Halochromic coordination polymers based on a triarylmethane dye for reversible detection of acids

Chromeazurol B (Na₂HL) is a pH-sensitive (halochromic) dye based on a hydroxytriarylmethane core and two carboxylate functional groups, which makes it suitable for the synthesis of coordination polymers. Two new coordination polymers [NaZn₄(H₂O)₃(L)₃]·3THF·3H₂O (1) and [Zn₃(H₂O)₃(μ₂-OH₂)(μ₃-OH)(HL)₂(H₂L)]·2THF·3H₂O (2) incorporating Chromeazurol B linkers have been prepared and characterised. The structure of 1 comprises pentanuclear heterometallic {Zn₄Na} nodes linked by six L³⁻ anions to give a layered structure with a honeycomb topology. 2 crystallizes as a double-chain ribbon (ladder) structure with two types of metal node: a mononuclear Zn(ii) cation and tetranuclear {Zn(ii)}₄ cluster. Chromeazurol B anions link each tetranuclear cluster to four individual Zn(ii) cations and each Zn(ii) cation with four tetranuclear clusters. Both compounds show pH-sensitivity in water solution which can be observed visually, giving the first example of a halochromic coordination polymer. The halochromic properties of 1 towards HCl vapors were systematically investigated. As-synthesized violet-grey 1 reversibly changes color from orange to pink in the presence of vapors of 2 M and 7 M HCl, respectively. The coordination of the Chromeazurol B anion at each color stage was examined by diffuse reflectance spectroscopy and FT-IR measurements. The remarkable stability of 1 to acid and the observed reversible and reproducible color changes provide a new design for multifunctional sensor materials.