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

I n 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(〈 β 0 〉) in most nano-graphenes except those with high symmetry( e.g. , a C 2 v nano-graphene has octupolar contributions Φ J =3 up to 59.0% of the 〈 β 0 〉). Nano-graphenes containing two parallel orientating azulenes ( i.e. , Out-P and Out-P s ) 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-P s to closed-shell singlet, and the Out-P s -2N has a large 〈 β 0 〉 of 1621.67×10 −30 esu. Further addition of an electron donor(NH 2 ) at the pentagon end enhances the 〈 β 0 〉 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.