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 pi-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 pi-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 pi-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 (beta(eff)) 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.