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-f]­[1,10]­phenan­throline (L1), 1,3-dithiole-2-thiono­[4,5-f]­[1,10]­phenanthroline (L2), and 1,3-dithiole-2-ono­[4,5-f]­[1,10]­phenanthroline (L3), 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(L1)]­(PF6)2 (1) is
located on the tetrathiafulvalene (TTF) subunit and appears ∼0.6
eV above the three Ru-centered d orbitals. In agreement with this
finding, 1 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(L2)]­(PF6)2 (2) and [Ru­(bpy)2(L3)]­(PF6)2 (3) 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 1 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 L1, according to time-dependent
DFT calculations. The substantial decrease in the luminescence quantum
yield of 1 compared to those of 2 and 3 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 1 undergoes a photoinduced oxidative cleavage
of the central CC bond of the TTF fragment, resulting in complete
transformation to 3. This photodegradation process was
studied with 13C NMR and optical absorption spectroscopy.