Ferrocene-appended donor--acceptor Schiff base: Structural, nonlinear optical, aggregation-induced emission and density functional theory studies

New ferrocenyl Schiff bases [FcC(H)NC6H3(OH)(R)] (R=H (1), NO2 (2)) have been synthesized and characterized using various techniques. Compound 2 was further confirmed using single-crystal X-ray diffraction analysis. Solvatochromism studies of 2 showed redshift from nonpolar to polar solvents. In addition, results of fluorescence studies indicated excellent aggregation-induced emission properties. The quasi-reversible redox wave in electrochemical studies of the Schiff bases evidenced the electron transfer ability of ferrocene to the Schiff base (CN) conjugation group. The second-order nonlinear optical (NLO) properties of 1 and 2 were investigated using the Kurtz-Perry powder technique and 2 showed an effect 1.46 times greater than that of urea reference. Although 2 crystallized in the P2(1)/c centrosymmetric space group, NLO property was observed, due to non-covalent interactions (CH). The band gaps were calculated using the diffuse reflectance spectroscopic method and 2 exhibited a low band gap of 2.9eV which is due to the more electron-withdrawing nature of the nitro group. Quantum chemical calculations were performed on the synthesized compounds using the density functional theory (DFT) and time-dependent DFT approach. The theoretical studies showed that the band gap for the Schiff bases was 3.8eV (1) and 3.2eV (2) and they can be considered as candidates for use in optical applications.