The Expanded Side-Chain Conjugation Strategy Markedly Improves the Third-Order Nonlinear Optical Performance of Triazine-Based Covalent Organic Frameworks

Covalent organic frameworks (COFs) are considered to be the preferred materials for building high-performance optical devices in the field of nonlinear optics. However, an increasing number of studies have shown that the alteration of the side chains of COFs has a significant impact on the electronic structure, spatial structure, charge transfer efficiency, and stability of the materials, thus affecting the nonlinear properties. This study employs an extended side-chain coupling strategy to enhance the third-order nonlinear optical (NLO) properties of triazine-based COFs. By meticulously designing the expansion and modification of the side chains in COFs materials, four novel 2D COFs materials (P1, P2, P3, and P4) are designed and synthesized. The experimental results show that the NLO performance of P2, P3, and P4 films is significantly improved compared with that of unmodified P1, and the third-order NLO performance of P4-containing films is 1.4 times higher than that of P1, with a nonlinear absorption coefficient as high as -6.35 x 10-6 m/W. This study not only elucidates the positive effect of side-chain modification on the NLO performance of COFs, but also provides new insights for the design and synthesis of COFs with higher NLO activity in the future.