A subtle change in substituent enabled multi-ways fluorine anion signals including paper-strip colorimetric detection using urea-functionalized push–pull chromophore receptor

Abstract

A simple color change detection by the naked-eye using paper-strip for a biologically relevant fluorine (F–) anion in water is a challenge. New non-planar push–pull chromophore involving intramolecular charge-transfer (ICT) from urea donor in 2,3-disubstituted-1,1,4,4-tetracyanobuta-1,3-diene (TCBD) turns out to be an efficient system for detecting F– ion giving various output signals. But, replacing phenyl (Ph) at C3-position with 4-(dimethylamino)phenyl (DMA) as well as hydrogen (H) have led to the absence of colorimetric and fluorometric detections due to the masking and quenching nature of the ICT by the DMA. NMR and electrochemical studies revealed that the sensing mechanism is governed by H-bonding as well as the deprotonation of NH attached with TCBD moiety which reduces the HOMO–LUMO gap and causes the dramatic color change. Coupled with excellent sensitivity (3 ppm) and specificity towards F–, a successful demonstration of cheap tissue paper-based visual strip-detection in aqueous phase is presented.