[5]Helicene-based chiral triarylboranes with large luminescence dissymmetry factors over a 10-2 level: synthesis and design strategy via isomeric tuning of steric substitutions.

ABSTRACT

Constructing chiral luminescent systems with both large luminescence dissymmetry factor (glum) and high luminous efficiency has been considered a great challenge. We herein describe a highly efficient approach to sterically stabilize the helical configurations of carbo[5]helicenes for improved CPL properties in a series of π-donor and π-acceptor substituted [5]helicenes (1, 2, 3, 4 and 5). Enabled by the ortho-installation of methyl groups as well as the steric effects of triarylamine (Ar3N) and triarylborane (Ar3B) handles in meta-substituted [5]helicenes, their optical resolution into enantiomers has been accomplished using preparative chiral HPLC. The molecular chirality of [5]helicenes can be transferred to Ar3B and Ar3N as light emitters, which allowed further investigations of their chiroptics, including optical rotation, circular dichroism (CD) and circularly polarized luminescence (CPL). Remarkably, 4 has been demonstrated to display dramatically enhanced CPL performance with a much larger glum (>1.2 × 10-2) and an increased emission quantum efficiency (ΦS = 0.75) compared with the other analogues, as a result of the isomeric tuning of substitutions with differential steric and electronic effects. These experimentally observed CPL activities were rationalized by TD-DFT computations for the angle (θµ,m) between electric and magnetic transition dipole moments in the excited states. In addition, the conspicuous intramolecular donor-acceptor charge transfer led to thermal responses in the emissions of 2 and 4 over a broad temperature range.