Utilization of Hypervalent Bonds as a Novel Strategy for Manipulating Energy Levels of π‐Conjugated Systems

Abstract The unique optical properties created by the combination of azobenzene‐based π‐conjugated systems and hypervalent bonds are summarized. The hypervalent bond, a polarized three‐center four‐electron (3c−4e) bond, simultaneously provides electron‐donating and ‐withdrawing effects to the directly attached π‐conjugated system. The resulting hypervalent compounds have a dramatically narrow energy gap showing near‐infrared (NIR) emission despite the limited size of π‐conjugated systems. In addition, the hypervalent compounds also exhibit stimuli‐responsive chromism through the reversible coordination‐number shift of the central main‐group elements in both crystals and π‐conjugated polymer films. We also explain that the nature of hypervalent bonds depends on the element type, which is particularly effective in bringing out the properties of heavy elements. In this review, we describe that the use of hypervalent bonds is a novel strategy not only for manipulating the energy levels of the π‐conjugated system but also for developing a variety of functional materials based on the newly rising properties from hypervalent element.