Chromogenic calixarene sensors for amine detection

Spun films of nitrophenylazo-calix[4]arenes were investigated as gas-phase optical sensors. A red shift of the visible absorption bands was observed as these films were exposed to amine vapors. The magnitude of chromogenic effect depends on the basicity and the steric hindrance of the amine. The highest sensitivity is observed for n-hexylamine, as compared to amines with weaker basicity, e.g. n-pentylamine, n-butylamine and aniline, or amines with branched alkyl groups, e.g. dipropylamine, triethylamine, iso-pentylamine and tert-pentylamine. The chromogenic effect towards n-hexylamine also depends on substitutions on the upper and lower rims of calix[4]arene. A mechanism based on deprotonation of the azophenol group is proposed. This mechanism is consistent with experimental observations as well as results from molecular simulations.