Aggregation induced emission (AIE)-active ferrocene conjugated linear π−extended multi donor−π−acceptor (D-D′-π-A) chromophores: Synthesis, structural, theoretical, linear and nonlinear optical studies

Abstract

A new series of ferrocene (D) and methoxyphenyl (D′) conjugated D-D′-π-A push-pull chromophores [Fc-(RPh)C=CH–CH=CN-R’{R, R’ = H, CN (1), OCH3, CN (2), H, C6H4NO2 (3), OCH3, C6H4NO2 (4)}] were synthesized and spectroscopically characterized. The structure of the chromophore 3 was confirmed by single crystal X-ray diffraction studies, and it was crystallized in triclinic crystal system with P-1 centrosymmetric space group. In the crystal packing, several non-covalent interactions (H-bonding and C–H·····π) were observed. The thermal behaviour of the chromophores 3 and 4 has been investigated using thermogravimetric analysis and was found to be stable up to 230 °C. The solvatochromic studies of the chromophores 1–4 showed a positive solvatochromism with red-shift from non-polar to polar solvents due to the high dipolar excited state in the presence of strong electron-donor and acceptor moieties. The electrochemical performance of the chromophores revealed the one-electron transfer from the ferrocene to ferrocenium ion Fe2+⇌Fe3+. The chromophores 1–4 exhibited low fluorescence intensity and quantum yield as a consequence of the quenching nature of ferrocene, it was further enhanced through the aggregation-induced emission (AIE) process in THF/H2O mixtures by the restriction of intramolecular rotation (RIR), and the chromophores 1–4 showed 2 times higher quantum yield in the aggregated state. The second-order nonlinear optical response was attained by Kurtz-Perry powder technique. The second harmonic generation (SHG) efficiencies of the chromophore 4 was close to the standard KDP, owing to the presence of multi-donor [Ferrocene (D) and methoxyphenyl (D′)] and acceptor groups with extended π-conjugation. In addition, the mesomeric effect (+M) of the methoxy group makes an effective charge transfer process. The optical and nonlinear optical properties of the chromophores were further analyzed by density functional theory (DFT) with different functionals (B3LYP, CAM-B3LYP and LC-BLYP). The hyperpolarizability values were underestimates in the B3LYP-hybrid functional because of the corrected long-range charge transfer between donor and acceptor in the D-π-A systems. The frontier molecular orbital (FMO) level helps for better understanding the charge transfer, nonlinear optical (NLO) properties, and the calculated HOMO-LUMO energy levels were in good agreement with the experimental results.