Non-symmetric thieno[3,2-b]thiophene-fused BODIPYs: synthesis, spectroscopic properties and providing a functional strategy for NIR probes

Aromatic conjugation at the [β] position is an efficient strategy to construct NIR BODIPY dyes; functionalization of such BODIPYs is difficult because of fewer modifiable positions. The methyl group at the 3,5-position can be versatilely functionalized by the Knoevenagel condensation reaction. In this paper, a small series of non-symmetric thieno[3,2-b]thiophene-fused BODIPYs have been prepared via the condensation of 7H-thieno[2′,3′:4,5]thieno[3,2-b]pyrrole-6-carbaldehydes and various pyrroles, followed by coordination with BF2. The resulting BODIPY dyes exhibited red absorption and emission with extremely high absorption coefficients (2 × 105 M−1 cm−1) and fluorescence quantum yields up to 1.00. Importantly, the remaining methyl group at the 3-position can be further functionalized to achieve NIR probes; we then synthesized an organosilicon-based probe for F− and evaluated its imaging in living cells. The structure–property relationships were interpreted on the basis of X-ray crystallography, optical spectroscopy and theoretical calculations.