Tetraanionic Oligo-Dioxaborines: Strongly Absorbing Near-Infrared Dyes

Polymethine dyes of tetraanionic nature comprising 1,3,2-dioxaborine rings in the polymethine chain and end-groups of different electron-accepting abilities have been synthesized. They can be considered as oligomeric polymethines, where a linear conjugated π-system passes through three 1,3,2-dioxaborine units and a number of tri- and dimethine π-bridges between two end-groups. The obtained dyes exhibit near-infrared absorption and fluorescence, with molar absorption coefficients reaching as high as 564000 M^-1 cm^-1 in DMF, rendering them among the strongest absorbers known. The novel compounds are bright NIR fluorophores, with fluorescence quantum yields up to 0.13 in DMF. A comparative analysis of the electronic structure of the obtained dyes with respective dianionic and trianionic oligomers was conducted through quantum chemical calculations.