Chiral Cycloparaarylenes Incorporating Binaphthyl Units: Dramatic Effects of Peripheral Functionalization on (Chir)Optical Properties.

Abstract

Axially chiral [12]cycloparaarylenes 1 and 2, consisting of three each of 1,1'-binaphthyl and biphenyl units, were synthesized. Their characteristics were distinct from those of [12]cycloparaphenylene and depended on whether the 2,2'-substituents on the binaphthyls were methoxy groups or a methylenedioxy bridge. 1 with methoxy groups adopted a D3-symmetric structure with transoid binaphthyls, whereas 2 with methylenedioxy groups existed as an equilibrium mixture of C3- and C1-symmetric structures with cisoid binaphthyls. This difference arises from modulation in binaphthyl dihedral angles and the intrinsic preference of cycloparaarylenes (CPAs) for cylindrical arene arrangements. 1 displayed strong aggregation-induced emission enhancement (AIEE) with Φfl of 0.85 at 5°C and blue emission in the solid state. 2 showed weaker but more redshifted AIEE and solid-state green emission. Trifluoroacetic acid molecules stabilized the highly symmetric conformations of 1 in the excited state, leading to a ninefold enhancement in fluorescence (FL) and intense circularly polarized luminescence (CPL) with |glum| of 0.028 and BCPL of 820 M-1 cm-1 at -60°C.