Pyrazole-Based Metallogels Showing an Unprecedented Colorimetric Ammonia Gas Sensing through Gel-to-Gel Transformation with a Rare Event of Time-Dependent Morphology Transformation

A series of novel pyrazole-based bis- and tris-amide molecules have been synthesized and characterized which were found to gel copper chloride only among other first row transition metals or other copper salts. All the gels have been thoroughly characterized using infrared spectroscopy, UV–visible spectroscopy, scanning electron microscopy, transmission electron microscopy, and rheological studies. The gels were composed of an infinitely long nanofibrillar network which displayed an interesting morphological transformation to nanoscale metal–organic particles (NMOPs), further corroborated by microscopic techniques and X-ray powder diffraction data. Although NMOPs, hybrid organic–inorganic nanoparticles, are widely known for their immense applications in the field of drug discovery and medicine, NMOP-based metallogels are not so common. Furthermore, this is a very rare example, to the best of our knowledge, in which such a transformation is observed in metallogelation. Gel 3 shows another extremely rare and fascinating external chemical stimuli responsive gel-to-gel transformation with a potential application of colorimetric ammonia gas sensing.