Azine bridges enhance hyperpolarizability in ferrocenyl donor–π–acceptor chromophores

The effect of replacing the hydrazone (–CH=N–NH–) with an azine (–CH=N–N=CH–) bridge on the nonlinear optical (NLO) response of ferrocene-based donor–π–acceptor chromophores was investigated using DFT and TD-DFT. Four hydrazone (FcH1–FcH4) and four azine (FcA1–FcA4) derivatives were optimized at B3LYP/def2-TZVP; UV–Vis and NLO properties were evaluated at CAM-B3LYP/def2-TZVP(Fe)/6-31 + G(d,p)(C,H,N,O,S) in the gas phase and dichloromethane (DCM). The azine series shows narrower HOMO–LUMO energy gaps and an extended intramolecular charge-transfer (ICT) network involving multiple HOMO–n → LUMO pathways, compared with the single HOMO→LUMO pathway in hydrazones. Consequently, FcA4 shows 1.87-fold (β₀) and 2.98-fold (γ₀) enhancements over FcH4, exceeds para-nitroaniline by 16.7-fold (β₀) and 52.0-fold (γ₀) in the gas phase, and by 11.7-fold (β₀) and 44.9-fold (γ₀) in DCM. This is the first DFT/TD-DFT study demonstrating that hydrazone-to-azine bridge conversion simultaneously enhances β₀ and γ₀ in ferrocene-based NLO chromophores.