Low-Bandgap Organic Conjugated Copolymers with Ultra-Broadband Optical Limiting Properties Extending to Near-Infrared-II.

Developing organic conjugated copolymers with an intramolecular charge transfer (ICT) system is an efficient strategy for expanding their applications but is still a challenge for broadband optical limiting (OL). Herein, three thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A) type copolymers P1, P2, and P3 with different electron-donating groups of thiophene, bithiophene, and di(thiophen-2-yl)ethene, respectively, were reported. Copolymers exhibit low bandgaps of 0.70-0.83 eV attributed to their extended π-conjugation and strong ICT. Interestingly, these copolymers display the nonlinear optical response of reversed saturable absorption (RSA) extending from the visible to near-infrared-II region. P3 exhibits the most excellent RSA property with a nonlinear absorption coefficient (β) value of 17.8 cm GW-1, over double that of P1 and P2. Consequently, P3 displays the most remarkable OL ability with a limiting threshold (Fth) of 8.9 J cm-2 under a laser wavelength of 1600 nm. This study provides a new strategy for developing ultra-broadband OL materials based on well-identified D-A copolymers.