Impact of Diradical/Ionic Character on Third‐Order Nonlinear Optical Property in Asymmetric Phenalenyl Dimers

Abstract

Abstract The dissociation behavior of the asymmetric hetero‐phenalenyl dimers is theoretically investigated from the viewpoint of the open‐shell singlet third‐order nonlinear optical (NLO) systems. It is found that the second hyperpolarizability γ of all the investigated phenalenyl dimers exhibit bell‐shape behavior with maximum at the intermediate dissociation region, and that the γ value of the most effective asymmetric dimer reaches about 20 times the maximum of the original phenalenyl dimer. This behavior is clarified by the new definition of diradical/ionic characters, identifying that the significant enhancement of γ occurs in the intermediate ionic region as well as in the intermediate diradical region. The present result extends the concept of diradical character based design of NLO molecules toward ionic closed‐shell region in a realistic molecular framework, and demonstrates the intermediate ionic systems as a new target in the NLO molecules with open‐shell molecular frameworks.