Sulfur-bridged annulated BODIPYs: synthesis, structure and photophysical properties

Abstract

In this work, we present the synthesis of a novel class of sulfur-bridged ring-fused BODIPY dyes (2a and 2b) and the investigation of their photophysical properties. The introduction of a unilateral sulphur-bridged ring fusion at the zig-zag site of the BODIPY core results in redshifts in absorption and emission. Furthermore, incorporation of a sulfur-bridged ring substantially enhances both the fluorescence quantum yield (ΦF) and fluorescence lifetime (τ) compared to the unfused precursors (compounds 3 and 4), suggesting that the sulfur-bridged ring increases the molecular rigidity and suppresses non-radiative transition pathways. Theoretical calculations demonstrate that the sulfur-bridged ring effectively narrows the HOMO-LUMO energy gap and significantly improves the electronic delocalization of the molecule, which in turn profoundly affects its optoelectronic properties. Single-crystal X-ray diffraction analysis further confirms that the formation of the sulfur-bridged ring limits the conformational freedom of meso-phenyl group, enhances molecular rigidity, and promotes π-electron delocalization.