Synthesis, crystal structures, photophysical, electrochemical studies, DFT and TD-DFT calculations and Hirshfeld analysis of new 2,2′:6′,2′′-terpyridine ligands with pendant 4′-(trimethoxyphenyl) groups and their homoleptic ruthenium complexes

Abstract

Two new ligands, 4′-(2,3,4-trimethoxyphenyl)-2,2′:6′,2′′-terpyridine (L¹) and 4′(2,4,5-trimethoxyphenyl)-2,2′:6′,2′′-terpyridine (L²), and their homoleptic ruthenium(II) complexes, [Ru(L¹)₂](PF₆)₂ (1) and [Ru(L²)₂](PF₆)₂ (2), were synthesized and characterized by elemental analysis, ¹H and ¹³C{¹H} NMR spectroscopy, FT-IR spectroscopy and single crystal X-ray diffraction. The crystals of these compounds contain CH⋯F, OH⋯F, CH⋯O, CH⋯π and/or π⋯π intermolecular secondary interactions, resulting in supramolecular architectures. Hirshfeld surface analysis of all four compounds was also carried out to determine the intermolecular interactions. We investigated the photophysical properties of L¹, L², 1 and 2 by UV-Visible absorption and fluorescence emission spectroscopy and studied their electrochemical properties by cyclic voltammetry. In addition to n–π* and π–π* bands in the UV-Visible spectra of both ligands and complexes, a fairly strong and relatively broad ¹MLCT band appeared at λ_max ∼ 500 nm in complexes 1 and 2. The emission bands in ligands L¹ and L² appeared at λ_em 429 and 426 nm, respectively, whereas in the complexes 1 and 2, these bands showed red-shifts of 34 and 82 nm, respectively. Both L¹ and L² showed two oxidation peaks; however, only L² showed a significant reduction peak. Complexes 1 and 2 showed two oxidation peaks and two reduction peaks. The effects of increasing scan rate (100 to 1000 mV s⁻¹) on the electrochemical behaviour of complexes 1 and 2 were also investigated. The experimentally observed structural and photophysical properties were supported by DFT and TD-DFT calculations.