A Subtle Change in Substituent Enabled Multi-Ways Fluoride Signals Including Paper-Strip Colorimetric Detection Using Urea Receptor Functionalized 1,1,4,4-Tetracyanobuta-1,3-Diene-Based Push-Pull Chromophore

A simple color change detection by the naked-eye using untreated paper for a biologically relevant fluoride (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 Fion giving various output signals. But, replacing phenyl (Ph) at C3-position with 4-(dimethylamino)phenyl (DMA) led to the absence of colorimetric and fluorometric detections due to the masking and quenching, nature of strong ICT by the DMA. NMR and electrochemical studies revealed that the sensing mechanism is governed by H-bonding as well as the deprotonation of N–H 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 is presented.