Near-infrared fluorescein dyes containing a tricoordinate boron atom

Bora-fluoresceins ( BF s), fluorescein analogues containing a tricoordinate boron atom instead of an oxygen atom at the 10-position of the fluorescein skeleton, were synthesized as a new family of fluorescein analogues. The deprotonated BF s exhibited absorption and fluorescence in the near-infrared region, which were significantly red-shifted relative to those of hitherto-known heteroatom-substituted fluorescein analogues on account of the orbital interaction between the tricoordinate boron atom and the fluorescein skeleton. BF s also showed multi-stage changes resulting from a Lewis acid-base equilibrium at the boron center in combination with a Brønsted acid-base equilibrium at the phenolic hydroxy group. Tricoordinate boron imparts near-infrared absorption/emission and unusual multi-stage changes in the photophysical properties to fluorescein dyes.