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

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 μβEFISH 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 (JSC = 1.51 mA cm−2, VOC = 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 (μg and μe), 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.