Asymmetric restriction of intramolecular rotation in chiral solvents

Commercially available molecular rotor (MR) compounds were recrystallized using chiral monoterpenes as solvents. The resulting crystals exhibited large circular dichroism signals with opposing signs according to the handedness of the chiral solvent used. X-ray crystallographic analysis showed that the chirality originated from asymmetric restriction of intramolecular rotation in the crystals. The crystals were also highly emissive due to restricted bond rotation, while solutions of the materials were almost nonemissive. The solvent-to-MR chirality transfer approach to crystallization discussed herein should be a convenient, universal way to obtain highly emissive chiral crystals.