Ultrafast Excited State Dynamics and Nonlinear Optical Properties of Ferrocene-Functionalized Chalcone Derivatives

Four ferrocene-functionalized chalcone derivatives, (3-oxo-3-phenyl-1-propen-1-yl)-ferrocene (I), [3-(3-bromophenyl)-3-oxo-1-propenyl]-ferrocene (II), [3-Oxo-3-(2-pyridinyl)-2-propen-1-yl]ferrocene (III), and [3-(6-Bromo-2-pyridinyl)-3-oxo-1-propenyl]-ferrocene (IV), incorporating benzene or pyridine acceptor units were synthesized and comprehensively investigated to establish structure-property correlations in ultrafast excited-state dynamics and nonlinear optical (NLO) responses. The compounds were fully characterized by NMR, FTIR, mass spectrometry, electrochemistry, and steady-state absorption spectroscopy. Femtosecond transient absorption (fs-TAS) measurements revealed rapid internal conversion from the S₂ state to a stabilized intramolecular charge-transfer (ICT) state within picoseconds, followed by slower recovery to the ground state. Global kinetic analysis confirmed that ICT lifetimes are strongly modulated by the nature of the acceptor moiety. ICT-driven delocalization can have a significant effect in enhancing NLO activity. Open-aperture Z-Scan experiments at 800 and 1060 nm demonstrated pronounced reverse saturable absorption arising from two-photon absorption (TPA). Among the series, the 3-bromophenyl derivative exhibited the highest TPA coefficient (β ∼10^{- 5} cm/GW), attributed to enhanced ICT stabilization and extended π-conjugation. These results highlight halogen substitution as a critical design principle, with bromine enhancing electronic polarization and amplifying NLO performance. The combined spectroscopic and optical studies establish ferrocene-chalcone hybrids as versatile molecular platforms for rational design of photonic materials, offering valuable guidelines for optical limiting and optoelectronic applications.