Construction of Narrow-Bandgap Copolymers with Pt(II)-Enhanced Near-Infrared Optical-Limiting Properties

The construction of electron donor–acceptor (D-A) material systems is one of the efficient strategies to accomplish optical limiting (OL) toward near-infrared (NIR) laser pulses, which can be attributed to the facilitated reverse saturable absorption (RSA) through intramolecular charge transfer (ICT) process. However, exploring OL materials with outstanding RSA performance in the NIR region remains challenging. Herein, a series of new thiadiazoloquinoxaline (TQ)-based D-A copolymers P1–P4 and the corresponding Pt(II)-incorporated copolymers P1Pt–P4Pt are designed and synthesized via a rational three-component polymerization strategy. Copolymers P1–P4 exhibit broadband ICT absorption in the NIR region with ultranarrow bandgaps (0.72–0.82 eV). In comparison, the ICT absorption redshifts after Pt(II) incorporation on the backbone of copolymers, endowing P1Pt–P4Pt with narrower bandgaps (0.66–0.79 eV) than those of the corresponding copolymers without Pt(II). Z-scan experiments reveal that all copolymers exhibit OL behavior under both 532 and 1064 nm laser pulses, benefiting from the extended π-conjugation and efficient ICT absorption in the NIR region. Among P1–P4, copolymer P2, containing thiophene as the electron donor, shows the best-performing RSA property with the highest nonlinear extinction coefficient (βeff) of 91 cm GW–1 and the lowest OL onset fluence (Fon) of 0.14 J cm–2 at 532 nm. Interestingly, Pt(II) incorporation can significantly boost the RSA performance of all copolymers under 1064 nm laser pulses due to the heavy-atom effect of Pt(II). Among them, Pt(II)-incorporated copolymer P2Pt shows the best RSA ability with the largest βeff of 66 cm GW–1 and the lowest Fon of 0.28 J cm–2, both of which are superior to the parent copolymer P2 (βeff = 17 cm GW–1, Fon = 0.91 J cm–2). Above all, this work not only establishes TQ-based D-A copolymers as a new class of broadband OL materials but also reveals Pt(II) coordination as a versatile approach for these types of copolymers featuring improved RSA properties under NIR laser pulses.