Fused-ring electron acceptor molecules with a narrow bandgap for near-infrared broadband ultrafast laser absorption†

With the development of near-infrared (NIR) laser technologies, there is an urgent demand to develop novel optical limiting materials for laser protection. Fused-ring electron acceptors (FREAs), as NIR dyes, hold great potential in optical limiting due to their strong electron mobilities. In this study, optical nonlinearities of FREAs (IEICO-4F and ITIC-4F) are investigated within the NIR window (850–1550 nm). Under the combined contributions of two-photon absorption (TPA), TPA-induced excited state absorption or three-photon absorption (3PA), both compounds exhibit broadband reverse saturation absorption, which extends to 1550 nm. Especially for IEICO-4F, its TPA cross-section reaches 26 700 GM at 950 nm under 190 fs and it has an ultralow optical limiting threshold (0.44 mJ cm⁻²). In comparison with ITIC-4F, increasing the conjugated length in IEICO-4F by non-covalent interaction between ‘O’ and ‘S’ atoms effectively modulates its molecular band gap and remarkably enhances its optical nonlinear response. Additionally, the nonlinear refraction of FREAs displays obvious self-defocusing in the near-resonance region, which is primarily governed by a bound electron response (Kerr effect). Our results indicate that FREAs with electron-rich cores are greatly promising as optical limiting materials and warrant further exploration in nonlinear optics.