Dibenzothieno and dibenzothieno[2,3-d]thieno [a]-fused BODIPYs: synthesis, unique structure and photophysical properties†

Abstract

Aromatic annulation on the periphery of BODIPY has proven to be particularly effective in expanding its π-conjugated system, thereby enabling near-infrared (NIR) absorption and emission. However, achieving this shift in BODIPY's absorption and emission to the NIR region without compromising fluorescence quantum yield remains a significant challenge. Herein, we report a new family of symmetrical dibenzothieno and dibenzothieno[2,3-d]thieno[a]-fused BODIPYs (5a–c and 8a–c) synthesized via a simple four-step route involving regioselective 1,7-bromination to selectively enhance the reactivity specifically at the 1,7 positions. These aromatic heterocyclic [a]-fused BODIPYs dyes exhibit “butterfly-shaped” crystal structures, with a tunable dihedral angle between the indacene plane and its “wings” ranging from 2.47° to 11.49°. The symmetrical [a]-fusion of dibenzothieno and dibenzothieno[2,3-d]thieno rings results in NIR absorption and emission with high fluorescence quantum yields (Φ_F > 0.8), and effectively narrows the HOMO–LUMO gap by elevating the energy level of the HOMO. The high fluorescence quantum yields and tunable HOMO levels of these aromatic heterocyclic [a]-fused BODIPY dyes make them highly promising as functional chromophores, particularly in the field of bioimaging. This work offers valuable insights for developing NIR fluorescent dyes with high fluorescence quantum yields for use as photosensitizers.