Gabriel Canard

Anisotropic Organization and Microscopic Manipulation of Self-Assembling Synthetic Porphyrin Microrods That Mimic Chlorosomes: Bacterial Light-Harvesting Systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChls). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems. They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChls; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChls and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces.

meso-Ester Corroles.

The introduction of ester groups on the 5- and 15-meso positions of corroles stabilizes them against oxidation and induces a redshift of their absorption and emission spectra. These effects are studied through the photophysical and electrochemical characterization of up to 16 different 5,15-diester corroles, in which the third meso position is free or occupied by an aryl group, a long alkyl chain, or an ester moiety. Single-crystal X-ray structure analysis of five 5,15-diestercorroles and DFT and time-dependent DFT calculations show that the strong electron-withdrawing character of the 5,15 ester substituents is reinforced by their π overlap with the macrocyclic aromatic system. The crystal packing of corroles 2, 4, 6, 9, and 15 features short distances between chromophores that are stacked into columns thanks to the low steric hindrance of meso-ester groups. This close packing is partially due to intermolecular interactions that involve inner hydrogen and nitrogen atoms, and thereby, stabilize a single, identical corrole tautomeric form.

An Efficient Synthesis of Porphyrins with Different meso Substituents that Avoids Scrambling in Aqueous Media

We have developed new conditions that afford regioisomerically pure trans-A2B2-, A3B-, and trans-AB2C-porphyrins bearing aryl and arylethynyl substituents. The porphyrins were prepared by the acid-catalyzed condensation of dipyrromethanes with aldehydes followed by oxidation with p-chloranil or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Optimal conditions for the condensation were identified after examining various reaction parameters such as solvent composition, acid concentration, and reaction time. The conditions identified (for aromatic aldehydes: EtOH/H2O 4:1, [DPM] = 4 mM, [aldehyde] = 4 mM, [HCl] = 38 mM, 16 h; for arylethynyl aldehydes: THF/H2O 2:1, [DPM] = 13 mM, [aldehyde] = 13 mM, [HCl] = 150 mM, 3 h) resulted in the formation of porphyrins in yields of 9-38% without detectable scrambling. This synthesis is compatible with diverse functionalities such as ester or nitrile. In total, 20 new trans-A2B2-, A3B-, and trans-AB2C-porphyrins were prepared. The scope and limitations of the two sets of reaction conditions have been explored. The methodological advantage of this approach is its straightforward access to building blocks and the formation of the porphyrin core in higher yields than by any other methodology and by using environmentally benign and nonhazardous chemicals.

Influence of the electron donor groups on the optical and electrochemical properties of borondifluoride complexes of curcuminoid derivatives: a joint theoretical and experimental study

Molecules displaying a p-conjugated D–A–D structure, in which D and A are electron donor and acceptor groups, respectively, represent an important class of dyes. In this work, we present a series of borondifluoride complexes of curcuminoid derivatives in which the central dioxaborine ring acts as a strong electron acceptor unit. Compounds 1–15 differ by the nature of the terminal D groups. We have also studied unsymmetrical compounds 16–23 that contain two different terminal donor groups, as well as compounds 24–32 in which an electron acceptor or donor unit has been introduced at the meso position of the dioxaborine ring. We describe the synthesis of the dyes that have not yet been reported in the literature, and report the electrochemical, absorption and fluorescence properties of all the compounds in dichloromethane. To gain insights into the electronic structures and optical properties, we performed DFT/TD-DFT calculations for a panel of representative compounds. We established correlations between the reduction potential and the Hammett s and s + constants, as well as between the redox gap and the emission wavelength. The electron donor character of one terminal D unit strongly influences (88 mV per unit of s) the reduction potential due to the strong resonance interaction along the curcuminoid backbone. The correlation points to the smaller effect of the meso substituent (50 mV per unit of s), which we relate to the twisted ground-state geometry of the meso aryl substituent. The correlation models established in this study may be useful to anticipate the optical and electrochemical properties of borondifluoride complexes of curcuminoids with good reliability.

N‐Substituted Azacalixphyrins: Synthesis, Properties, and Self‐Assembly

Pre- and postintroduction of substituents with respect to the macrocyclization step leads to previously unknown N-substituted azacalixphyrins. The stepwise synthetic approach has been studied in detail to highlight the key role of the N-substituents of the precursors and/or intermediates in terms of reactivity. Based on a combined experimental and theoretical investigation, the relationship between the properties of the macrocycles and their degree of substitution is rationalized. Depending on the nature of the N-substituents, the formation of supramolecular ribbon-like structures could also be observed, as demonstrated by combined TEM, SEM, AFM, and FTIR experiments.

Structural, Spectroscopic, and Thermodynamic Consequences of Anti-Chelate Effect in Nine-Coordinate Lanthanide Podates

The connection of three tridentate 2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine binding units to an extended sulfur-containing tripodal anchor in the ligand L9 yields nine-coordinate podates [Ln(L9)](3+) upon reaction with trivalent lanthanides, Ln(III). Structural analysis of [Eu(L9)](ClO(4))(3) in the solid state with the help of the bond valence method shows that the peripheral ethyl groups are responsible for a specific distortion of the triple-helical structure, which allows a closer approach of the nitrogen donors toward the central metal, while minimizing interstrand repulsion. The consequences of this distortion on the Eu(III) luminescent probe are investigated by high-resolution emission spectroscopy, while paramagnetic NMR data collected in acetonitrile demonstrate that [Ln(L9)](3+) adopts a single relaxed C(3)-symmetrical structure along the complete lanthanide series. The persistence of the triple-helical structure in solution is obtained at the cost of severe constraints in the helically wrapped organic tripod, which strongly disfavor intramolecular cyclization processes. The resulting antichelate effect can be exploited for the selective preparation of polynuclear complexes with tripodal ligands.

Synthesis, electrochemical and photophysical studies of the borondifluoride complex of a meta-linked biscurcuminoid

The synthesis of the borondifluoride complex of a biscurcuminoid system is described, and its electrochemical and photophysical properties are compared to those of a series of monochromophoric models. The data show that the meta-linked biscurcuminoid exhibits enhanced optical properties in solution, such as high optical brightnesses obtained at one- and two-photon excitation (B1 = 87 000 M−1 cm−1 and B2 = 313 GM, respectively). UV/visible absorption of solid-state particles formed in water solution revealed that the four investigated dyes are strongly aggregated and fluorescence spectroscopy showed that they are emissive in the NIR with fluorescence quantum yields spanning from 1.5 to 12.5%. In the case of the meta-linked biscurcuminoid, a high brightness is obtained using one- and two-photon excitation (B1 = 6952 M−1 cm−1 and B2 = 70 GM, respectively). Such red-shifted emission combined with a yet significant brightness in the solid-state is believed to arise from the choice of the meta-linkage which limits strong intermolecular packing.

Polymerization Initiated by Organic Electron Donors

Polymerization reactions with organic electron donors (OED) as initiators are presented herein. The metal-free polymerization of various activated alkene and cyclic ester monomers was performed in short reaction times, under mild conditions, with small amounts of organic reducing agents, and without the need for co-initiators or activation by photochemical, electrochemical, or other methods. Hence, OED initiators enabled the development of an efficient, rapid, room-temperature process that meets the technical standards expected for industrial processes, such as energy savings, cost-effectiveness and safety. Mechanistic investigations support an electron-transfer initiation pathway that leads to the reduction of the monomer.

1,3-Alternate Tetraamido-Azacalix[4]arenes as Selective Anion Receptors.

Six tetraaza[1.1.1.1]cyclophane derivatives bearing peripheral amide groups were prepared according to two distinct synthetic strategies that depend on the connection pattern between the aryl units. NMR experiments combined with the X-ray structures of two tetraamide derivatives 4 b and 10 show that these cavitands adopt a 1,3-alternate conformation both in solution and in the solid state. Consequently, the four amide groups of the aza[1.1.1.1]-m,m,m,m-cyclophane isomer 10 can contribute to the same recognition process towards neutral water molecules or anion guests. NMR experiments, mass spectrometry analyses and single-crystal X-ray structures confirm the anion-binding ability of this receptor. Absorption spectrophotometric titrations in nonpolar solvents provided evidence for the selectivity of 10 to chloride anions in the halide series, with a corresponding association constant Ka reaching 2.5 × 10(6) m(-1).

Light absorption and hole-transport properties of copper corroles: from aggregates to a liquid crystal mesophase

The synthesis of the corrole-based liquid crystal phase CorLC is described together with its full characterization using a combination of Polarised Optical Microscopy (POM), ThermoGravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) measurements and Small-Angle X-Ray Diffraction (SA-XRD), which shows that CorLC has a hexagonal columnar organization at room temperature. The light absorption and hole-transport properties of the mesophase CorLC are compared with those featured by assemblies of its parent meso-triaryl (Cor1) and per-aryl (Cor2) derivatives. J-Aggregates are replaced by H-aggregates when the intermolecular distance is increased by a higher peripheral steric hindrance producing lower hole-transport mobilities.