Nonlinear optical-active ferrocene conjugated Y-shaped imidazole donor–π–acceptor [(D–π)2–IM–π–A] compounds for dye-sensitized solar cells using non-corrosive copper complexes as a redox mediator

Abstract

Two new bis-ferrocene (Fc) conjugated Y-shaped imidazole (IM) based donor–π–acceptor [(D–π)₂–IM–π–A] type “push–pull” compounds [(Fc–CHCH)₂–IM–C₈H₁₇–C₆H₄–R] {R = COOC₂H₅ (1); COOH (2)} have been synthesized for second-order nonlinear optics (1, ester) and dye-sensitized solar cell (2, acid) applications. The redox potentials of the compounds were examined using cyclic voltammetry, which shows one-electron charge transfer from the ferrocene to ferrocenium ion (Fe²⁺ ⇌ Fe³⁺), and these potentials were employed to calculate the related energy gap. The second-order nonlinear optical (NLO) properties of the ester compound 1 were explored using the Electric-Field-Induced Second Harmonic generation (EFISH) technique in CHCl₃ solution with an incident wavelength of 1907 nm, and it shows an interesting μβ_EFISH value of −780 × 10⁻⁴⁸ esu. Besides, the carboxylic acid compound 2, adequately functionalized for anchoring to TiO₂, was tested as a photosensitizer in dye-sensitized solar cells (DSSCs) with non-corrosive Cu(I)/Cu(II) complexes as the redox mediator. Although the photovoltaic performance of 2 attached to TiO₂ in a DSSC device is rather limited (J_SC = 1.51 mA cm⁻², V_OC = 404 mV, FF = 42.7, η = 0.26%), it confirms the potential of ferrocenyl conjugated D–π–A systems as photosensitizers. In addition, frontier molecular orbital levels, ground and excited state dipole moments (μ_g and μ_e), and the origin of electronic absorption spectra were studied by using density functional theory (DFT) with the B3LYP method using 6-31+G** as the basis set.