Abstract
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.