Propeller Chirality of Boron Heptaaryldipyrromethene: Unprecedented Supramolecular Dimerization and Chiroptical Properties

Chiral boron dipyrromethenes (BPs) enjoy high fluorescence efficiency at visible to near-IR wavelength regions with a reasonable range of dissymmetry factors. Here, we demonstrate that the (quasi)­propeller chirality, similarly to hexagonal propeller in hexaarylbenzene, can be effectively induced in heptaarylated BP. In addition, supramolecular dimer was formed at low temperatures in nonpolar solvent, which exhibits strong bisignate Cotton effects at BP transitions (the couplet amplitude A = 193 M^–1 cm^–1) in the circular dichroism (CD). Due to the bulky substituents on the propeller blades, but with void space around boron atoms, BP chromophores in the dimer are aligned in a head-to-tail manner with a small torsion (φ ≈ 15°), to avoid fluorescence quenching usually observed in H-type dimer of BPs, exhibiting strong circularly polarized luminescence (CPL) signals (g_lum = 2.0 × 10^–3, Φ_lum = 0.45). Such supramolecular dimer formation would be viewed as an alternative approach for designing and developing novel chiroptical materials.