Unveiling the effects of conjugated gradient bridges: Inducing intramolecular charge transfer significantly enhances nonlinear response

Abstract

As an important π-conjugated bridge, (E)-2-styrylthiophene possesses excellent electronic transport and optical properties, showing great potential in nonlinear optical (NLO) applications. In order to investigate the influence of different donor units on (E)-2-styrylthiophene in NLO, we synthesized three thiophene-based D-π-A conjugates with dicyanoacetylene as the acceptor and different donors (MS, PS, FS) by inducing intramolecular charge transfer (ICT). Their third-order nonlinear absorption was studied using Z-scan experiments and transient absorption spectroscopy. Due to the significant ICT, MS exhibited the highest nonlinear absorption coefficient (β = 1.5 × 10⁻⁹ m W⁻¹) and effective third order refractive index (n₂ = -12 × 10⁻¹⁷ m² W⁻¹). Taking the strongest performing MS molecule as an example, we analyzed four types of conjugated bridges through theoretical calculations, which indicated that (E)-2-styrylthiophene has the strongest dipole moment difference (Δμ = 34.73 D). Finally, transient absorption (TA) spectroscopy revealed that the nonlinear absorption of these molecules is primarily caused by reverse saturable absorption (RSA).