External electric field driven electronic structures: tunable nonlinear optical properties of helical bilayer nanographenes

Abstract

Helical bilayer nanographenes (HBNGs) possess a unique structure and versatile properties, which are of great interest in experimental and theoretical research. This study applied an external electric field to regulate the electronic properties of the HBNG with a helicene moiety length of [9] ([9]HBNG). The direction of the external electric field (F_z) is the positive z-axis along the nanographenes. Intriguingly, the interplanar angle of [9]HBNG is regulated under the F_z. Furthermore, molecular electrostatic potential (ESP) maps demonstrate that the F_z regulates charge distribution effectively. Notably, when F_z = 100 × 10⁻⁴ a.u., the highest occupied molecular orbital (HOMO) is primarily located on the lower nanographene, while the lowest unoccupied molecular orbital (LUMO) is concentrated on the upper nanographene. Remarkably, the F_z induces a substantial increase in the β_tot values, ranging from 6.53 × 10² (F_z = 0 a.u.) to 1.26 × 10⁴ a.u. (F_z = 100 × 10⁻⁴ a.u.). This study presents an efficient approach for developing high-performance nonlinear optical (NLO) materials through the application of external electric fields, with potential applications in NLO switch devices.