Donor−Acceptor Engineering in Porphyrin-Based Two-Dimensional Polymers for Ultrafast Nonlinear Optical Response

Leveraging their extended π-conjugation, structurally tunable backbones, and superior film-forming processability, two-dimensional (2D) organic polymers have emerged as a highly promising class of materials for third-order nonlinear optical (NLO) applications. Herein, a series of porphyrin-based donor−acceptor (D−A) 2D polymer films, namely, TAPP-TAPZ, TAPP-BATD, TAPP-TDAT, and TAPP-BDTA, are successfully fabricated via a liquid−liquid interfacial polymerization approach. These films exhibit broad absorption profiles across the visible region alongside markedly narrowed optical bandgaps, underscoring the extensive π-electron delocalization inherent to the 2D polymeric frameworks. Femtosecond open-aperture Z-scan measurements conducted at 515 and 800 nm unambiguously reveal pronounced reverse saturable absorption in all samples. Strikingly, the TAPP-TAPZ film delivers exceptional nonlinear absorption coefficients (β) of 8.8 × 103 cm GW−1 at 800 nm and 5.2 × 103 cm GW−1 at 515 nm, values that not only substantially outperform the analogous TAPP-BATD, TAPP-TDAT, and TAPP-BDTA counterparts but also rival or surpass those of numerous reported covalent organic frameworks, 2D inorganic nanomaterials, and small-molecule NLO systems. Complementary density functional theory calculations ascribe this remarkable performance to the incorporation of benzothiadiazole units, which stabilize a donor−donor−acceptor (D−D−A) electronic configuration and facilitate efficient intramolecular charge transfer. Collectively, our findings establish interfacial polymerization as a robust and generalizable route for constructing ordered, defect-minimized 2D films, thereby offering actionable design principles for the development of next-generation ultrafast photonic devices.