Regulation of Nonlinear Optical Properties of Covalent Organic Framework Materials by Imine Bond Orientation Isomerism

Subtle structural variations in covalent organic frameworks (COFs) can significantly alter their electronic structures and optoelectronic properties, thereby modulating their third-order nonlinear optical (NLO) performance. However, the systematic investigation of how imine bond orientation affects the third-order NLO properties of COFs remains unexplored. To address this, two pairs of imine bond orientational isomers (DA-COF1/DA-COF2 and DA-COF3/DA-COF4) were designed and synthesized. The experimental results demonstrate that DA-COF1 films exhibited saturation absorption (SA) at all pulse energies, while DA-COF2, DA-COF3, and DA-COF4 films exhibited optical switching characteristics and were inverse saturation absorption (RSA) at low energy, which was transformed into SA with increasing energy. At a pulse energy of 5 μJ, the nonlinear absorption coefficient |β| of DA-COF1 (23.4 × 10–7 m/W) and DA-COF3 (10.2 × 10–7 m/W) is 2.31 and 2.04 times that of DA-COF2 (10.1 × 10–7 m/W) and DA-COF4 (5.0 × 10–7 m/W), respectively. Besides, the third-order nonlinear polarizability χ(3) of DA-COF1 (6.22 × 10–7 esu) and DA-COF3 (5.06 × 10–7 esu) is 1.31 and 1.66 times that of DA-COF2 (4.76 × 10–7 esu) and DA-COF4 (3.05 × 10–7 esu), respectively. These results further illustrate that compared with D-C═N-A configured COFs (DA-COF2 and DA-COF4), A-C═N-D configured COFs (DA-COF1 and DA-COF3) exhibit narrower bandgaps, lower fluorescence intensity, highly symmetrical electron cloud density, and stronger intramolecular charge transfer efficiency, consequently demonstrating superior third-order NLO performance. In summary, subtle differences in imine bond orientation significantly regulate third-order NLO performance, providing references for COFs’ applications in NLO and chemical bond orientation design.