Hydrogen-bonding receptor substituted BODIPYs as selective ON-OFF fluorimetric sensors for fluoride ions in polar aprotic organic solvents – A molecular-level understanding based on experimental and theoretical studies

Abstract

We report the facile synthesis of three meso-substituted BODIPYs which can act as fluorimetric chemosensors for detecting fluoride ions in polar aprotic organic solvents with a micromolar detection limit. The substituents at the meso-positions of synthesized BODIPYs are acetamidophenyl (B1), trihydroxyphenyl (B2), and hydroxyquinoline (B3) which show strong absorption (490–510 nm) and emission (515–550 nm) in the visible region. The spectral and quantum mechanical studies indicate that the fluoride ions can interact with the acidic proton of the BODIPYs viz., NH in B1 and OH in B2 and B3 respectively, and results in 1:1 BODIPY-fluoride [BODIPY----HF] adduct. The acidic protons of these BODIPYs are abstracted by fluoride ions resulting in non-fluorescent deprotonated species (turn-OFF state). The time-dependent density functional theory (TDDFT) calculations indicate that the deprotonated adduct is responsible for the fluorescence turn-OFF via the photoinduced electron transfer (PET) mechanism. These non-fluorescent deprotonated adducts can return to the fluorescent BODIPY state by abstracting H+ ions from any protic solvents. Hence, these molecules can act as ON-OFF-ON reversible sensors. The deprotonated adducts show significantly different UV–visible responses for B1, B2 and B3. The binding constant of fluoride ions with BODIPYs (Kb) is calculated to be in the range of 105 –106 M−1 with a limit of detection (LOD) in the range of 0.73–9 ppm. The ON-OFF-ON sensing behaviour of B1 is utilized to construct NOR molecular logic gate based on the two inputs of F- and OH– ions, and the emission intensity at 520 nm as the output. Among these BODIPYs, B2 has the lowest LOD for fluoride ions and shows distinct color change upon deprotonation in the visible region. This property has been utilized to develop portable test-paper strips for the qualitative colorimetric detection of fluoride ions.