A color version of the Hinsberg test: permethylated cyclodextrin and crown-appended azophenol for highly selective sensing of amines

Azophenol dyes 1-5 having the permethylated cyclodextrin and/OF crown moieties have been synthesized. Compounds 1 and 2 provide critical information for discrimination among 1 degrees-3 degrees amines by unique color changes. Addition of 1 degrees and 2 degrees amines to solutions of I or 2 in chloroform shifts the absorbance maximum of the initial solutions from 380 nm to similar to 580 nm and similar to 530 nm, respectively, but no change is observed with 3 degrees amines. The high selectivity of I is mainly due to H-bonding between the ammonium H atoms of the amine and oxygen atoms of the crown-6. The selectivity of 1 possessing the beta-cycloclextrin moiety toward amines was higher than that of 2 possessing the alpha-cyclodextrin moiety. On the other hand, chloroform solutions of 3 or 4, which lack the crown ether moiety, changed from yellow (380 nm) to pink (500 nm) with the addition of 1 degrees and 2 degrees amines, but with no selectivity. These results indicate that the crown unit in 1 or 2 plays an important role in discriminating among the types of amines. (C) 2008 Elsevier Ltd. All rights reserved.