Molecular design and synthesis of bithiophene copolymers for advanced nonlinear optical response

In this paper, a series of low band gap bithiophene-based copolymers were designed and synthesized through direct arylation polymerization using a palladium catalyst. The band structures of the five polymers were determined by quantum mechanical calculation employing Density Functional Theory (DFT) in the periodic Boundary Condition (PBC) using the HSE06/6-31G basis set. The polymers were characterized using ¹H NMR, FT-IR, GPC, UV-vis and cyclic voltammetry. The five polymers have an energy gap below 2.95 eV and have broad absorption in the visible region. The experimental results support the theoretical prediction. The fluorescence lifetime of the polymers, P(BT-PH), P(BT-CZ), P(BT-FLN), P(BT-ANT) and P(BT-TPA) was monitored using Time-Correlated Single Photon Counting (TCSPC) in CHCl₃. We report on the third-order nonlinear optical (NLO) properties of the copolymers, which were assessed using the Z-scan method with a nanosecond laser beam at 532 nm. The copolymers P(BT-PH), P(BT-CZ), P(BT-FLN), and P(BT-TPA) were found to have nonlinear absorption coefficients of 3.85 × 10^-10, 2.87 × 10^-10, 2.99 × 10^-10, and 3.01 × 10^-10 m W^-1, respectively. The polymers exhibit better optical power limiting behavior at 532 nm because of the donor-acceptor scheme. This optical power limiting behavior positions these materials as promising candidates for integration into next-generation photonic devices, including laser protection systems, optical sensors, and nonlinear optical switches.