Enhanced Third-Order Nonlinear Optical Response in Cross-Conjugated Pyrrole-Based <i>ortho</i> -Halo Arylhydrazono-β-Diketones

The growing demand for high-performance nonlinear optical (NLO) materials has driven the development of novel organic systems with enhanced third-order NLO responses. Herein, we report the design and synthesis of a new series of <i>ortho</i>-halogen-substituted arylhydrazono dipyrrolyldiketones <b>1</b>-<b>4</b>, featuring a cross-conjugated π-system tailored to optimize the π-electron delocalization, molecular planarity, and intramolecular charge transfer. Pyrrole substitution on the β-diketone core was employed to redshift the electronic transitions and boost the NLO activity. Combined experimental and theoretical analyses revealed that both halogen and pyrrole modifications significantly altered the electronic polarization and third-order NLO responses. Open-aperture Z-scan measurements, performed using a 532 nm nanosecond pulsed Nd: YAG laser, revealed clear reverse saturable absorption (RSA) behavior in all four compounds. Among the series, the iodo-substituted compound <b>4</b> exhibited the highest nonlinear absorption coefficient (β = 4.51 × 10^-11 m/W) and the lowest optical limiting (OL) threshold (4.85 × 10¹² W/m²), confirming its superior RSA and OL performance. To the best of our knowledge, this is the first study showing that halogen engineering and pyrrole incorporation synergistically enhance RSA and OL performance in hydrazono β-diketones, offering structure-property insights and design guidelines for advanced photonic materials.