Tunable Electronic Effects in Indole–Boron Difluoride Fluorochromes: Hammett Correlations, TD-DFT Insights, and Hypochlorous Acid Detection in Live Cells

The 6-substituted indole-boron difluoride fluorochromes bearing electronically modulating substituents were synthesized and fully characterized via NMR and HRMS analyses. These complexes exhibited remarkable photophysical properties, including substantial quantum yields and large Stokes shifts. Systematic quantitative analyses using Hammett constants (<i>σ</i>_p) revealed strong correlations between substituent electronic properties and photophysical parameters such as ¹⁹F NMR chemical shifts, absorption maxima (λ_max^abs), and emission maxima (λ^em). Notably, an indole-boron-difluoride complex bearing a nitro group exhibited significant solvatochromic behavior, attributable to pronounced intramolecular charge transfer (ICT), as confirmed by Lippert-Mataga analysis. Time-dependent density functional theory (TD-DFT) calculations were consistent with experimental findings and provided detailed insight into the electronic transitions and spectroscopic properties of the complexes. Furthermore, the developed fluorescent probe bearing an oxidizable catechol moiety demonstrated selective turn-on fluorescence toward hypochlorous acid (HOCl) and successfully visualized endogenous HOCl generation in living HeLa cells, indicating its promising utility as a selective HOCl-sensing probe.