Silapillar[ n ]arenes: Their Enhanced Electronic Conjugation and Conformational Versatility.

During recent decades, methylene-bridged macrocyclic arenes have been widely used in supramolecular chemistry. However, their π-conjugations are very weak, as the methylene bridges disrupt the electronic communication between π orbitals of the aromatic units. Herein, we successfully synthesized a series of silapillar[ n ]arenes ( n = 4, 6, and 8) using silylene bridging. These showed enhanced electronic conjugation compared with the parent pillar[ n ]arenes because of σ*-π* conjugation between σ* (Si-C) orbitals and π* orbitals of the benzenes. Owing to the longer Si-C bond compared with the C-C bond, silylene-bridging provides additional structural flexibility into the pillar[ n ]arene scaffolds; a strained silapillar[4]arene was formed, which is unavailable in the parent pillar[ n ]arenes because of the steric requirements. Furthermore, silapillar[ n ]arenes displayed interesting size-dependent structural and optical properties.