Theoretical Studies on the Electronic Structure of Nano-graphenes for Applications in Nonlinear Optics

Abstract

In this work, azulene is introduced into nano-graphene with coronene center to enhance the second-order nonlinear optical (NLO) properties. The sum-over-states(SOS) model based calculations demonstrate that dipolar contributions are larger than octupolar contributions to the static first hyperpolarizability(〈β0〉) in most nano-graphenes except those with high symmetry(e.g., a C2v nano-graphene has octupolar contributions ΦJ=3 up to 59.0% of the 〈β0〉). Nano-graphenes containing two parallel orientating azulenes (i.e., Out-P and Out-Ps) have large dipole moments, while their ground state is triplet. Introducing B/N/BN atoms into the positions with a high spin density transfers the ground state of Out-P and Out-Ps to closed-shell singlet, and the Out-Ps-2N has a large 〈β0〉 of 1621.67×10−30 esu. Further addition of an electron donor(NH2) at the pentagon end enhances the 〈β0〉 to 1906.22×10−30 esu. The two-dimensional second-order NLO spectra predicted by using the SOS model find strong sum frequency generations and difference frequency generations, especially in the near-infrared and visible regions. The strategies to stabilize the electronic structure and improve the NLO properties of azulene-defect carbon nanomaterials are proposed, and those strategies to engineer nano-graphenes to be semiconducting while maintaining the π-framework are extendable to other similar systems.