Single‐crystal structures and photophysical properties of brucine complexes with axially chiral biphenyl derivatives

Biphenyl derivatives that are substituted at the 2,2',4,4',6,6'‐positions exhibit axial chirality due to restricted rotation around the central bond, rendering them crucial chiral sources for various functional materials. The present study reports the optical resolution and crystal structure determination of the brucine complexes ( R )‐ 1 •brucine and ( S )‐ 1 •brucine, and the spectroscopic characterization of ( R )‐ 1 •brucine, ( S )‐ 1 •brucine, ( R )‐ 1 , and ( S )‐ 1 . The optical resolution was achieved through the diastereomeric complexation of rac ‐ 1 with brucine, and the absolute configurations of ( R )‐ 1 •brucine and ( S )‐ 1 •brucine were confirmed via single‐crystal X‐ray diffraction analysis. In the complexes, the brucine molecule was tightly bonded to ( R )‐ 1 and ( S )‐ 1 by the ionic intermolecular hydrogen‐bonding interactions. The additional intermolecular CH–π interaction is exclusive to be present between the brucine CH and the aromatic ring of ( R )‐ 1 , which most likely facilitates the crystallization of ( R )‐ 1 •brucine over ( S )‐ 1 •brucine, leading to the efficient optical resolution. The use of UV–vis absorption, fluorescence, and circular dichroism (CD) spectroscopy resulted in distinct spectral signatures arising from a difference in the structures of the intermolecular hydrogen‐bonding interactions in these complexes. Time‐dependent density functional theory (TD‐DFT) calculations employing chloroform solvation models (CPCM) successfully reproduced experimental absorption spectroscopies, thereby demonstrating a charge‐transfer HOMO‐LUMO electron transition from the brucine unit to the biphenyl unit.