ABSTRACT
Low-dimensional nanographenes such as graphene nano-akes (GNFs) are known to display the unique spin states with nonbonding molecular orbitals localized on their zigzag edges, that is, “open-shell singlet” electronic state. The open-shell singlet nature presents a key concept for designing a new class of functional materials such as highly efcient nonlinear optical (NLO) and singlet ssion materials. As an example, we have theoretically investigated the open-shell natures of GNFs with various architectures and sizes using the diradical characters, dened by quantum chemical calculations, and have revealed several structure-open-shell property relationships for the GNFs. It has been found that several zigzag-edged GNFs exhibit intermediate and large diradical characters, while GNFs having only armchair edges are closed-shell systems. Also, large GNFs are shown to display multiradical characters beyond the diradical as increasing the zigzag-edge lengths. We have further found unique structural dependences of multiradical characters in antidot hexagonal GNFs and linear GNFs composed of trigonal fused-ring units. These GNFs are expected to be possible candidates for highly efcient and tunable open-shell singlet NLO materials, which exhibit strong diradical character dependences of NLO properties and give large NLO properties as compared to conventional closed-shell NLO systems. The mechanism and rational design principle of various GNFs for highly ef- cient NLO materials are presented in this chapter.
