Effect of substitution on second-order nonlinear optical properties of ferrocene appended donor–π–acceptor Y-shaped trifluoromethyl imidazole chromophores

Abstract

New Y-shaped ferrocene (Fc)-conjugated trifluoromethyl-substituted imidazole (IM) donor–π–acceptor [(D–π)₂–IM–D′–π–CF₃] “push–pull” chromophores [(Fc–CHCH)₂–IM–C₆H₄–R–C₆H₄–CF₃] {R= –OCH₃ (1); –N(C₂H₅)₂ (2)} were synthesized and structurally characterized. The structure of the chromophores was confirmed via single-crystal X-ray diffraction studies. They crystallized in the triclinic P (for 1) and monoclinic P2₁/c (for 2) space groups. Thermogravimetric analysis (TGA) was used to study the thermal stability of chromophores 1 and 2, and it was found that both were stable at temperatures of about 300 °C. The redox properties were studied through cyclic voltammetry (CV), which revealed one-electron transfer from the ferrocene to ferrocenium ion (Fe²⁺ ⇌ Fe³⁺), and their potentials were utilized to calculate their energy gaps, which were 2.16 eV for 1 and 2.12 eV for 2. The second-order nonlinear optical (NLO) properties of chromophores 1 and 2 were explored via electric-field-induced second-harmonic generation (EFISH) technique in CHCl₃ solution with an incident wavelength of 1907 nm. μβ_EFISH values were −725 × 10⁻⁴⁸ esu and −806 × 10⁻⁴⁸ esu for 1 and 2, respectively. The μβ_EFISH values slightly decreased compared with other reported ferrocene-conjugated Y-shaped imidazole chromophores owing to their substitution on imidazole nitrogen, which increases steric hindrance and reduces the ICT process in the chromophores. In addition, their frontier molecular orbital levels, excited and ground state dipole moments (μ_e and μ_g), and electronic absorption spectra were studied using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with the B3LYP method using the 6-31+G** basis set.