Bridge-induced taming of the visible electronic circular dichroism signatures of helicoBODIPYs

Helically-flexible bis(BODIPY)s based on chiral ethane-1,2-diamine or -1,2-diol bridges (named helicoBODIPYs) are an interesting family of accessible and tunable small multichromophoric organic dyes enabling visible electronic circular dichroism (ECD) and, even visible circularly polarized luminescence (CPL). However, their CPL brightness (BCPL) is still limited (up to ca. 10 M-1cm- 1), due to low fluorescence efficiency owing to a non-optimal participation of intramolecular charge transfer (CT) emission. Herein, we analyze in depth the origin and factors controlling this CT, demonstrating the key participation of a BODIPY-to-BODIPY symmetry-breaking induced CT (SBCT), beyond the expected CT within each single BODIPY chromophore due to their push-pull character. It is also demonstrated that increasing the overcrowding at the flexible helicoBODIPY ethane bridges leads to BODIPY-BODIPY faced dispositions, promoting the SBCT and enabling the detection of both CT absorption and CT emission bands under specific conditions. Interestingly, this SBCT enhances the visible ECD (absolute gabs values), supporting the key role of the involved forbidden CT transitions in the chiroptical activity of these dyes, as well as the possibility of modulating their valuable visible chiroptical signatures (e.g., visible CPL brightness) by easily playing with steric factors at the dye bridge.