Donor–Acceptor Covalent Organic Frameworks with Modulated Excitonic Effects for Enhanced Nonlinear Optical Responses

Covalent organic frameworks (COFs) have emerged as promising candidates for third-order nonlinear optical (NLO) applications, owing to their extended π-conjugation and high structural designability. However, the NLO response mechanism of donor–acceptor (D–A) type COFs remains insufficiently understood, particularly regarding the electronic effects of the donor or acceptor units. In this work, D–A type COFs were constructed using triazine as the acceptor unit and 2,5-dichloroterephthalaldehyde as the donor, namely, TA-DC; additionally, TA-TP and TA-DM were synthesized by progressively increasing the D–A interaction strength within the D–A moieties. The NLO properties of the synthesized D–A COFs were investigated under nanosecond 532 nm laser excitation, revealing saturable absorption behavior. Among them, TA-DM exhibited the highest nonlinear absorption coefficient of −1.69 × 10 –10 m W –1, which is 1.97 times that of TA-DC (−0.86 × 10 –10 m W –1 ). The band structures and charge transfer characteristics of three COFs were calculated using density functional theory (DFT). Results reveal that the energy gap ( E g ) can be tuned according to the D–A interaction strength, with minimum E g in TA-DM . Notably, TA-DM also exhibits the best activity in electron transfer, and the conductivity trend of the three COFs is associated with the calculated E g values of the D–A pairs. UV–vis spectroscopy results are consistent with DFT and Z-scan findings. This work provides an effective strategy for clarifying the relationship between the structural design of the D–A COFs and their NLO properties, offering guidance for the development of COF materials with optimal NLO performance.