1,3,2-Dioxaborine-based mero-polyanionic dyes: high-extinction NIR-absorbing polymethines

Polymethine dyes incorporating multiple meso-cyano-2,2-difluoro-1,3,2-dioxaborine units as integral components of the π-chain and 1,3,3-trimethyl-2H-indole moieties as donor end-groups were synthesized and characterized. The obtained dyes exhibit an unusual electronic structure that combines merocyanine-like (D-π-A) and polyanionic ([A-π-A'] _n ) π-conjugated systems within a single mero-polyanionic chromophore, realized in both symmetric (D-π-A-π[-A-π] _n -D) and unsymmetric (D-π[-A-π] _m -A) architectures. The molecular structures of representative monoanionic dyes from both series were elucidated by X-ray analysis, suggesting a non-uniform distribution of the electron density along the π-conjugated system. These dyes exhibit NIR absorption extending up to 940 nm, with remarkably high molar absorption coefficients of up to 468 000 M^-1 cm^-1 in DMF. The position of the long-wavelength absorption maximum shows only weak solvent dependence. The shortest-wavelength oligomers of the studied dyes are strongly fluorescent compounds, with quantum yields of 0.10-0.16 for emission around at 800 nm, and display photostability comparable to that of classical cyanine dyes such as indotricarbocyanine iodide (HITC) and indocyanine green (ICG).