Rational design of an acid-sensitive fluorophore from 8-hydroxy quinoline derivative exhibiting proton activated charge transfer characteristics

A pH-responsive small organic molecule 8OMeQBI has been synthesized by functionalizing 8-hydroxy quinoline and benzimidazole derivatives in an organo-aqueous medium. The pH-responsive fluorescence behavior in a semi-aqueous environment was studied using UV–visible and fluorescence spectral analysis. The newly prepared fluorophores show a protonation-activated fluorescence enhancement in an acidic environment. An increase in pH in an alkaline environment has caused deprotonation, resulting in diminished fluorescence intensity. Additionally, the protonation-induced fluorescence was confirmed through the proton-nuclear magnetic resonance titration analysis. Further, the photophysical studies reveal an enhancement in the relative quantum yield (ɸ = 0.3) and fluorescence lifetime (increases to 3 ns at pH 3) in an acidic environment. Also, the fluorophore shows fluorescence “On” with acids such as hydrochloric acid and fluorescence “Off” with bases such as triethylamine (TEA). This fluorescence “on-off” behavior is reversible and repeatable, which helps to develop a molecular-level logic gate and sequential memory unit with “Writing-Reading-Erasing-Reading” behavior. Furthermore, solid supportive experiments were carried out by preparing fluorophore-based films and paper strips. Also, the fluorophore demonstrates the practical applicability of potable water samples.