Branched Alkyl Chain-Guided Modulation of BODIPY Packing Polymorphism for Engineering Thin-Film J-Aggregates

Systematic modulation of the volume fractions between the boron dipyrrin (BODIPY) unit and the tethered 3,4,5-tri(alkoxy)phenyl group was explored to establish a rational crystal-engineering strategy toward thin-film J-aggregate formation. We propose that the branched alkyl chains play two distinct roles: (i) inducing steric hindrance that enforces a large offset in the cofacial arrangement of chromophore units and (ii) imparting substantial conformational entropy that increases nucleation probability and relieves packing strain, thus promoting the tessellation of BODIPYs within crystalline thin films. In this proof-of-concept study, 3,4,5-tri(isopentyloxy)phenyl and 3,4,5-tris((<i>S</i>)-dihydrocitronellyloxy)phenyl groups were introduced as the branched alkyl chains to the BODIPY skeleton. The BODIPY core was further fine-tuned by introducing iodine and nitrile substituents. Eventually, the formation of a J-aggregate exhibiting exceptionally sharp absorption and fluorescence bands was achieved in a spin-coated film.