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

A coordination
polymer gel that is self-assembled to form a network structure between
a thiacalix[4]­arene derivative (L) 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.