Ferrocene Appended Linear Chromophores for Aggregation-Induced Emission (AIE) and Nonlinear Optics (NLO): Combined Experimental and Theoretical Studies

Abstract

New nonlinear optical (NLO)-active chromophores, featuring phenyl and methoxy phenyl substitutions at the D-π-A motif [(Fc-C(C₆H₄-R) = CH-CH = C(CN)-C₆H₄-Br)] [R = H (1), OCH₃ (2)] are synthesized and structurally analyzed. Chromophore 2 crystallized in a triclinic system (P-1), consistent with DFT-optimized structures. Non-covalent interactions in the crystal packing suppress antiparallel alignment, enhancing SHG efficiencies. Molecular electrostatic potential (MEP) maps reveal structure-property relationship and electronic communication between donor–acceptor moieties. Both chromophores exhibit suppressed emission in solution due to twisted intramolecular charge transfer (TICT) facilitated by the cyano vinylene group. However, Upon aggregation-induced emission in a THF/H₂O mixture, fluorescence significantly increases, attributed to restricted intramolecular rotation (RIR). Second Harmonic Generation (SHG) efficiencies, measured using the Kurtz–Perry powder technique with potassium dihydrogen phosphate (KDP) as a reference, show chromophore 2 is 1.1 times higher efficiency than chromophore 1. Density functional theory (DFT) derived hyperpolarizability values follow this trend, with chromophore 2 (β₀ = 40.39 × 10⁻³⁰ esu in B3LYP functional) surpassing chromophore 1. DFT and time-dependent density functional theory (TD-DFT) calculations employing B3LYP, CAM-B3LYP, and LC-BLYP functionals determined second-order nonlinear optical parameters, B3LYP and CAM-B3LYP produced values with minimal differences and a close correlation with the experimental results.