Water-soluble 4-hydroxystyryl and 4-hydroxyphenyl-butadienyls dyes with switchable fluorescence

Fluorescent 4-hydroxystyryl dyes are useful for many biomedical and pharmaceutical assays, and other analyte sensing applications. However, these dyes often suffer from limited applicability in analytical and bioanalytical utilization due to insufficient water-solubility, high acid-ionization constants (pK_a) and short-wavelength absorption and emission. To solve these issues, a series of new, water-soluble 4-hydroxystyryl and longer-wavelength 4-hydroxyphenyl-butadienyl dyes were synthesized and the spectral and protolytic properties were studied. These new dyes contain electron-withdrawing substituents ortho to the triggering 4-hydroxyl group. The introduction of the cyano and formyl groups was found to decrease the pK_a and extend the pH-sensing region of these fluorophores. In respect of the molecular structure, these dyes exhibit either a “turn-on” activatable fluorescence, or a dual-fluorescence that enables ratiometric measurements. The 4-hydroxyphenyl-butadienyl dye was evaluated for fluorescence monitoring of drug delivery. These new dyes are promising fluorophores for acidity measurements and other sensing applications.