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

A pH-sensitive coordination polymer reversibly changes its color in air or acidic atmosphere.


Experimental
Materials and Methods. Chromeazurol B was purchased from TCI and used as delivered.
Spectrophotometric experiments were carried out on ThermoScientific Helios Hamma Spectrophotometer with 1cm acrylic cuvettes. FT-IR spectra of samples (KBr pellets) were recorded on a Scimitar FTS 2000 instrument. Elemental analysis was carried out on a EURO EA 3000 instrument (EuroVector).

Synthesis of [NaZn 4 (H 2 O) 3 (L) 3 ]·3THF·3H 2 O.
Zn(OAc) 2 ·2H 2 O (11mg, 0.05 mmol) and Na 2 HL (25 mg, 0.05 mmol) were dissolved in water/THF mixture (5 ml, 2:1). The mixture was heated at sealed glass tube at 100°C for 3 days after which black rhombohedral crystals ( Fig. S1a)  It is also possible to obtain crystals 1 not only from THF/water mixture, but also from water/dioxane solutions. In this case the resulting product is contaminated with red needle crystals which could not be separated and gave no diffraction. To obtain pure 1, we used THF/water solutions only. and Na 2 HL (25 mg, 0.05 mmol) was suspended in water/THF mixture (5 ml, 4:1). The mixture was heated in a sealed glass tube at 100°C for 3 days. After cooling and standing for 2 days black octahedral crystals (with green sheen; red in transmitted light) were collected (

X-Ray Crystallography
Diffraction data for a single crystal of 1 were obtained at 130 K on an automated Agilent Xcalibur diffractometer equipped with a two-dimensional AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å). Diffraction data for a single crystal of compound 2 were obtained at 120 K on an Rigaku Oxford Diffraction SuperNovaII diffractometer equipped with a two-dimensional Atlas detector (rotating anode, λ(CuKα) = 1.54184 Å). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [1]. The structures were solved by direct methods and refined by the full-matrix least squares technique in the anisotropic approximation (except hydrogen ν, S5 atoms) using the SHELX-2013 software [2]. Positions of hydrogen atoms of the organic ligands were calculated geometrically and refined in a riding model. The diffraction data for 2 were weak, and the values of I / σ(I) at a resolution higher than 1.2 does not exceed 3 (R int > 0.20). However, the structure of the material is sufficiently reliable and clear. Table S1 contains the crystal data and structure refinement details for 1 and 2. Selected bond lengths and angles for 1 and 2 are provided in Tables S2 and S3,   Hydrogen atoms are omitted. Symmetry transformations used to generate equivalent atoms: i) −x + 3/2, y, −z + 1; ii) x + ½, −y + ½, −z + 1.

Investigation of Halochromism.
Diffuse reflectance spectra of powders were recorded on a Shimadzu UV-3101 PC spectrophotometer in the range 240 nm to 800 nm at room temperature. Kubelka-Munk (KM) spectra were recalculated from the diffuse reflectance spectra of the samples containing 5% of compound dispersed in the BaSO 4 matrix. The reflectance data was converted to Kubelka-Munk units, which are proportional to the sample absorbance, by the following equation:  Measurements were carried out using a Mettler Toledo SevenCompact pH/Ion S220 pH meter at 25°C.
Samples of powdered 1 were treated by vapors of 7M and 2M HCl for 1 h. The samples were then exposed to air for several minutes to remove surface-adsorbed HCl. The pH was measured after the addition of distilled water to the resulting solids. The number of protons was calculated by the formula: where V is the volume of water added, m is the weight of the sample, and 1978 is the molar mass of [NaZn 4 (H 2 O) 3 (L) 3 ]·3THF·3H 2 O,. As could be seen from the data in Table S1, the number of acidic protons drops after the first minute of exposure to air and stabilizes at 2.8 and 3.9 H + per ligand for the number of the protons in 1 after the exposure to vapors 2M and 7M HCl, respectively.