Ferrocene appended Y-shaped imidazole aldehyde chromophores: Synthesis, the effect of alkyl chain in the second order nonlinear optical properties and theoretical studies

Abstract

Two new Y-shaped ferrocene (Fc) conjugated imidazole (IM) aldehyde [(D-π)2-IM-π-A] type chromophores [(Fc-CH=CH)2-IM-R-C6H4-CHO] {R= H (1); C8H17 (2)} have been synthesized and characterized with the aid of analytical and spectroscopic techniques [FT-IR, ESI-Mass, 1H and 13C NMR]. The thermal stability of the chromophores 1 and 2 were investigated using thermogravimetric analysis (TGA), and the chromophores were stable up to 160 ᵒC. The solvatochromic studies of the chromophores 1 and 2 show a positive solvatochromism due to the more polarizable and greater stabilization in the excited state than ground state, when solvent polarity was increased from non-polar to polar. The redox behavior of the chromophores were determined by cyclic voltammetry, and the experimentally observed HOMO and LUMO energy levels were good agreement with the DFT. The second harmonic generation efficiencies (SHG) were investigated by the Kurtz-Perry powder technique using potassium dihydrogen phosphate (KDP) as a standard. The SHG efficiencies of the chromophore 2 is 2.5 times greater than that of the chromophore 1, because of the presence of n-octyl group which reduces the anti-parallel alignments in molecular packing of the chromophore 2. In addition, the dipole moment (μ) and hyperpolarizability (β0) were calculated using different functionals (B3LYP, CAM-B3LYP and LC-BLYP), and the B3LYP-hybrid functional is overestimated when compared with long-range CAM-B3LYP and LC-BLYP functionals, because of the incorrected long-range charge transfer behaviour between donor and acceptor in the d-π-A systems. The chromophore 2 shows higher β0 values in all the different functionals than the 1, due to the presence of the n-octyl group, which restricts the molecular mobility and makes effective charge transfer process of the chromophore 2. The theoretically observed results followed the same trend as experimental results.