Zinc Coordination Polymers Containing Isomeric Forms of p-(Thiazolyl)benzoic Acid: Blue-Emitting Materials with a Solvatochromic Response to Water

Two coordination polymers of assorted dimensionality (1D, 2D) have been prepared, namely [Zn-3(L-2Th)(4)(OH)(2)2(HL2Th)] (1) and [Zn(L-5Th)(OAc)] (2), starting from Zn-II salts and the isomeric forms of the organic linker p-(thiazolyl)benzoic acid: p-(2-thiazolyl)benzoic acid (HL2Th) and p-(5-thiazolyl)benzoic acid (HL5Th). The isomers have been prepared ad hoc, following straightforward Pd-catalyzed C-C coupling reaction protocols. In 1, the deprotonated ligand is coordinated through its carboxylate group only, with dangling thiazole groups. The -COO- units are bridging adjacent metal centers, thus creating a 1D chain. The Zn-3 cluster is made of one six-coordinate (O-h) and two four-coordinate (T-d) Zn-II ions; triple-bridging mu(3)-OH groups are balancing the overall positive charge. The structure of 2 is instead made of Zn-2(carboxylate)(4) paddle-wheel dimers as the constituting inorganic node. The octahedral metal coordination sphere includes two mu-(-COO) benzoate spacers, two mu-(-COO) acetate ions, the thiazole N atoms coming from adjacent building blocks, and a weak ZnZn axial interaction. The resulting final assembly is two-dimensional (2D), where p-(5-thiazolyl)benzoate adopts a genuine mu-[(COO):(N)] bridging coordination mode. The luminescent properties of both polymers have been analyzed in the solid state; they feature ligand-centered emissions at = 434 nm (1) and = 427 nm (2). These electronic transitions fall in the visible region, giving the samples a characteristic blue color under an ordinary UV lamp (excitation at = 254 nm). The theoretical analysis of the electronic features of the ligands and related molecular orbitals reveals that the observed transitions are mainly of * nature, involving orbitals delocalized on both aromatic cycles. A significant (reversible) blueshift of the emission maximum of ca. 60 nm, from the visible to the UV region, has been observed for 1 when suspended in water.