Advanced Functional Luminescent Metallomesogens: The Key Role of the Metal Center

The use of liquid crystals for the fabrication of displays incorporated in technological devices (TVs, calculators, screens of eBook's, tablets, watches) demonstrates the relevance that these materials have had in our way of living. However, society evolves, and improved devices are looked for as we create a more efficient and safe technology. In this context, metallomesogens can behave as multifunctional materials because they can combine the fluidic state of the mesophases with properties such as photo and electroluminescence, which offers new exciting possibilities in the field of optoelectronics, energy, environment, and even biomedicine. Herein, it has been established the role of the molecular geometry induced by the metal center in metallomesogens to achieve the self-assembly required in the liquid-crystalline mesophase. Likewise, the effect of the coordination environment in metallomesogens has been further analyzed because of its importance to induce mesomorphism. The structural analysis has been combined with an in-depth discussion of the properties of these materials, including their current and potential future applications. This review will provide a solid background to stimulate the development of novel and attractive metallomesogens that allow designing improved optoelectronic and microelectronic components. Additionally, nanoscience and nanotechnology could be used as a tool to approach the design of nanosystems based on luminescent metallomesogens for use in bioimaging or drug delivery.

Coordination behaviour of new dipyridylpyrazole ligands towards ZnCl2and PdCl2fragments. Crystalline structural characterization and multinuclear NMR studies as evidence of linkage and conformational isomers

Novel metal complexes containing dipyridylpyrazole ligands as efficient examples of isomerism are investigated.

Silver-pyrazole complexes as hybrid multifunctional materials with metallomesogenic and photoluminescent behaviour

New pyridine-functionalised pyrazole compounds [Hpz(R(n)py)] (R(n) = C(6)H(4)OC(n)H(2n+1); n = 12, 14, 16, 18; 1-4) and their corresponding silver complexes [Ag(Hpz(R(n)py))(2)][A] ([A] = NO(3)(-), BF(4)(-); ) have been synthesised and characterised. All of them, with the exception of 1, are liquid crystal materials exhibiting monotropic or enantiotropic SmA mesophases, in contrast to the non-mesomorphic related R(n)-monosubstituted compounds. Because the molecular shape is a factor determinant in the organisation of molecules in the liquid crystal phase, we were interested in solving the crystal structure of representative examples of the mentioned compounds, such as 1 and 6. So, the X-ray crystal structure of [Hpz(R(12)py)] 1 shows the presence of dimeric units through N-H···N hydrogen bonds, which conform to an elongated molecular shape containing a double chain length. On the other hand, the structure of [Ag(Hpz(R(14)py))(2)][NO(3)] 6 also evidenced Ag-Ag bonded dimers from 'U'-shaped cationic entities. These dimers exhibit four chains, two by two alternated, so giving rise to a longer molecular length. Of particular interest was to observe that in both structures, the dimers are layer-like packed, their lamellar structures being related to that of the mesophases found in both kinds of compounds. Furthermore, the analysis of the optical data of the compounds 2 and 4 and the silver compounds 5, 6, 9 and 10 as representative examples pointed out their luminescent behaviour as well as their good ability to act as fluorescent probes for Zn(2+), Cu(2+) and Ag(+). An increase in the fluorescence quantum yields is observed in the final complexes produced in the titrations, this fact being specially notable when 9 was used as the starting compound.

Synthesis and characterization of azolate gold(I) phosphane complexes as thioredoxin reductase inhibiting antitumor agents

Following an increasing interest in the gold drug therapy field, nine new neutral azolate gold(I) phosphane compounds have been synthesized and tested as anticancer agents. The azolate ligands used in this study are pyrazolates and imidazolates substituted with deactivating groups such as trifluoromethyl, nitro or chloride moieties, whereas the phosphane co-ligand is the triphenylphosphane or the more hydrophilic TPA (TPA = 1,3,5-triazaphosphaadamantane). The studied gold(I) complexes are: (3,5-bis-trifluoromethyl-1H-pyrazolate-1-yl)-triphenylphosphane-gold(I) (1), (3,5-dinitro-1H-pyrazolate-1-yl)-triphenylphosphane-gold(I) (2), (4-nitro-1H-pyrazolate-1-yl)-triphenylphosphane-gold(I) (5), (4,5-dichloro-1H-imidazolate-1-yl)-triphenylphosphane-gold(I) (7), with the related TPA complexes (3), (4), (6) and (8) and (1-benzyl-4,5-di-chloro-2H-imidazolate-2-yl)-triphenylphosphane-gold(I) (9). The presence of deactivating groups on the azole rings improves the solubility of these complexes in polar media. Compounds 1-8 contain the N-Au-P environment, whilst compound 9 is the only one to contain a C-Au-P environment. Crystal structures for compounds 1 and 2 have been obtained and discussed. Interestingly, the newly synthesized gold(I) compounds were found to possess a pronounced cytotoxic activity on several human cancer cells, some of which were endowed with cis-platin or multidrug resistance. In particular, among azolate gold(I) complexes, 1 and 2 proved to be the most promising derivatives eliciting an antiproliferative effect up to 70 times higher than cis-platin. Mechanistic experiments indicated that the inhibition of thioredoxin reductase (TrxR) might be involved in the pharmacodynamic behavior of these gold species.

Mesomorphism and luminescence properties of platinum(II) complexes with tris(alkoxy)phenyl-functionalized pyridyl pyrazolate chelates

A series of new mesomorphic platinum(II) complexes 1-4 bearing pyridyl pyrazolate chelates are reported herein. In this approach, pyridyl azolate ligands have been strategically functionalized with tris(alkoxy)phenyl groups with various alkyl chain lengths. As a result, they are ascribed to a class of luminescent metallomesogens that possess distinctive morphological properties, such as their intermolecular packing arrangement and their associated photophysical behavior. In CH(2) Cl(2), independent of the applied concentration in the range 10(-6)-10(-3)  M, all Pt(II) complexes exhibit bright phosphorescence centered at around 520 nm, which is characteristic for monomeric Pt(II) complexes. In stark contrast, the single-crystal X-ray structure determination of [Pt(C4pz)(2)] (1) shows the formation of a dimeric aggregate with a notable Pt⋅⋅⋅Pt contact of 3.258 Å. Upon heating, all Pt(II) complexes 1-4 melted to form columnar suprastructures, for which similar intracolumnar Pt⋅⋅⋅Pt distances of approx. 3.4-3.5 Å are observed within an exceptionally wide temperature range (>250 °C), according to the powder XRD data. Upon casting into a neat thin film at RT, the luminescence of 1-4 is dominated by a red emission that spans 630-660 nm, which originates from the one-dimensional, chainlike structure with Pt-Pt interaction in the ground state. Taking complex 4 as a representative, the emission intensity and wavelength were significantly decreased and blueshifted, respectively, on heating from RT to 250 °C. Further heating to liquefy the sample alters the red emission back to the green phosphorescence of the monomer. The results highlight the pivotal role of tris(alkoxy)phenyl groups in the structural versus luminescence behavior of these Pt(II) complexes.