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

Abstract

New pyridine-functionalised pyrazole compounds [Hpz^R(n)py] (R(n) = C₆H₄OC_nH_2n+1; n = 12, 14, 16, 18; 1–4) and their corresponding silver complexes [Ag(Hpz^R(n)py)₂][A] ([A] = NO₃⁻, BF₄⁻; 5–12) 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][NO₃] 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 Zn²⁺, Cu²⁺ 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.