly different from those observed in MNCO(z-1)+ although azide ion pairs are less stable. The forbidden ν1 vibration of N3- at 1315 cm-1 becomes allowed in ion pairs. Δν3 and Δν1 wavenumber shifts are both proportional to the polarizing power of the cation. Longitudinal force constants are calculated by assuming that in an N1N2N3M ion pair the force constant of the N2N3 bond is insensitive to ion pairing. The mean polarizability of N3- and the anisotropy of polarizability are measured by refractive index and by Rayleigh diffusion measurements. From these results the integrated intensity of the ν1 band is interpreted by calculating the variation of dipole moments induced by the Mz+ in N3-. Theoretical calculations of equilibrium geometry, polarizability, wavenumbers and IR intensity are made by the DFT (Density Functional Theory) method on some typical azide species: free N3-, unsolvated ion pairs LiN3, NaN3 and KN3, solvent shared ion pair LiN3.3H2O and triple anion N3LiF-.

Linearly extended hybrid tetrathiafulvalene analogues with bridged dithienylethyleneπ-conjugating spacers

New linearly extended π-electron donors based on bridged dithienylethylene (DTE) end-capped with diversely substituted 1,3-dithiol-2-ylidene electron-releasing groups have been synthesized by Wittig–Horner olefination of appropriate aldehydes. Cyclic voltammetry shows that whereas the unbridged analogues are directly oxidized to the dication state through a two-electron transfer, rigidification of the DTE spacer leads to the splitting of the two-electron wave into two successive one-electron steps due to the decrease of the potential corresponding to the formation of the cation radical. This unusual electrochemical behaviour is interpreted with the help of theoretical calculations which suggest that these effects are related to the enhanced electron delocalization resulting from the rigidification of the DTE spacer. This conclusion is supported by an X-ray diffraction structural analysis which reveals that in addition to a fully planar conformation stabilized by intramolecular S‥S interactions, the bridging of the DTE spacer leads to a significant reduction of bond length alternation.

A way to compare experimental and SCRF electronic static dipole polarizability of pure liquids

Abstract When the polarizability tensor is computed through Self-Consistent Reaction Field Models (the interactions between solute and solvent are incorporated into calculation), there still exists a need to take into account the modifications of the applied macroscopic fields at the local site of the solvated molecule (the phenomenon historically indicated as ‘local field effects’). Another way is to remove local field factors from experimental mean polarizability ( α ) and polarizability anisotropy (Δα) values. The present paper focuses on this way. Seven liquids (CS2, C6H6, (CH3)2CO, CHCl3, CH2Cl2, CH3CN, CCl4) have been studied. In a first step, the α tensor components have been computed at the B3LYP and HF levels with Sadlej basis set using a finite field procedure. The solvent effect is introduced into the calculation through the Self-Consistent Isodensity Polarizable Continuum Model (SCIPCM). In a second step, experimental literature values of refractive index, density, isothermal compressibility coefficient and depolarization ratio have been carefully chosen and introduced in modified experimental equations (where local field factors have been removed) to determine α and Δα. Concerning α , there is a good agreement between experimental and B3LYP values for dipolar molecules. In the case of non-dipolar molecules calculated values are always lower than experimental ones. Such a behaviour had not been observed in a previous study concerning molecules in the gas state, therefore it seems that the SCIPCM induces a deficiency over calculated α (0) in the case of non-dipolar molecules.

Unguiculin A and Ptilomycalins E–H, Antimalarial Guanidine Alkaloids from the Marine Sponge Monanchora unguiculata

Chemical study of the CH2Cl2-MeOH (1:1) extract from the sponge Monanchora unguiculata collected in Madagascar highlighted five new compounds, one acyclic guanidine alkaloid, unguiculin A (1) and four pentacyclic alkaloids, ptilomycalins E-H (2-5), along with four known compounds: crambescidin 800 (6) and crambescidin 359 (7), crambescidic acid (8), and fromiamycalin (9). Their structures were elucidated by 1D and 2D NMR spectra and HRESIMS data. All compounds were evaluated for their cytotoxicity against KB cells and their antiplasmodial activity. The new ptilomycalin E (2) and the mixture of the new ptilomycalins G (4) and H (5) showed promising cytotoxicity against KB cells with IC50 values of 0.85 and 0.92 μM, respectively. Ptilomycalin F (3) and fromiamycalin (9) exhibited promising activity against Plasmodium falciparum with IC50 values of 0.23 and 0.24 μM, respectively.

Intramolecular charge transfer in 5-phenyl-3H-1,2-dithiole-3-thione and 5-phenyl-3H-1,2-dithiole-3-one derivative molecules for quadratic nonlinear optics

SummaryThis paper deals with the theoretical and experimental studies of new sulfur-containing NLO chromophores: 5-phenyl-3H-1,2-dithiole-3-thione and 5-phenyl-3H-1,2-dithiole-3-one derivatives; ground-state and first1(ππ*) excited states characteristics are determined within the framework of MNDO-PM3 and CNDO/S-CI methods. These results are confronted with the NLO response of these molecules. The intramolecular charge transfer is identified and its contribution to the quadratic nonlinear susceptibility estimated and shown to be the main feature. Theoretical and experimental solvent effects on π → π* absorption bands are discussed. MNDO-PM3 finite-field first-order hyperpolarizabilities are compared to EFISH measurements. A preliminary CNDO/S parametrization of the sulfur atom in thio-carbonyl compounds is proposed and shown to be adequate for the description of electronic spectra.

Contribution of trans-aconitic acid to DPPH scavenging ability in different media

The antioxidant properties of trans-aconitic acid (TAA) alone or in the presence of usual antioxidants were assessed by DPPH assay. The IC50 value equal to 70mM was very high compared to usual antioxidants (vitamin C and trolox). A joint experimental/theoretical study suggested that hydrogen atom abstraction in TAA by DPPH was located on -CH2- methylene bridge because the corresponding radical was more stabilized than COO(·) and CC(·) radicals. In combination with antioxidants (vitamin C, gallic acid, caffeic acid, trolox), synergy or additivity effects were noticed. The magnitude of the synergistic effect varied between 1.06 and 1.24 depending on the type and concentration of antioxidant for a concentration of TAA equal to 22.3mM. Especially, the addition of TAA at a concentration below 32mM to a solution containing 20μM of vitamin C had a synergy effect. Beyond this concentration, TAA showed an additive effect.