Ultrafast optical nonlinearity in porphyrin-based covalent organic framework membrane via Cu(II)-mediated interfacial polymerization

Abstract

Covalent organic frameworks (COFs) are emerging as attractive candidates for nonlinear optical (NLO) materials, owing to their high chemical stability, structural diversity, and design flexibility. However, the inherent insolubility, poor dispersion in common solvents, and pronounced light scattering of bulk COFs are extremely detrimental to their practical applications in NLO fields. Herein, we report the successful one-step synthesis of a Cu-porphyrin-based COF membrane, via liquid-liquid interfacial polymerization, without pre- or post-metallization modification (donated as TAPP-BDA-Cu COF). Under 35-fs (fs) ultrafast pulsed laser excitation, the TAPP-BDA-Cu COF membrane demonstrates outstanding NLO performance, exhibiting remarkable nonlinear optical absorption coefficients (β_eff) of 1557 ± 40.3 cm GW⁻¹ at 800 nm and 909 ± 23.1 cm GW⁻¹ at 515 nm, which surpass those of previously reported typical two-dimensional (2D) inorganic materials and 2D polymers. Furthermore, the TAPP-BDA-Cu COF membrane exhibits exceptional optical limiting (OL) performance, with ultralow threshold values of 2.75 mJ cm⁻² at 800 nm and 6.85 mJ cm⁻² at 515 nm, indicating its significant potential for practical OL applications. These superior NLO properties of the TAPP-BDA-Cu COF membrane can be attributed not only to its extended π-conjugated framework of the COF structure, which enhances π-electron delocalization across the system, but also to the coordination of Cu(II) ions with porphyrin units, enabling efficient metal-to-ligand electron transfer. This work develops a novel and straightforward synthetic strategy for fabricating porphyrin-based COF membranes as advanced NLO materials, but also provides meaningful guidance for the future development of COF-based materials in next-generation ultrafast photonic devices.