Third-order nonlinear optical properties based on triphenylamine derivative-based covalent organic frameworks

Nonlinear optics (NLO) is an extremely critical research area. In recent years, covalent organic frameworks (COFs) have attracted much attention as a potential NLO material. In this study, a series of COF materials with different D-A effects were successfully synthesized based on the excellent electron-donating properties of the triphenylamine group and were prepared into D-A COFs\KBr sheet materials that can be used for third-order NLO by the KBr−pressing method. The results of the open-aperture Z-scan technique showed that all COFs\KBr sheets exhibited saturable absorption (SA) behavior at 3 μJ, with nonlinear absorption coefficients (β) of −1.64, −1.23, and −2.02 × 10−9 mW−1 for PT-PA, PT-BD, and PT-TZ, respectively. Theoretical calculations show that the electron transport distance of PT-PA is shorter than that of PT-BD, while the introduction of S atoms makes the D-A effect of PT-TZ stronger than that of PT-BD, and thus the SA properties of PT-PA and PT-TZ are better than those of PT-BD. These works indicate that the COFs\KBr sheets have a potential application as high-performance NLO materials and also provide an option for COFs powder application in the field of third-order NLO. In this study, we successfully prepared three COF/KBr composite films for Z-scan experiments using potassium bromide compression method. In experiments, COF materials exhibit significant donor-acceptor (D-A) interactions. By shortening the charge transport distance in the COF structure, or introducing sulfur (S) atoms to enhance the electron donor capacity, we effectively improve the optical nonlinear properties of COF materials. [Display omitted] •Three kinds of D-A COF/KBr sheets were prepared by the potassium bromide pressing method.•Due to varying electron donors and electron transport distances, COFs' electronic structures significantly alter.•PT-TZ/KBr sheet have a high nonlinear absorption coefficient of −2.02 × 10−9 m W−1.•Improve the NLO properties of COFs by reducing charge transport distance or incorporating S to boost electron donation.