Elisabeta I. Szerb

Tuning solid state luminescent properties in a hydrogen bonding-directed supramolecular assembly of bis-cyclometalated iridium(III) ethylenediamine complexes.

Synthesis, crystal structural determination and photophysical properties of a series of heteroleptic cationic cyclometalated iridium(III) derivatives of general formula [(ppy)(2)Ir(en)]X (X = ClO(4)(-) (1), PF(6)(-) (2), Cl(-) (3), BPh(4)(-) (4)), are described. The assembly of the common molecular building block allows to get highly luminescent crystalline materials or to assemble poorly luminescent supramolecular channelled architectures, for which the additional contribution of oxygen quenching effects has been observed. Moreover, the high reproducibility of the preparations of the crystalline materials in their specific crystalline phases, makes the control of the supramolecular organization of photo-active iridium(III) complexes within the crystalline structures a useful synthetic procedure for the construction of highly luminescent materials.

Self-incorporation of a luminescent neutral iridium(III) complex in different mesoporous micelle-templated silicas

The neutral luminescent tris-cyclometallated 2-phenylpyridine iridium(III) complex, (fac-Ir(ppy)3, [Ir]) following a self-assembling procedure, has been successfully located in the cavities of mesostructured silica materials through a surfactant-mediated process. Two different structure-directing agents, the cationic cetyltrimethyl ammonium bromide (CTAB) and the non-ionic poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) were tested. The structural features induced by the metal complex incorporation can be explained by comparing the newly synthesized hybrid mesostructured materials with the corresponding undoped samples which were similarly prepared. X-Ray powder diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and UV-Vis spectroscopy were used to characterize the investigated materials. Medium-sized spherical particles of 800 nm were obtained using CTAB as a structure-directing agent whereas larger monolithic aggregates with a minimum dimension of 5–8 μm were obtained using P123. The new hybrids showed the typical hexagonal symmetry of analogously prepared materials. Moreover high luminescence quantum yield values were obtained for both hybrids as a result of a very good dispersion of the chromophore in the mesostructured matrices, thereby avoiding dramatic self-quenching phenomena. The approach described in this paper provides a simple synthetic way to prepare new luminescent silica-based materials by the inclusion of neutral metal-containing luminophores into the pores of a mesoporous hosting skeleton.

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.

Supramolecular Columnar Mesomorphism Induced by Silver(I) Coordination of 2,2′-bipyridine-4,4′-diamides

ABSTRACT The synthesis and characterization of novel ionic liquid crystals displaying columnar mesomorphism induced upon complexation of 2,2′-bipyridine-4,4′-disubstituted ligands ( L n ) are reported. A tetracoordination around the silver( I ) centre has been obtained for the first time in metallomesogens based on chelated bipyridine ligands. The molecular organization in the mesophase for these [ Ag ( L n ) 2 ][ OTf ] derivatives is strongly dependent on the coordination and ionic bonds which drive the supramolecular assembly.

Unconventionally shaped chromonic liquid crystals formed by novel silver(I) complexes

The synthesis of the first chiral Ag(I) bis chelated bipyridine ionic complexes showing chromonic liquid crystalline behaviour in water is reported. Single crystal X-ray analysis revealed a pre-chromonic organization in the crystalline state, with silver(I) cations stacking into columns reinforced by hydrogen bonding with the acetate counterions and water molecules. Due to the presence of molecules with an unconventional shape for the generation of typical chromonic phases, a full characterization based on phase diagram analysis, completely resolved by POM, DSC and X-ray powder diffraction measurements, has been conducted. Moreover, cryo-TEM experiments have been carried out in order to confirm the appearance of nematic columnar phases. A “route” to drive the alignment of the liquid crystalline phases of these compounds has been developed, which is particularly important for future application in biophotonic devices. Finally, the presence of transient twisted periodical stripe structures has been observed, when an initial planar alignment is destroyed in favour of a homeotropic configuration of the molecules dictated by the confining surfaces.

Thermotropic iridium(III)-based liquid crystal in amphiphilic environment

In the search of new functional templating agents for the construction of mesostructured materials, the study of the behaviour in water and F127–water mixture of a new ionic Ir(III) liquid crystalline complex is reported. The complex was solubilized in the water medium using tetrahydrofuran as the co-solvent. The resulting mixtures were completely characterized by rheological techniques and photophysical comprehensive investigations. The position of the chromophore inside the micelles formed by F127 in water was successfully detected and moreover, a displacement towards the polar corona with increasing concentrations of the Ir(III) complex was observed.

Rheological and photophysical investigations of chromonic-like supramolecular mesophases formed by luminescent iridium(III) ionic complexes in water

ABSTRACT Luminescent complexes with general formula [(C^N)2Ir(N^N)]OCOCH3, where C^N = 2-phenylpyridine (ppy) and N^N = 2,2ʹ-bipyridine (bpy), self-assemble into ordered ‘soft’ viscous mesophases of chromonic type. By changing the ancillary ligand with 1,10-phenanthroline (phen), a new mesomorphic complex was obtained. The self-assembly into ordered liquid crystalline phases of chromonic type in water was investigated by comprehensive rheological and photophysical studies. By changing the ancillary ligand bpy with phen, assembly into smaller, more symetric aggregates is favoured, resulting in lower viscosity and more dynamic mesophase. Graphical Abstract

“Green light” for Zn(II) mesogens

Zn(II) complexes of different geometries have been synthesized starting from hexacatenar 2,2′-bipyridine or 1,10-phenanthroline ligands and their photophysical behaviour has been studied as a function of the temperature, starting from the solid state up to the isotropic liquid, in order to correlate the aggregation state to the luminescence properties.

Europium(III) and Terbium(III) Luminescent Lanthanidomesogens

Liquid-crystalline columnar phenanthroline-based ligand, for the first time substituted in 4,7-positions, is synthesized and used to induce low temperature mesomorphism in his corresponding lanthanide complexes. Stable liquid crystalline Eu(III) and Tb(III) complexes with low transition temperatures and low viscosity mesophases are obtained and their thermal and photophysical properties investigated as a function of their different solid, soft and liquid crystalline organisations. The eight-coordination around the metal lanthanide centre is further fulfilled by six oxygen atoms of three bidentate 1-phenyl-3-methyl-4-benzoylpyrazol-5-one proligands.

2,2′-Bipyridine Zn(II) complexes: effect of the 4,4′ substituents on the crystalline solid state properties

The syntheses, structures and photophysical characterizations of new tris-chelated Zn(II) ionic complexes based on substituted bipyridine ligands (L1 = 4,4′-bis(hydroxymethyl)-2,2′-bipyridine; L2 = 4,4′-bis(methoxy)-2,2′-bipyridine) are reported. Owing to the different hydrogen bonding ability and steric hindrance of the 4,4′-bipyridine substituent groups, the L1 and L2 ligands cause a substantial difference in terms of stoichiometry, supramolecular organizations and solid state photophysical properties of the final products. The reaction performed in solution between the Zn(CH3COO)2 salt and the ligand L2 in a 1:3.5 metal-to-ligand stoichiometric ratio afforded the expected octahedral complex [Zn(L2)3](PF6)22 as white crystalline powder. Conversely, the species {[Zn(L1)3](PF6)2}2·L1·2H2O (1a) in the form of pink single crystals is the exclusive product of the reaction performed under the same conditions as those for complex 2 by using L1, the compound including un-coordinated ligands into the crystalline framework. The resulting molecular structure of the new product 1a consists of a sort of supramolecular tetra-cationic adduct, the intercalation of the un-coordinated L1 ligand between two crystallographically equivalent [Zn(L1)3]2+ cations being mainly supported by stacking interactions and cooperative hydrogen bonds. Additionally, compound {[Zn(L2)3](PF6)2}2·L1·2H2O (2a) has also been prepared through targeted synthetic approaches by combining complex 2 with the free L1 ligand. In particular, the liquid-assisted grinding (LAG) method has been successfully applied by mixing complex 2 with a twofold molar amount of L1.