Dual Anion Sensing Enabled by Synergistic Radical Formation and Hydrogen Bonding in a Bisindolyl-NDI Probe

The simultaneous detection of structurally diverse anions remains a significant analytical challenge, particularly for strongly basic species such as fluoride (F – ) and cyanide (CN – ). Their comparable charge densities and strong hydrogen-bonding tendencies often lead to overlapping binding profiles and competing interactions, complicating selective recognition. To address this, we designed an NDI-based bis(indolyl)methane derivative (probe 1 ) capable of dual anion detection. The probe enables differentiation between F – and CN – ions via distinct optical signatures, providing insights into their underlying recognition mechanisms. Upon the addition of CN – ions to the probe solution (in DMSO), prominent absorption features corresponding to the NDI •– radical anion were observed, indicating an electron-transfer process. Similarly, F – ions also induced the formation of radical anion peaks but with comparatively lower intensity, alongside spectral changes predominantly arising from strong hydrogen-bonding interactions between the bisindolyl-NH and F – ions. These observations demonstrate that while both H-bonding and electron-transfer processes contribute to anion recognition, H-bonding dominates in the case of F –, whereas electron transfer is the primary pathway for CN – . Time-course experiments showed that the interaction between probe 1 and the anions completed in ∼4 min, and the NDI •– radical remains stable at ambient conditions in DMSO. Furthermore, probe 1 demonstrated high sensitivity toward CN –, with a limit of detection of 0.42 μM, well below the WHO permissible limit. To assess practical applicability, paper-based test strips impregnated with probe 1 were fabricated, enabling rapid, instrument-free, and visually distinguishable colorimetric detection of both anions under ambient conditions.