Collective In-Plane Molecular Rotator Based on Dibromoiodomesitylene π‑Stacks

Interest
in artificial solid-state molecular rotator systems is growing as
they enable systems to be designed for achieving specific physical
functions. The phase transition behavior of four halomesitylene crystals
indicated dynamic in-plane molecular rotator characteristics in dibromoiodomesitylene,
tribromomesitylene, and dibromomesitylene crystals. Such molecular
rotation in diiodomesitylene crystals was suppressed by effective
I···I intermolecular interactions. The in-plane molecular
rotation accompanied by a change in dipole moment resulted in dielectric
phase transitions in polar dibromoiodomesitylene and dibromomesitylene
crystals. No dielectric anomaly was observed for the in-plane molecular
rotation of tribromomesitylene in the absence of this dipole moment
change. Typical antiferroelectric–paraelectric phase transitions
were observed in the dibromomesitylene crystal, whereas the dielectric
anomaly of dibromoiodomesitylene crystals was associated with the
collective in-plane molecular rotation of polar π-molecules
in the π-stack. We found that the single-rope-like collective
in-plane molecular rotator was dominated by intermolecular I···I
interactions along the π-stacking column of polar dibromoiodomesitylene.