Modulation of the nonlinear optical response in squaraine derivatives via alkyl cyclization: Near-infrared ultrafast nonlinear refraction and absorption

Squaraines (SQs) are a significant family of near-infrared dyes with applications in fluorescence bioimaging, photodynamic therapy, etc. Most efforts have been devoted to modulating their nonlinear absorption while their nonlinear refraction has been neglected. Here, near-infrared broadband nonlinear absorption and refraction in two squaraine derivatives (SQ-640 and SQ-670) are investigated with Z-scan (720–1300 nm) and time-resolved pump-probe method. With essentially similar conjugation and planarity in the two molecules, the effect of alkyl cyclization on the nonlinear optical response is discussed. A larger transition dipole moment is reached via alkyl cyclization and results in a wider range of nonlinear optical responses for SQ-670. In addition to conventional transient absorption measurement, transient nonlinear refraction based on phase contrast pump-probe method are used for the study of ultrafast dynamics. Kinetic details from both measurements are compared and analyzed for further details of ultrafast photophysical mechanisms in these molecules. Besides, an intensity-dependent transition from saturable absorption to reverse saturable absorption in SQ-670 is observed at 720 nm which is mainly due to the combined contributions of one-photon absorption and TPA. Both samples display good nonlinear absorption (NLA) at the near-infrared region (750–1300 nm) which originates from two-photon absorption (TPA) and TPA induced excited state absorption. Bound electron response and negative excited state refraction dominate the nonlinear refraction of both molecules. Our work gains a deeper understanding of the NLO properties of squaraine derivatives from an alternative perspective and enriches the experimental research on squaraine derivatives. It may provide a useful reference for the future design of organic materials with excellent nonlinear optical responses. [Display omitted] •Investigate the near-infrared nonlinear optical properties of a squaraine after alkyl cyclization.•Discuss the intensity-dependent nonlinear absorption at 720 nm and the nonlinear optical properties in 750–1300 nm.•An increase in transition dipole moment results in a wider range of nonlinear optical responses.•Kinetic details from both measurements are compared and analyzed.