Improving the optical nonlinearity of covalent organic frameworks through spatial electron transport channels within the pore environment.

The influence of spatial electron transport channels in covalent organic frameworks (COFs) on third-order nonlinear optical (NLO) performance remains largely unknown. Herein, we present a method for preparing host-guest (H-G) COF composites with diverse pore environments to investigate their NLO performance. Pt nanoparticles (NPs) were selected as guests and confined into two novel azo/ethylene-decorated highly crystalline Azo-COF and Et-COF. The synthesized Pt NPs@Azo-COF and Pt NPs@Et-COF exhibit better NLO performance than Azo-COF and Et-COF under laser irradiation in the near-infrared to the visible range. In the near-infrared range (1064 nm), the reverse saturable absorption (RSA) of Pt NPs@Azo-COF and Pt NPs@Et-COF has increased by 4.62-fold and 3.25-fold, respectively, and the corresponding self-defocusing properties have also increased by 3.01-fold and 2.17-fold. Moreover, with the confinement of Pt NPs, the NLO absorption of Azo-COF and Et-COF changed from saturable absorption (SA) to RSA in the visible range (532 nm). Theoretical calculations and transient absorption demonstrate that the superior NLO performance of Pt NPs@Azo-COF is attributed to the reduction of the band-filling effect of excited states caused by the charge transfer between Pt NPs and Azo-COF, thereby optimizing the absorption cross-section of the ground state and excited state. This study expands the application range of COFs in the NLO field and opens a new avenue for improving NLO properties by modulating the pore environment.