Electron localization engineering to construct Cu single-atom bridges between g-C3N4 layers for regulating third-order nonlinear optical properties

The weak van der Waals force between 2D g-C₃N₄ layers suppresses the charge transfer between layers and hinders the application development in nonlinear optics (NLO) field. Herein, the covalently functionalized g-C₃N₄ is wet-impregnated into CuCl₂ solution through a post-synthesis strategy, and the engineering of Cu single atoms bridging the 2D layers is achieved through coordination effect of Cu-N. The conjugated modification of 8-aminoquinoline (8-AQ) within layers and the synergistic effect of interlayer Cu single atom bridges induces electron cross plane delocalization, thereby stimulating changes in the higher-order polarizability of medium. The third-order NLO absorption behavior of g-C₃N₄-8AQ transitions from saturated absorption (SA) of g-C₃N₄ to reverse saturated absorption (RSA), and g-C₃N₄-8AQ-Cu to exhibits a more significant RSA signal (with a β_eff of 4.0 × 10⁻¹⁰ m W⁻¹). It is notable that the third-order NLO refractive behavior undergoes a striking transition from self-focusing (g-C₃N₄ with n₂ of 1.3 × 10⁻¹⁷ m² W⁻¹ and g-C₃N₄-8AQ-18 with n₂ of 3.1 × 10⁻¹⁷ m² W⁻¹) to self-defocusing (g-C₃N₄-8AQ-18-Cu with n₂ of −3.3 × 10⁻¹⁷ m² W⁻¹). This work realizes the cross-layer transfer of charges in g-C₃N₄, thereby distorting the electron cloud and making it potentially promising in light-limiting applications as well.