Quinodimethanes Incorporated in Non-Benzenoid Aromatic or Antiaromatic Frameworks

Three isomers of quinodimethanes (QDMs) adopt different electronic configurations and geometries, generating their own characteristic physical properties. Incorporation of QDMs into non-benzenoid aromatic or antiaromatic frameworks not only planarizes the whole π system optimizing conjugation, but also changes the electronic properties inherent to QDMs, sometimes drastically, due to the topology of the π system, through interaction with the remaining part of the molecules. In non-benzenoid systems, molecular orbital levels and orbital distribution are uneven compared to benzenoid systems, thereby polarizing the ground state and leading to unique behavior in excited states. In antiaromatic systems, open-shell, diradical character, which is inherent to QDMs, may be enhanced due to small HOMO-LUMO energy gap. In this chapter, effects of incorporating QDMs into non-benzenoid aromatic or antiaromatic frameworks are discussed focusing on the open-shell, diradical character with respect to their molecular structures, antiaromaticity, and physical properties related to the open-shell character and molecular orbital levels and materials applications, as well as covering historical works to the current state-of-the-art achievements.