Trianionic 1,3,2-Dioxaborine-Containing Polymethines: Bright Near-Infrared Fluorophores

Trianionic polymethines of the A'-π-A-π-A-π-A' type comprising dioxaborine rings (A) and different electron-accepting end groups (A') have been synthesized. The obtained dyes absorb and emit light in the near-infrared region with remarkably high molar attenuation coefficients (ϵ up to 495 000 M^-1  cm^-1 in DMF) and fluorescence quantum yields (Φ_f up to 0.73 in DMF). Thus, the novel trianionic dyes stand among the brightest individual fluorophores known to date - with a magnitude of fluorescence brightness (ϵ⋅Φ_f ) of 313 000 M^-1  cm^-1 in DMF. The synthesized dyes demonstrate a minor negative solvatochromism and small Stokes shifts. X-ray data reveal the nearly planar geometry of the trianionic chromophore. All the obtained compounds are stable in the solid state and in a solution, although the relative stability is much higher in polar aprotic than in protic solvents.