Bright far-red emitting BODIPYs via extension with conjugated fluorene and carbazole motifs

Six new BODIPY dyes have been synthesized exhibiting fluorescence emission in the far red part of the electromagnetic spectrum. The compounds were prepared via an acid catalyzed reaction of rigidified pyrroles with benzaldehyde derivatives. This reaction tolerates a variety of aldehydes and therefore analyte responsive parts can be easily introduced. As an example, indicators with a crown ether receptor, sensitive for potassium, and a pH sensitive group for the determination of CO₂, were obtained via modification of a BODIPY bearing a carbazole motif. Apart from the bathochromic shift of the absorption and emission spectra (about 130–200 nm compared to 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene) most of the new dyes possess excellent photophysical properties, such as high molar absorption coefficients (130 000–230 000 M⁻¹ cm⁻¹), sharp absorption and emission bands as well as high quantum yields (0.48–0.95). In case of the potassium fluoroionophore both fluorescence intensity and decay time are enhanced in presence of the analyte making it potentially suitable for the self-referenced read-out.