Thiacopillar[5]arenes in Supramolecular Assemblies and Chromogenic Detection of Aliphatic Diamines: A New Approach

This paper demonstrates the synthesis, crystal structures, and amine recognition properties of a series of thiacopillar[5]arenes 1-4. A simple route for the thiolation of pillar[5]arene that allows for the easy synthesis of bis-pillar[5]arene structures is provided. Hence, synthesis of disulfide-linked bis-pillar[5]arenes 2 and 2a is reported, for the first time, from our newly developed copillar[5]arene precursor 1. Michael addition of the thiols to the quinone motif yields thiacopillar[5]arenes 3 and 4 from pillar quinone. In the study, the chemical properties of the thiocarbonate ring and phenolic ─OH in 1 are undertaken to demonstrate amine recognition. It visually detects 1,3-diaminopropane and 1,4-diaminobutane (putrescine) in DMSO through host-guest interaction, phenol deprotonation, and cleavage of the cyclic thiocarbonate ring. In reference to this, bis-pillararene 2 exhibits color change with different amines (except aromatic amines) and anions, giving no selectivity. Due to the absence of a hydroquinone motif in the structure, 2a does not exhibit similar characteristics. Furthermore, bis-pillararene 2 is established to be excellent in the vapor phase detection of volatile amines. On the other hand, 3 and 4 are insensitive in amine recognition due to the lack of disulfide bonds. A DFT study has been used to interpret the results.