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

Abstract

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).