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 (λabs = 666 nm, λem = 687 nm), 4-H⁻ (λabs = 730 nm, non-fluorescent), and 4²⁻ (λabs = 769 nm, λem = 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 (λabs/λem = 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 (λabs = 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.