Photoinduced Energy vs Electron Transfer Between Fe(II) and Ru(II) Bisterpyridyl Complexes in Solution and Grafted on a Gold Surface

Quantitative oxidation of [Fe­(ttpy)₂]²⁺ (1) (ttpy = 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine) has been performed in CH₃CN by continuous photolysis experiments in the presence of [Ru­(ttpy)₂]²⁺ (2) and a large excess of a diazonium salt (ArN₂⁺) as sacrificial oxidant. The reaction occurs with a low quantum yield (ϕ = 10^–3). The photooxidation process is fully characterized and shows that (2) acts as an antenna and transfers the excitation energy to (1). The latter complex in its excited state is oxidized by ArN₂⁺. Thiolated derivatives of both complexes, i.e., [Fe­(ttpySH)₂]²⁺ (3) (ttpySH = 4′-(4-(2-mercaptoethoxy)­phenyl)-2,2′:6′,2″-terpyridine) and [Ru­(ttpySH)₂]²⁺ (4), have been self-assembled on gold. The reactivity of the [Ru­(ttpy)₂]²⁺ core is modified between solution and the self-assembled monolayer (SAM) structure. Whereas in solution under irradiation (2) is not oxidized by ArN₂⁺, a cathodic photocurrent generation is obtained from a SAM of (4) with ArN₂⁺ in solution. A bilayer assembly combining (4) as a photoactive unit and (3) as an electron transport relay has been obtained on a gold electrode by a two-step process: A self-assembled monolayer (SAM) of (3) is immobilized on gold, followed by the covalent attachment of (4) via an electroinduced S–S bond formation. A stable cathodic photocurrent is observed under visible irradiation when the potential applied to the electrode is fixed at 0.4 V vs Ag/AgNO₃ 10^–2 M. The system achieves the uphill transport of electron of 0.8 eV.