Complexes with Redox-Active Ligands: Synthesis, Structure, and Electrochemical and Photophysical Behavior of the Ru(II) Complex with TTF-Annulated Phenanthroline

Ru­(II) complexes with chelating ligands, 4′,5′-ethylene­dithio­tetra­thiafulvenyl­[4,5-<i>f</i>]­[1,10]­phenan­throline (<b>L1</b>), 1,3-dithiole-2-thiono­[4,5-<i>f</i>]­[1,10]­phenanthroline (<b>L2</b>), and 1,3-dithiole-2-ono­[4,5-<i>f</i>]­[1,10]­phenanthroline (<b>L3</b>), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru­(bpy)₂(<b>L1</b>)]­(PF₆)₂ (<b>1</b>) is located on the tetrathiafulvalene (TTF) subunit and appears ∼0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, <b>1</b> exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru³⁺/Ru²⁺ redox couple. Complexes [Ru­(bpy)₂(<b>L2</b>)]­(PF₆)₂ (<b>2</b>) and [Ru­(bpy)₂(<b>L3</b>)]­(PF₆)₂ (<b>3</b>) exhibit only the Ru³⁺/Ru²⁺-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of <b>1</b> the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of <b>L1</b>, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of <b>1</b> compared to those of <b>2</b> and <b>3</b> is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru³⁺ center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O₂, complex <b>1</b> undergoes a photoinduced oxidative cleavage of the central CC bond of the TTF fragment, resulting in complete transformation to <b>3</b>. This photodegradation process was studied with ¹³C NMR and optical absorption spectroscopy