AIE-Active Ferrocene Appended Linear (D-π-A) Aromatic Ester Chromophores: Structural, Theoretical and Effect on Phenyl Ring on Luminescence and Nonlinear Optical Properties

In the present research, a new ferrocene-appended linear donor-pi-acceptor (D-pi-A) ester chromophore Fc-(C6H4)2-COOC2H5 (2) was synthesized and characterized using various spectroscopic techniques. The molecular structure of chromophore 2 was determined using single-crystal X-ray diffraction analysis, revealing its crystallization in the monoclinic crystal system with the P21/n space group. The chromophore Fc-C6H4-COOC2H4 (1) was utilized to compare its photophysical and nonlinear optical characteristics with those of 2. Cyclic voltammetry revealed a single-electron transfer mechanism from Fe2+<-> Fe3+. The emission solvatochromism studies show a polarized excited state in response to increasing solvent polarity, indicating charge transfer processes. Interestingly, in mixed aqueous media (THF/H2O), the chromophores exhibit intensified fluorescence due to restricted intra-molecular rotation (RIR), despite weak emission in THF solvent. Chromophore 2 demonstrated 1.5 times higher second harmonic generation (SHG) efficiency than 1, attributed to its extended pi-conjugation from an additional phenyl ring and reduced antiparallel alignments. Despite its centrosymmetric crystal packing, chromophore 2 exhibited SHG efficiency due to non-covalent interactions (C-H & sdot;& sdot;& sdot;pi) in the crystal lattice. Density Functional Theory (DFT) calculations, employing various functionals, provided insights into dipole moment and first hyperpolarizability, with B3LYP-hybrid functional underestimating hyperpolarizability values owing to long-range charge transfer interactions within the D-pi-A systems, aligning closely with experimental findings. Enhanced NLO efficiency was observed in AIE-Active ferrocene appended linear aromatic ester chromophore Fc-(C6H4)2-COOC2H5 (2) due to extended pi-conjugation with non-covalent interactions.+ image