Periphery engineering to enhance the chiroptical response of β-isoindigo-based aza-BODIPY analogs

Helicene-type compounds possess appealing chiroptical features, but most helicenes typically show chiroptical response primarily in the ultraviolet region. In our earlier research, we developed a series of helical β-isoindigo-based B–O–B bridged aza-BODIPY analogs (BIABs), which exhibited strong absorbing/emission at the red/NIR region. However, BIABs with less π-extension had low Cotton effect (Δε = 1.3 M⁻¹cm⁻¹) and absorbance dissymmetry factor (gₐbₛ = 3 × 10⁻⁵) values. Herein, we reported a simple strategy to boost chiroptical response by introduction of alkyl chain (methyl or ethyl) at the periphery of the BIAB core. The resulting BIAB derivative displayed strong Cotton effect (Δε = 49 M⁻¹cm⁻¹) and absorbance dissymmetry factor (gₐbₛ = 1.69 × 10⁻³), resulting in 38 and 56-fold enhancement compared with BIAB core, respectively. This strategy is expected to offer a new opportunity to boost chiroptical response.