Radical Cation Salts of Hetera‐Buckybowls: Polar Crystals, Negative Thermal Expansion and Phase Transition

Radical cation salts of π‐conjugated polycycles are rich in physical properties. Herein, two kinds of hetera‐buckybowls, ethoxy‐substituted trithiasumanene (3SEt) and triselenasumanene (3SeEt), are synthesized as electron donors. Galvanostatic oxidation of them affords radical cation salts (3SEt)5(TTFMPB)3, (3SeEt)5(TTFMPB)3, (3SEt)4PMA, and (3SeEt)4PMA, where PMA is Keggin‐type phosphomolybdate and TTFMPB is tetrakis[3,5‐bis(trifluoromethyl)‐phenyl]borate. In these salts, 3SEt/3SeEt are partially charged and show distinct conformation change with the site charge and counter anions. In TTFMPB salts, (TTFMPB)− forms hexagonal channels that accommodate the packing columns of 3SEt/3SeEt. In particular, (3SEt)5(TTFMPB)3 adopts the R3c space group and is a polar crystal with the columns of 3SEt all in the up‐bowl direction. The PMA salts of 3SEt/3SeEt are polar crystals (C2 space group) with 3SEt/3SeEt being planar and forming columnar stacks. (3SeEt)4PMA shows a structural modulation below 200 K, namely, negative thermal expansion (NTE) of the unit cell volume and enlargement of the intermolecular distances between neighboring 3SeEt molecules. The four salts are semiconductors with an activation energy of 0.18–0.38 eV. The conductivity of (3SeEt)4PMA shows a reversible transition upon cooling and heating, in accordance to the NTE structural modulation. This work paves the way toward conducting materials based on hetera‐buckybowls. The electrocrystallization of hetera‐buckybowl trichalcogenasumanenes (TCSs) has afforded radical cation salts with excellent stability under ambient conditions. The resulting salts are mostly polar crystals, where the TCSs are partially charged and form columnar stacks. A reversible semiconductor‐to‐semiconductor phase transition is observed, which is caused by a negative thermal expansion of the unit cell.