Nicolas Godbert

Liaisons between photoconductivity and molecular frame in organometallic Pd(II) and Pt(II) complexes

The synthesis and a comprehensive experimental study of molecular structure, electrochemical properties, photoconductivity, absorption and emission properties of two homologous series of square planar cyclometallated Pd(II) and Pt(II) complexes are described. In these complexes, a five member (C,N) metallacycle is formed by either azobenzene, 2-phenylpyridine or benzo[h]quinoline, while the ancillary (O,N) ligand required to complete the metal coordination sphere is the Schiff base resulting from the condensation of 4-n-hexylaniline with 2-hydroxy-4-n-hexyloxybenzaldehyde. The study is complemented by Density Functional Theory calculations of molecular structure in the ground state, frontier orbitals distribution and energies. Absorption energies, intensities for the transitions and composition of the excited states are characterized by Time Dependent Density Functional Theory computations. Results show that for these complexes the HOMO is located on the Schiff base and the LUMO on the cyclometalated ligand. As also indicated by spectroscopic and photoconductivity results, the photogeneration of charge carriers might be associated with the spatial separation of HOMO and LUMO, while an additional contribution could derive from a conformational variation in the excited state. The separation of the frontier orbitals on different ligands provides simple synthetic routes towards band-gap tuning and energy matching with other molecular species and electrodes, both important features for several applications of molecular semiconductors. The results of the present investigation suggest that these square planar cyclometalated Pd(II) and Pt(II) complexes actually form a new class of photoconductors.

Room temperature columnar mesomorphism and high quantum yield phosphorescence in ionic ruthenium(II) 2,2′-bipyridine-based complexes

The induction of columnar mesomorphism in ionic heteroleptic octahedral tris-2,2′-bipyridine Ru(II) complexes, through a modulation in the ligand structure, produces a large effect on the phosphorescence quantum efficiency (Φ = 0.14) demonstrating that the synthesis of a liquid crystalline compound containing a hexacoordinated Ru(II) chromophore is a helpful strategy in the design of room temperature multifunctional soft materials, very appealing for light-emitting electrochemical cells or solid-state photovoltaic cells applications.

Electropolymerized highly photoconductive thin films of cyclopalladated and cycloplatinated complexes.

The complete characterization of novel electropolymerizable organometallic complexes is presented. These are newly synthesized cyclometalated complexes of general formula (PPy)M(O ∧ N)(n) (H(PPy) = 2-phenylpyridine, M = Pd(II) or Pt(II), H(O ∧ N)(n) = Schiff base). Polymeric thin films have been obtained from these complexes by electropolymerization of the triphenylamino group grafted onto the H(O ∧ N)(n) ancillary ligand. The redox behavior and the photoconductivity of both of the monomers (PPy)M(O ∧ N)(n) and the electropolymerized species have been investigated. The polymeric films of (PPy)M(O ∧ N)(n) have shown a very significant enhancement of photoconductivity when compared to their monomeric amorphous counterparts. The high stability of the obtained films strongly suggests that electropolymerization of cyclometalated complexes represents a viable deposition technique of quality thin films with improved photoconduction properties, hence opening the door to a new class of materials with suitable properties for optoelectronic applications.

Photoconductive Nile red cyclopalladated metallomesogens

The synthesis and the electrochemical, photophysical and photoconductive properties of two new cyclometallated metallomesogens are described. These two complexes were respectively obtained by covalent metallation of Nile red, or a polyalkylated Nile red derivative, to a palladium(II) ion and by using a polyalkylated Schiff base as an ancillary ligand to complete the coordination sphere of the metal centre. The introduction of long alkyl chains on either one or both ligands leads to disk-shaped metallomesogens with ordered room temperature columnar organisation induced, in both cases, by the formation of dimeric pairs. As shown by powder X-Ray diffraction studies, both complexes exhibit a columnar rectangular mesophase over a wide temperature range. These complexes show a redox solution behaviour characterised by one reversible single-electron reduction and two consecutive reversible oxidation waves, which give them an electrochemical amphoteric character. Energy levels and distributions of frontier orbitals have been estimated. Photoconductivity of the mesophase at room temperature has been observed from UV-Vis to near IR wavelength. The high thermal, electrochemical and mesophase stabilities of the studied Nile red complexes confirm that cyclometallated Pd(II) complexes form a new class of photoconductors with features suitable for molecular semiconductor applications.

Highly-charged organic nanoparticles: redox-active dendrimers incorporating 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene units

The incorporation of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene units into dendritic structures has been achieved by convergent routes. The key starting material is the 4-(hydroxymethyl) derivative 3. The branches contain aryl ester and aryl ether groups and the core reagent is benzene-1,3,5-tricarbonyl chloride. Dendrimers 10, 15 and 18 with 6 and 12 redox units at the periphery have been characterised by 1H NMR spectroscopy, elemental analysis, mass spectrometry, UV-vis spectroscopy and solution electrochemistry. The 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene redox chemistry is retained in the dendrimer structures: clean formation of hexa(dication) and dodeca(dication) species is observed upon electrochemical oxidation. These highly-charged species enjoy remarkable electrochemical stability as their reduction to the neutral species is thwarted by the loss of aromaticity and the marked conformational change which accompanies this process. Molecular modelling studies were performed on 10, 1012+, 18 and 1824+ by molecular mechanics.

Near-IR Electrochromism in Electrodeposited Thin Films of Cyclometalated Complexes

Homogeneous thin films of controlled thickness obtained from cyclometalated complexes of general formula [(C(∧)N)M(O(∧)N)], where M = Pd(II) or Pt(II), H(C(∧)N) = 2-phenylpyridine and, respectively, 2-thienylpyridine and H(O(∧)N) = a triphenylamine functionalized Schiff base, have been deposited by oxidative electropolymerization. The films have been electrochemically and morphologically characterized. The metallopolymeric thin films present stable reversible redox behavior and typical cauliflower-like textures in agreement with a nucleation-growth electropolymerization mechanism. However, the film growth is greatly influenced by the nature of the metal center, with a higher tendency of the Pt complexes to promote the 3D growth. Furthermore, a complete spectroelectrochemical study has been performed on electrodeposited films showing near-IR absorption in the oxidized state, high contrast ratios (up to 65%) and low response times.

Neutral and Cationic Cyclopalladated Nile Red Metallomesogens: Synthesis and Characterization In Memory of Dr. Teresa Pugliese

We present the synthesis and characterization of a series of neutral and cationic Nile Red cyclopalladated complexes, first examples of covalent coordination of Nile Red to a Pd(II) metal centre. The Pd(II) coordination sphere is completed by a series of ancillary substituted ligands (acetylacetonates or bipyridine derivatives) selected in order to tune the photophysical properties and/or to enhance the supramolecular order. Both neutral and cationic columnar rectangular metallomesogens have been obtained. Emission has been observed only for neutral complexes (both in solution and mesophase) but quenched in cationic complexes.

Thin Film Electrodeposition of Ir(III) Cyclometallated Complexes

Novel electropolymerizable Ir(III) cyclometallated complexes have been synthesized and characterized. In these complexes the cyclometallated ligands are either 2-phenylpyridine H(PhPy) or benzothiazole-triphenylamine H(BzTh-tpa), while the Ir(III) coordination sphere is completed by a Schiff base substituted with a triphenylamine fragment. A complete electrochemical study has been conducted on all complexes, in order to verify the feasibility of electropolymerization and to elucidate the role of the specific position of the triphenylamine moiety in the molecular structure. Homogeneous thin films of Ir(III) metallopolymers have been successfully obtained through electropolymerization process.

Enhancement of Exciton Dissociation Efficiency in Bulk Heterojunction Solar Cells by Using an Intrinsic Photoconductor Component

A new type of intrinsic photoconductor was used as a donor in bulk heterojunction solar cells, in blends with PCBM. The donor is a square planar cyclopalladated coordination complex, with a Nile red moiety and a Schiff base as ligands, coordinated by a Pd(II) metal center. The photoconducting properties of the complex were characterized. Photovoltaic devices exhibit low performance, but the analysis of the wavelength dependence of the efficiency reveals a much higher photocurrent upon absorption of the complex, when compared to the photocurrent obtained via absorption by PCBM. This effect is tied to the higher tendency of the intrinsic photoconductor towards exciton dissociation at the heterojunction.

New π-electron rich donors and cavities and their supramolecular assemblies: Synthesis, electrochemistry and crystal structures

The synthesis, solution redox properties and X-ray crystal structures of 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene derivatives are described. anthracene macrocycles cyclophanes supramolecular