Shape Effect of Graphene Quantum Dots on Enhancing Second-Order Nonlinear Optical Response and Spin Multiplicity in NH2–GQD–NO2Systems

Abstract

In the donor–bridge–acceptor [NH2–graphene quantum dot (GQD)–NO2] system, we found that, for GQDs with the same number of carbon atoms, changing the shape of the GQDs not only effectively enhances the first hyperpolarizability (β0) but also changes the spin multiplicity of the systems, which is useful to designing the smaller nanoscale spin and optical device. Here, special shaped species with a singlet diradical or multiradical character exhibit a very large β0 value, up to 1.7 × 105 au. Interestingly, it is found that the large β0 value is not presented in the high-spin trapezoidal 3-1 (3A) and triangle 3-2 (5A), but in the low-spin 3-1 (1A) and 3-2 (1A), which are unusual cases.