Significant Enhancement of Reverse Saturable Absorption Ability via Pt(II) Incorporation into the Conjugated Backbone of Benzo[1,2-b:4,5-b′]dithiophene-Based Donor–Acceptor Copolymers

Organic nonlinear optical (NLO) materials with π-conjugated electron donor (D) and/or acceptor (A) frameworks have attracted widespread attention because of their controllable performance through rationally structural tailoring, while optical limiting (OL) materials with reverse saturable absorption (RSA) ability is becoming a challenging focus in NLO research. Herein, 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) was employed as an electron-rich building block to construct donor–acceptor (D–A)-type π-conjugated copolymers P1 and P2 and corresponding Pt(II)-incorporated copolymers P1–Pt and P2–Pt toward NLO materials. All copolymers exhibit typical RSA behavior toward a 532 nm nanosecond laser pulse. Interestingly, copolymers P1–Pt and P2–Pt show significantly superior RSA ability than P1 and P2, which is attributed to the synergistic effect of the extension of π-conjugation, enhancement of the ICT process, and especially the efficient formation of a triplet state caused by the novel Pt(II)-incorporated structure. Furthermore, copolymers show decreased band gaps with the tendency of P1 (2.48 eV) > P1–Pt (2.21 eV) > P2 (2.18 eV) > P2–Pt (1.94 eV). Correspondingly, P1–Pt and P2–Pt exhibit larger nonlinear absorption coefficients (βeff) of 45 and 79 cm GW–1, respectively, ca. 2.6 and 2.9 times higher than those of P1 (17 cm GW–1) and P2 (27 cm GW–1), respectively. This work presents an efficient strategy toward excellent organic NLO materials, facilitates the understanding of copolymer structure–property relationship, and promotes their advanced applications.