Photophysical tuning of fluorinated benzimidazole fluorophores: Donor-acceptor systems and BSA interactions

Benzimidazole is a well-known medicinal scaffold widely present in biologically active heterocycles and therapeutic agents. Natural products, such as zoanthines, demonstrate antimicrobial activity, while synthetic benzimidazoles exhibit antimicrobial, antiparasitic, antiviral, anti-inflammatory, and anticancer activities. In addition, benzimidazole-based fluorophores have been explored as sensors, light-emitting diodes, solvent polarity indicators, and bioimaging agents. Incorporation of fluorine into fluorescent probes enhances their performance by enabling precise tuning of electronic properties, resulting in improved fluorescence efficiency and photostability. It also modulates intramolecular charge transfer (ICT) and solvatochromic behavior, increasing sensitivity to environmental factors such as polarity and hydrogen bonding. Owing to fluorine's ability to tune molecular properties and excellent polarity sensitivity, fluorinated benzimidazole donor-acceptor (D-A) chromophores were developed as environment-responsive probes. Three chromophores: F3Ph, F3Ph-CPr, and F4Ph were synthesized, each incorporating a dimethylamine donor and an electron-deficient imidazole acceptor to facilitate efficient ICT. Among them, F3Ph displayed the highest environmental sensitivity, as evidenced by pronounced polarity-dependent solvatochromism, enhanced fluorescence response, blue-shifted absorption, strong binding affinity, and distinct lifetime changes, supported by DFT analysis. F3Ph was further explored as a microenvironmental probe using bovine serum albumin as a model protein. Spectroscopic studies revealed its strong sensitivity to local polarity and protein-induced environmental changes. Molecular docking and molecular dynamics simulations corroborated these findings, confirming the stability and specificity of F3Ph-BSA interactions, in which the fluorine contributes to the stabilization of the F3Ph-BSA complex. Henceforth, the results establish F3Ph as a robust, highly sensitive fluorophore that would serve as a new tool for monitoring protein, membrane, and nucleic acid microenvironments and polarity variations.