Tuning Third-Order Nonlinear Optical Properties in Heterocycle-Dependent Hydrazono-1,3-Diketone Chromophores

Abstract Organic π-conjugated chromophores with tunable electronic structures are promising candidates for nonlinear optical (NLO) and optical limiting (OL) applications. Herein, we report a series of arylhydrazono-1,3-diketones incorporating pyrrole, N-methylpyrrole, thiophene, and furan units to investigate the effect of heterocycle substitution on their NLO response. The structural and open-aperture Z-scan measurements confirm that all four compounds exhibit reverse saturable absorption (RSA) type nonlinear attenuation. Under nanosecond excitation at 532 nm, the nonlinear absorption coefficients (β) enhanced with increasing structural flexibility among the compounds. Compound 2, containing the N-methylpyrrole unit, exhibits the highest β (5.22 × 10–11 m W–1) and the lowest OL start threshold, FS (3.13 × 1012 W m–2), indicating the markedly enhanced RSA and OL performance. The enhanced NLO response of compound 2 highlights the influence of heterocycle substitution on the excited-state electronic structure and nonlinear absorption behavior. In addition, the weaker intramolecular hydrogen-bond interaction in compound 2 is associated with a more pronounced torsional distortion, which may contribute to its distinct excited-state electronic response. This study establishes heterocycle substitution as an effective molecular parameter for tuning nonlinear absorption (NLA) and OL materials suitable for developing advanced laser safety devices.