Synthesis, crystal structures and spectroscopic characterizations of two difluoroboradiaza-s-indacene dyes

Two new 8-(4-dimethylaminophenyl) substituted boron-dipyrromethene (BDP) dyes: 1,2,6,7-tetraethyl-4,4-difluoro-3,5-dimethyl-8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene (5) and 4,4-difluoro-1,2,3,5,6,7-hexamethyl-8-(4-dimethylaminophenyl)-4-bora-3a,4a-diaza-s-indacene (6) have been synthesized and their crystal structures are determined by X-ray diffraction analysis. The torsion angles of 8-phenyl ring and the indacene plane are 79.61° for 5 and 87.56° for 6, respectively. The absorption and steady-state fluorescence properties in different solvents have been investigated. The fluorescence quantum yields of the two dyes are lower in more polar solvents, which can be ascribed to the intramolecular charge transfer from the aniline to the BDP unit. Protonation of the aniline moiety at low pH in MeOH–H2O (1:1 v/v) solution causes a large fluorescence enhancement. The pKa values calculated from the pH dependent fluorescence emission spectra are 3.14 for 5 and 3.09 for 6, respectively. They are suitable as pH probes excitable with visible light.