Structural Design and Study of the Nonlinear Optical Properties of Metal-Post-Modified Porphyrin-Based COFs

Abstract To elucidate how metal postmodulation tunes the third-order nonlinear optical (NLO) properties of porphyrin-based covalent organic frameworks (COFs), we fabricated porphyrin COF BCTB-PL via solvothermal synthesis. A postmetalation strategy was employed to incorporate Co2+ and Zn2+ into the framework separately, yielding the corresponding derivatives BCTB-PL@Co and BCTB-PL@Zn. A variety of characterization techniques were applied to systematically examine the microstructure, chemical composition, optical bandgap, and third-order NLO response of the obtained materials. Open-aperture Z-scan measurements demonstrate that all three porphyrin-based COFs exhibit typical reverse saturation absorption behavior. The metal-free sample BCTB-PL possesses the highest nonlinear absorption coefficient β in the range of 7.22 to 7.98 × 10–9 m/W, showing outstanding third-order nonlinear optical performance. The NLO performance of the metal-doped derivatives undergoes deterioration. The β value of BCTB-PL@Co decreases to 3.36 to 5.01 × 10–9 m/W. The performance suppression effect is more pronounced for BCTB-PL@Co than for BCTB-PL@Zn, which has a β value ranging from 5.31 to 7.69 × 10–9 m/W. Combined with the results of physical property analysis, metal coordination attenuates the NLO performance of the material by broadening the optical band gap. Furthermore, the strong spin–orbit coupling effect of Co2+ accelerates nonradiative relaxation in the excited state and further exacerbates performance degradation. These findings clarify the synergistic regulatory mechanism of metal ion species on the NLO performance of porphyrin-based COFs, and provide experimental support for the application of metalated COF materials in the NLO field.