Supramolecular Chiral J-Aggregates of an Amphiphilic Boron-Azadipyrromethene Dye: Kinetic Pathway and Circularly Polarized Near-Infrared Luminescence

Abstract

An amphiphilic boron-azadipyrromethene (aza-BODIPY) dye (R)-1 bearing two chiral hydrophobic side chains was synthesized, and its supramolecular chiral assembly properties were investigated in detail. The UV/vis absorption, emission, and CD spectroscopic studies indicated the pathway complexity in the aggregation process of dye (R)-1. In mixed solvent of MeOH/H₂O, the dye (R)-1 first formed kinetically favored metastable aggregates (Agg. I) upon cooling of the solution, which could slowly transform into the thermodynamically more stable aggregates, i.e., Agg. II by keeping the solution of Agg. I at appropriate temperature. Further kinetic analysis of the Agg. I to Agg. II transformation indicated that the Agg. I was an off-pathway aggregate species. The Agg. I and Agg. II displayed distinct spectral and morphological characteristics. While a broad absorption band between 656 and 707 nm was observed for the Agg. I, the Agg. II exhibited an extremely narrowed absorption band at 799 nm and an emission band at 804 nm for J-aggregates. Moreover, the AFM and TEM revealed that Agg. I was spherical nanoparticles and Agg. II had a morphology of helical nanoribbons. In further chiroptical studies, the thin film of J-aggregated dye (R)-1 exhibited circularly polarized luminescence with an emission maximum at 804 nm and g-factor of −4.7 × 10^–2.