Synthesis and characterization of a nitrophenol disubstituted BODIPY halochromic probe

We report the synthesis and characterization of a (3,5)-nitrophenol substituted BODIPY dye bearing a clickable handle. The dye was initially studied in dichloromethane, where it absorbs and emits in the red (666 nm and 687 nm respectively). Stepwise deprotonation of the phenols was achieved by addition of an organic base giving three species: 4-H₂ (λ_abs = 666 nm, λ_em = 687 nm), 4-H⁻ (λ_abs = 730 nm, non-fluorescent), and 4²⁻ (λ_abs = 769 nm, λ_em = 844 nm), enabling an on-off-on pH sensing behavior Spectral deconvolution, DFT and TDDFT studies were carried out to support the experimental observations. Solvatochromic studies in 19 solvents show four distinct behaviors: J-aggregate formation in apolar alkanes (λ_abs/λ_em = 749 nm), conventional BODIPY response in low-polarity solvents, coexistence of diphenol and mono-deprotonated forms in moderately polar solvents, and dominance of the phenolate in polar solvents (λ_abs = 831 nm in N, N-dimethylformamide). Analysis using the Catalan scale indicates deprotonation is promoted by solvent dipolarity, acidity, and basicity, while the diphenol form is mainly sensitive to polarizability. These properties make such BODIPY a versatile probe for pH and microenvironment sensing, with potential applications in photoacoustic imaging and cellular studies.