Exploring photophysical properties of new boron and palladium(ii) complexes with β-diketone pyridine type ligands: From liquid crystals to metal fluorescence probes

Pyridine-functionalized β-diketone ligands of the type 1-(2-pyridyl)-3-(4-n-alkyloxyphenyl)propane-1,3-dione [HL^R(n)py] (R = C₆H₄OC_nH_{2n + 1}, n = 12 (1), 14 (2)) were carried to react with the BF₂ fragment giving rise to the corresponding adducts [BF₂(L^R(n)py)] (n = 12 (5), 14 (6)) which were characterized and their luminescent and thermal behaviour studied. All the ligands and boron derivatives are photoluminescent both in the solid state and in solution. The related compounds 2-[3-(4-n-alkyloxyphenyl)propane-1, 3-dion-1-yl]pyridinium chloride [HL^R(n)pyH]Cl (n = 12 (3), 14 (4)) were also proved to have luminescent properties. On the other hand the organometallic compounds [Pd(η³-C₃H ₅)(HL^R(14)py)][PF₆] (7) and [Pd(η³-C₃H₅)(L^R(14)pyH)][PF ₆] (8), based on the coordination of the [Pd(η³-C ₃H₅)]⁺ fragment to the above neutral 2 or ionic 4 β-diketone derivatives, respectively, were also established to be luminescent materials, compound 8 exhibiting liquid crystalline properties. The luminescent behaviour of all compounds was also investigated as a function of the metal addition of Zn²⁺ and Cu²⁺ ions. As a result it is proven that the free β-diketone pyridine ligands [HL^R(n)py], the related chloride salts [HL^R(n)pyH]Cl and their palladium(ii) complexes behave as fluorescence chemosensors for Zn²⁺ and Cu ²⁺. The luminescent behaviour of the allyl-palladium complex 8, which exhibits enantiotropic liquid crystalline properties, was also studied as a function of the temperature. As a consequence it is established that the liquid crystal state does not quench the fluorescence emission.