Citation: Tianyang Dong, Xingyuan Wen, Xingzhi Wu, Ying Jiang, Chong Wang, Ruizhi Liu, Junyi Li, Wenfa Zhou, Yinglin Song, Xiaofeng Shi, Rui Wen, Chunru Wang, Li Jiang, Chunli Bai. Ionic covalent organic frameworks enable laser-pulse-duration-dependent high third-order nonlinear optical responses[J]. Chinese Journal of Structural Chemistry, ;2025, 44(10): 100733. doi: 10.1016/j.cjsc.2025.100733 shu

Ionic covalent organic frameworks enable laser-pulse-duration-dependent high third-order nonlinear optical responses

In this paper, the third-order nonlinear optical (NLO) properties of covalent organic framework (COF) materials with conjugated amphoteric ion structure are studied for the first time. A highly ordered crystalline ultrathin films of the ionic COF material PySQ-iCOF was successfully fabricated using a solid-liquid interface method, meanwhile the building units extracted to be independent small molecule, 1-PySA, were synthesized for comparative studies. Compared to 1-PySA, PySQ-iCOF possesses not only a larger conjugated system but also exhibits enhanced polarization and charge transfer capabilities. The NLO properties of PySQ-iCOF and the small molecule 1-PySA were investigated using Z-scan technique at a wavelength of 532 nm, revealing the PySQ-iCOF thin film exhibits outstanding NLO performance. Specifically, it demonstrates saturable absorption under nanosecond (ns) pulse laser irradiation (β = -9.59 × 10-6 m/W), while exhibiting reverse saturable absorption under femtosecond (fs) pulse conditions (β = 6.91 × 10-8 m/W). Furthermore, the PySQ-iCOF film exhibits strong negative refractive nonlinearity, -6 × 10-12 m2/W for ns and -3.8 × 10-13 m2/W for fs, respectively. Transient absorption spectroscopy studies indicate that the pulse-width-dependent nonlinear absorption characteristics of the PySQ-iCOF film originate from the generation of triplet excited states. Both nonlinear absorption coefficient and nonlinear refractive index of the PySQ-iCOF film surpass those of most reported organic materials measured under comparable conditions, which provides huge potential in all-optical manipulating and switching at the nanoscale as outstanding NLO materials.
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