Ultrafast charge transfer in amino-substituted boron dipyrromethene dyes and its inhibition by cation complexation: A new design concept for highly sensitive fluorescent probes

The photophysical behavior of a newly synthesized aza crown-substituted boron-dipyrromethene (BDP) dye and its dimethylamino analogue were investigated with steady-state and time-resolved fluorometry and compared to a reference compound. In solvents more polar than hexane, excitation of the dyes leads to a fast charge transfer from the locally excited (LE) state to a weakly emissive charge-transfer (CT) state. The donor-substituted compounds show dual emission from the LE and CT state, both fluorescence quantum yields being low. The rate constant of excited-stare charge separation, calculated from the global analysis of time-resolved emission data, was determined to 1.6 x 10(11) s(-1) in 1,4-dioxane. The crowned compound forms 1:1-complexes with various alkali and alkaline-earth metal ions, which exist as two conformers in solution. In these complexes, coordination of the cation to the nitrogen donor atom of the crown inhibits the charge-transfer process, leading to a cation-dependent enhancement of the LE emission and the fluorescence lifetimes by factors >10(3). This efficient "switching on" of the fluorescence renders the crowned BDP dye an extremely sensitive fluorescent probe for metal ions.