N -Heterocyclic Microenvironment Engineering in Imine Covalent Organic Frameworks: Toward Rational Tuning of Donor–Acceptor Architectures and Nonlinear Optical Performance

N -heterocyclic microenvironment engineering is critical for modulating the electronic structure and nonlinear optical (NLO) responses of imine-linked covalent organic frameworks (COFs). Using 5′-(4-aminophenyl)–2′,4′,6′-trimethyl-[1,1′:3′,1″-terphenyl]-4,4″-diamine (ATTD), we synthesized three COFs (ACOF-1, ACOF-2, and ACOF-3) via polycondensation with aldehydes bearing N0, N1, and N3 heterocycles, yielding nitrogen contents of 5.25, 6.24, and 8.91%, respectively. Systematic investigation shows that increasing N-content progressively narrows the optical bandgap and strengthens donor–acceptor charge transfer. Open-aperture Z-scan measurements reveal reversible reverse saturable absorption (RSA) to saturable absorption (SA) switching under varying pulse energies for all samples. Closed-aperture Z-scan indicates self-defocusing and RSA for ACOF-1, whereas ACOF-2 and ACOF-3 exhibit asymmetric twin-peak-valley curves, signifying third- and fifth-order nonlinear refractive contributions. Collectively, the N-heterocyclic microenvironment precisely tailors electronic distribution and charge-transfer dynamics, enabling deliberate NLO performance tuning. This work offers a rational guideline for designing high-performance COF-based NLO materials.