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

Aromatic conjugation at the [beta] 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 x 10(5) 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.