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 pi-conjugations are very weak, as the methylene bridges disrupt the electronic communication between pi 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 sigma*-pi* conjugation between sigma* (Si-C) orbitals and pi* 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.