[Cr(ttpy)2]3+ as a multi-electron reservoir for photoinduced charge accumulation

Abstract

[Cr(ttpy)₂]³⁺ (ttpy = 4′-(4-methylphenyl)-2,2′:6,2′′-terpyridine) exhibits rich electrochemical and photophysical properties. Cyclic voltammetry performed in CH₃CN shows in the cathodic part the presence of three one-electron reversible systems at −0.47, −0.85 and −1.35 V vs. Ag/AgNO₃ 10⁻² M. These systems are attributed to the reduction of the terpyridine ligands with a partial delocalization of the charge on the tolyl for the last reduction event. The three different reduced species were generated by exhaustive electrolysis and characterized by EPR and UV-visible spectroscopy; DFT calculations were performed to locate the spin density of the electrons added during the reduction. Visible light irradiation of [Cr(ttpy)₂]³⁺ induces the population of a luminescent metal-centered excited state with a lifetime of 270 ns in deoxygenated CH₃CN. This excited state can be quenched by an electron transfer process with triphenylphosphine (PPh₃) or triethanolamine (TEOA). Using TEOA as a sacrificial electron donor, the doubly reduced species (i.e.[Cr(ttpy)₂] ⁺) can be generated under continuous irradiation. In the presence of [Ru(bpy)₃]²⁺ as an additional photosensitizer, the photoreduction of [Cr(ttpy)₂]³⁺ towards [Cr(ttpy)₂]⁺ is accelerated. The trinuclear [{Ru^II(bpy)₂(bpy-O-tpy)}₂Cr^III]⁷⁺ complex ([Ru₂Cr]⁷⁺) in which a Cr^III-bis-terpyridine centre is covalently linked to two Ru^II-tris-bipyridine moieties by oxo bridges has been synthesised. Its electrochemical, photophysical and photochemical properties were investigated in deoxygenated CH₃CN. Cyclic voltammetry indicates only a poor electronic communication between the different subunits, whereas luminescence experiments show a strong quenching of the Ru^II* excited state by an intramolecular process. Continuous irradiation of [Ru₂Cr]⁷⁺ under visible conditions in the presence of TEOA leads to [Ru₂Cr]⁴⁺ where three electrons are stored on the [Cr(ttpy)] subunit.