Investigation of ultrafast broadband optical nonlinearity and intensity-dependent nonlinear absorption in novel hydrazone derivatives

Two novel hydrazone derivatives (BTH-1 and BTH-2) were designed and synthesized to investigate the modulation of intramolecular charge transfer (ICT) and planar π conjugation on optical nonlinearities. Femtosecond Z-scan measurements show that trithiophene-substituted BTH-2 exhibits stronger broadband (600–900 nm) reverse saturable absorption (RSA) resulting from two-photon absorption (TPA)-induced excited state absorption (ESA). Combined with quantum chemical calculations, the results indicate that the local excitation and spatial delocalization enhanced by planarized trithiophene are more conducive to promoting the nonlinear optical (NLO) response of hydrazone molecules than the ICT introduced by twisted triphenylamine. In addition, the intensity-dependent transition from saturable absorption to RSA of BTH-2 at 532 nm under multiple pulse widths (fs, ps, ns) was explored. Transient absorption spectroscopy provided information on the excited state dynamics of the compounds. This work extends NLO research on hydrazone derivatives and provides a reference for the design of hydrazone molecules with high nonlinearity. The results also suggest that BTH-2 is a promising candidate for optical limiting applications.