Controllable mechanical properties of anthraquinone-urea gel depending on the catalyst effect and their sensing ability for fluoride anion

Abstract

We used 2,6–diaminoanthraquinone (1), which is a famous dye derivative, and hexamethylene diisocyanate (2) for the urea formation. In general, the urea moieties between amine and cyanate are formed without a catalyst. In recent research, however, there have been several reports on urea reactions using a catalyst. In this paper, we studied the effect on gel formation of using dibutyltin dilaurate (DBTDL) as a catalyst. We observed the NMR measurement to determine the gelation time with and without the catalyst. In the presence of the catalyst, the reaction time was faster, as the gel formation shortened two fold. New NH groups of urea moieties generated by the reaction of amine groups of 1 with diisocyanate groups of 2 were also confirmed via 1H NMR observation, which indicate that urea moieties acted as a driving force of their self-assembly. Interestingly, the integral ratio of the new NH peak generated during the reaction in the presence of the catalyst was about seven times larger than that without the catalyst, indicating that the former urea reaction was larger. The anthraquinone-urea gels formed in the presence of catalyst showed fibrous structures with 2-µm lengths. However, the anthraquinone-urea gel formed in the absence of the catalyst showed honeycomb structures. The effect of the catalyst on the mechanical properties of the anthraquinone-urea gel was investigated by rheological measurement. In this case, the anthraquinone-urea gel showed values of viscosity and elasticity that were approximately 100 times larger in the presence of the catalyst. Furthermore, we suggest the anthraquinone-urea gel acted as a sensor for detecting fluoride ions by using the spectroscopic properties.