Cristián Cuerva

Columnar discotic Pt(II) metallomesogens as luminescence multifunctional materials with chemo and thermosensor abilities

A new family of Pt(II) luminescent metallomesogens based on dicatenar pyridylpyrazolate ligands [Pt(pzR(n,n)py)2] (R(n,n) = C6H3(OCnH2n+1)2, n = 4–18) has been prepared, and their mesomorphic and photophysical properties are described. The compounds were isolated as red (n = 4–8) or yellow (n = 10–18) solids at room temperature, but the first ones were converted to yellow crystals by slow evaporation of a chloroform–acetone mixed solution. All of them behave as discotic liquid crystal materials, exhibiting hexagonal columnar mesophases (Colh) in a wide range of temperatures. Photoluminescence studies in the solid state at variable temperatures showed a high emission in the liquid crystalline phase, which was significantly red-shifted with respect to the yellow-green emission of the solid state. This photophysical change was attributed to the formation of aggregates through Pt(dz2)–Pt(dz2) interactions, thereby giving rise to the metal–metal-to-ligand charge transfers (3MMLCT) responsible for the luminescence observed. Taking advantage of these properties we have fabricated polymeric solid supports doped with the platinum complex [Pt(pzR(10,10)py)2], which can be used as temperature sensors for real technological applications. In addition, the Pt–bispyrazolate complexes and their corresponding pyrazole ligands have been proved to be useful as chemosensors towards Pd2+, Zn2+, Cd2+ and Hg2+ metal ions.

Platinum(II) Metallomesogens: New External-Stimuli-Responsive Photoluminescence Materials.

New dicatenar isoquinoline-functionalized pyrazoles, [Hpz(R(n,n)iq) ] (R(n,n)=C6 H3 (OCn H2n+1 )2 ; n=4, 6, 8, 10, 12, 14, 16, 18), have been strategically designed and synthesized to induce mesomorphic and luminescence properties into the corresponding bis(isoquinolinylpyrazolate)platinum(II) complexes [Pt(pz(R(n,n)iq) )2 ]. Thermal studies reveal that all platinum(II) compounds exhibit columnar mesophases over an exceptionally wide temperature range, above 300 °C in most cases. The photophysical behavior was also investigated in solution and in the solid state. As a consequence of the formation of Pt⋅⋅⋅Pt interactions, the weak greenish emission of the platinum derivatives turns bright orange in the mesophase. Additionally, the complexes are sensitive to a great variety of external inputs, such as temperature, mechanical grinding, pressure, solvents, and vapors. On this basis, they are used as dopant agents of a polyvinylpyrrolidone or poly(methyl methacrylate) polymer matrix to achieve stimuli-responsive thin films.

Bis(pyridylpyrazolate)platinum(II): a mechanochromic complex useful as a dopant for colour-tunable polymer OLEDs

The photoluminescence and mechanochromic behaviour of the bis(3-(3,5-bis(dodecyloxy)phenyl)-(5-pyridin-2-yl)pyrazolate)platinum(II) complex PT12, selected from a series of Pt(II) compounds with N,N′-pyridylpyrazolate ligands, has been investigated. The electroluminescence properties of polymer OLEDs based on PT12-doped polyfluorene (PFO) are also described. Addition of PT12 induces the formation of the PFO β-phase, a much more ordered phase with enhanced colour stability at high bias and high photoluminescence quantum efficiency. Additionally, the characteristic blue emission of PFO is red-shifted for PT12 concentrations equal to or higher than 3%. Depending on the dopant concentration and the applied current, the colour coordinates of these polymer OLEDs undergo a broad shift in the CIE colour space. In addition, through the strategic use of keto defect sites, white-emitting devices can be fabricated with a very small addition of PT12.

Water-Free Proton Conduction in Discotic Pyridylpyrazolate-based Pt(II) and Pd(II) Metallomesogens.

In this work we report on water-free proton conductivity in liquid-crystal pyridylpyrazolate-based Pt(II) and Pd(II) complexes [M(pz(R(n,n)py))2] (pz(R(n,n)py) = 3-(3,5-dialkyloxyphenyl)-5-(pyridin-2-yl)pyrazolate, R(n,n) = C6H3(OCnH2n+1)2; n = 4, 12, 16, M = Pd; n = 12, M = Pt) with potential application as electrolyte materials in proton exchange membrane fuel cells. The columnar ordering of the complexes in the liquid-crystalline phase opens nanochannels, which are used for fast proton exchange as detected by impedance spectroscopy and NMR. The NMR spectra indicate that the proton conduction mechanism is associated with a novel C-H···N proton transfer, which persists above the clearing point of the material. The highest conductivity of ∼0.5 μS cm(-1) at 180 °C with an activation energy of 1.2 eV is found for the Pt(II) compound in the mesophase. The Pd(II) complexes with different chain length (n = 4, 12, and 16) show lower conductivity but smaller activation energies, in the range of 0.74-0.93 eV.

Diketonylpyridinium Cations as a Support of New Ionic Liquid Crystals and Ion-Conductive Materials: Analysis of Counter-Ion Effects

Ionic liquid crystals (ILCs) allow the combination of the high ionic conductivity of ionic liquids (ILs) with the supramolecular organization of liquid crystals (LCs). ILCs salts were obtained by the assembly of long-chained diketonylpyridinium cations of the type [HOOR(n)pyH]+ and BF4−, ReO4−, NO3−, CF3SO3−, CuCl42− counter-ions. We have studied the thermal behavior of five series of compounds by differential scanning calorimetry (DSC) and hot stage polarized light optical microscopy (POM). All materials show thermotropic mesomorphism as well as crystalline polymorphism. X-ray diffraction of the [HOOR(12)pyH][ReO4] crystal reveals a layered structure with alternating polar and apolar sublayers. The mesophases also exhibit a lamellar arrangement detected by variable temperature powder X-ray diffraction. The CuCl42− salts exhibit the best LC properties followed by the ReO4− ones due to low melting temperature and wide range of existence. The conductivity was probed for the mesophases in one species each from the ReO4−, and CuCl42− families, and for the solid phase in one of the non-mesomorphic Cl− salts. The highest ionic conductivity was found for the smectic mesophase of the ReO4− containing salt, whereas the solid phases of all salts were dominated by electronic contributions. The ionic conductivity may be favored by the mesophase lamellar structure.

Tetrahedral and octahedral metallomesogenic Zn(ii) complexes supported by pyridine-functionalised pyrazole ligands

Pyridylpyrazoles with long-chained alkyloxyphenyl substituents [HpzR(n)py] (R(n) = C6H4OCnH2n+1, n = 12, 14, 16, 18) are versatile ligands for designing new zinc metallomesogens [Zn(HpzR(n)py)m][X]2 (m = 2, X = NO3−; m = 3, X = BF4−) with tetrahedral and octahedral coordination geometry, respectively. Molar conductivity measurements reveal the non-bonding character of the nitrate and tetrafluoroborate anions to Zn(II) and confirm the ionic nature of the complexes in solution. Polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD) studies show that all of them are enantiotropic liquid crystals, exhibiting SmA mesophases. A layered packing with interdigitation of the alkyl chains is proposed on the basis of XRD results to explain the organisation in the SmA mesophase. TG experiments indicate that the zinc complexes are stable up to ca. 175 °C, the temperature at which mass loss is observed. On the basis of computational models using hyperchem-7 program the molecular geometry has been examined. The semi-empirical calculations suggest that, in the solid state, the counteranions could be involved in hydrogen bonds, which would contribute to obtaining the stable electronic structure.

Nanostructured discotic Pd(II) metallomesogens as one-dimensional proton conductors

A novel family of square-planar Pd(ii) complexes based on isoquinoline-functionalised pyrazolate ligands [Pd(pzR(n,n)iq)2] (R(n,n) = C6H3(OCnH2n+1)2, n = 4, 6, 8, 10, 12, 14, 16, 18) has been synthesised and characterised. The new complexes show mesomorphic properties and exhibit columnar mesophases that are highly-stable in exceptionally wide temperature ranges of up to 345 °C. The formation of nanochannels in the fluid liquid crystal phases generates continuous pathways for one-dimensional proton conduction on the basis of C-HN proton transfer. The complex with an intermediate chain length (n = 12) shows the highest proton conductivity of 1.34 × 10-4 S m-1 at 269 °C in the hexagonal columnar mesophase, and an activation energy of 0.84 eV. The influence of both the terminal alkyl chain length and the mesophase columnar organisation on the proton conduction mechanism is demonstrated. The series of Pd(ii) complexes investigated in this work constitutes one of the first examples of proton-conducting metallomesogens with potential applications in PEM fuel cells.

Multifunctional Pt(II) metallomesogens exhibiting luminescence and proton conductivity in the mesophase near room temperature

New asymmetrical pyridine- and isoquinoline-functionalized bis(pyrazolate) Pt(II) metallomesogens have been engineered and designed to exhibit multifunctional properties in terms of mesomorphism, photoluminescence and ionic conductivity near room temperature. The molecules are self-assembled in hexagonal columnar mesophases via Pt⋯Pt interactions, which generates a strong orange-red emission as a result of the formation of triplet metal–metal-to-ligand charge-transfer (3MMLCT) excited states. Concomitantly, the hexagonal columnar mesophases open continuous nanochannels that have been used for proton conduction over a wide range of temperatures. It is also noteworthy that the luminescence typically decreases with temperature, whereas the ionic conductivity increases to values of the order of 10−5 Ω−1 cm−1 at ca. 300 °C in the mesophase.