Introduction
Planar chirality is a chirality without chiral centers that can distinguish the two sides of an aromatic ring, and a typical planar chiral molecule is the polysubstituted [2.2]paracyclophane compound.  We have developed novel optical resolution methods for six kinds of di- and tetra-substituted [2.2]paracyclophane isomers and synthesized planar chiral molecules using these isomers as key building units.  In addition, we reported that the planar chiral molecules exhibit circularly polarized luminescence (CPL), which is chiral luminescence.  These are excellent CPL-emitting materials with both high brightness and high anisotropy.
We have recently synthesized a chiral X-shaped molecule consisting of tetrasubstituted [2.2]paracyclophane. When it was crystallized under appropriate conditions, bowl-shaped single crystals with uniform shape and size was formed, which can actually be used as micro-sized vessels to hold liquids; we reported it in Science in 2022.  These single crystals are classified as skeletal crystals, which consist of concave polyhedral as can be seen in snow and Bi crystals.  In this study, electron-withdrawing substituents (trifluoromethyl, cyano, and nitro groups) were introduced into the X-shaped molecule to investigate their optical properties and crystallization behaviors.

Results
The diastereomers were separated by converting one bromo group of 4,7,12,15-tetrabromo[2.2]paracyclophane to a hydroxy group to give a chiral auxiliary group.  The chiral auxiliary was then removed to isolate the optically pure enantiomer, and three types of X-shaped molecules were successfully synthesized by cross-coupling reactions catalyzed by a palladium complex.  Namely, molecules with trifluoromethyl, cyano, and nitro groups were synthesized.  The X-shaped molecules with trifluoromethyl and cyano groups emit fluorescence with high efficiency and CPL with high anisotropy.  The X-shaped molecule with a nitro group showed almost no fluorescence emission.
Single crystals were obtained from all three X-shaped molecules, and the molecular structures were confirmed by X-ray crystallography.  In particular, the X-shaped molecule with a trifluoromethyl group was found to be a single crystal of the same space group as the X-shaped molecule that formed the bowl-shaped skeletal crystal reported previously.  We attempted to crystallize this molecule from saturated ethanol solution on a quartz plate and found that it formed a concave hexahedral bowl-shaped crystal, which is considered to be a micrometer-sized skeletal crystal.

Conclusion
In summary, we succeeded in synthesizing three types of X-shaped molecules with electron-withdrawing groups and elucidating their optical properties.  We attempted to crystallize them and found that bowl-shaped skeleton crystals could be obtained by changing one of the functional groups.  The X-shaped molecule with a trifluoromethyl group has a different luminescence color from the base X-shaped molecule; therefore, we crystallized the X-shaped molecule with a trifluoromethyl group in the bowl-shaped skeleton to make two or more layers of the skeleton, and observed its luminescence behaviors such as energy transfer.  We would also like to try co-crystallization with X-shaped molecules having nitro groups that do not emit luminescence.  We also plan to synthesize and crystallize X-shaped molecules with electron-donating groups in order to investigate the key factors that lead to the formation of skeletal crystals.