Ratiometric, fluorescent BODIPY dye with aza crown ether functionality: Synthesis, solvatochromism, and metal ion complex formation

A new pH and metal ion-responsive BODIPY-based fluorescent probe with an aza crown ether subunit has been synthesized via condensation of 4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-benzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted boron dipyrromethene moiety. Steady-state and time-resolved fluorometries have been used to study the spectroscopic and photophysical characteristics of this probe in various solvents. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalan solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. Fluorescence decay profiles of the dye can be described by a single-exponential fit in nonprotic solvents, whereas two decay times are found in alcohols. Protonation as well as complex formation with several metal ions are investigated in acetonitrile as solvent via fluorometric titrations. The aza crown ether dye undergoes a reversible (de)protonation reaction (pK_a = 0.09) and shows a ∼50 nm blue shift in the excitation spectra and a 10-fold fluorescence increase upon protonation. The compound also forms 1:1 complexes with several metal ions (Li⁺, Na⁺, Mg²⁺, Ca²⁺, Ba ²⁺, Zn²⁺), producing large blue shifts in the excitation spectra and significant cation-induced fluorescence amplifications.