Highly Fluorescent Dianionic Polymethines with a 1,3,2-Dioxaborine Core

The difluoroboron β-diketonate ring is ever more used for creating bright polymethine-type fluorophores for the visible and NIR range. Here, we report the synthesis and spectral properties of a series of dianionic cyanine dyes of the rare A¹-π-A-π-A¹ type, with the central dioxaborine ring (A) embedded into the polymethine chain and various electron-acceptor terminal groups A¹. Depending on the nature of the end group, the maxima of their intensive (with molar extinctions up to 380 000 M^-1 cm^-1) and narrow long-wavelength absorption band lie in the range of 530-770 nm. Their absorption and fluorescence bands are nearly mirror-like and characterized by weak solvatochromism; the marked hypsochromic shifts are observed only when going from polar aprotic solvents to methanol. The designed dianionic dyes have fluorescence quantum yields up to 92 % in the visible range, and even for the NIR dyes, the values of 18-37 % are observed in DMF.