Azo and imine functionalized 2-naphthols: promising supramolecular gelators for selective detection of Fe3+ and Cu2+, reactive oxygen species and halides

Abstract

Azo and imine functionalised 2-naphthols 1–3 have been established as low molecular weight supramolecular gelators (LMWGs). Hydrogen bonding and π–π stacking between the naphthalene units contribute a weak force for gelation. The gels have sufficient mechanical strength and are responsive to multiple ions. While the gel of 1 in CH₃CN : H₂O (1 : 1, v/v) is responsive to Fe³⁺ and Cu²⁺ ions, under identical conditions the gel of 2 is sensitive to Fe³⁺ ions involving a gel-to-sol transition. The imine compound 3 in its gel state (DMSO : H₂O; 1 : 1 v/v) displays selective interaction with Fe³⁺ ions over a series of other metal ions through a color change of the gel state from yellow to dark brown. Compound 3, in this context, is an LMWG that selectively detects Fe³⁺ and also Fe²⁺ in the presence of an oxidising agent. Oxidation of Fe²⁺ by reactive oxygen species (ROS) and its subsequent colorimetric effect on the gel of 3 finds application in ROS detection. Moreover, the Fe³⁺ ion-induced colored gel of 3 successfully senses F⁻ ions visually from other halides. For real-life application, a test strip of 3 for the detection of these analytes and also its use as a rewritable display material and combinational logic gate system are demonstrated.