Geometric Change of a Thiacalix[4]arene Supramolecular Gel with Volatile Gases and Its Chromogenic Detection for Rapid Analysis

Abstract

A coordination polymer gel that is self-assembled to form a network structure between a thiacalix[4]­arene derivative (<b>L</b>) and Co²⁺ has been prepared. This gel is capable of selectively changing color in the presence of gases that yield hydrogen chloride upon hydrolysis. The UV–vis absorption spectrum of a coordination polymer gel derived from Co­(NO₃)₂ exhibits an absorption band at 527 nm and is colored red, indicating the formation of an octahedral Co²⁺ complex. Treatment with a small amount of volatile gases containing a chlorine atom (VGCl) causes a red shift of ∼150 nm, resulting in a new strong band with a maximum at 670 nm and a color change to blue. In addition, the red color of the filter paper coated with a Co­(NO₃)₂ coordination polymer gel changed to blue upon exposure to VGCl, reflecting a change in the coordination geometry. Red and blue colors of single crystals of Co²⁺ complexes were obtained from a basic solution. From X-ray crystallographic analysis, the red Co²⁺ complex corresponds to an octahedral structure, while the blue Co²⁺ complex reflects the presence of a tetrahedral structure. Thus, the induced color change of Co²⁺ gel from red to blue upon exposure to VGCl is due to the coordination geometry. The quantitative concentration of VGCl was calculated by employing the RGB histogram available in a smartphone application