Fast, through-bond mediated energy transfer from Ir(iii) to Ru(ii) in di- and tetranuclear heterometallic assemblies: elucidation of a two-step Ir → Ir → Ru energy transfer process

Abstract

Energy transfer processes triggered by light excitation (λ_exc = 278 nm) have been investigated in a heterometallic complex, Ir–Ru, and in a tetranuclear assembly, (Ir^F4)₂–Ir–Ru, containing iridium(III) and ruthenium(II) centres {Ir represents the unit [(ppy)₂Ir(bpy)]⁺, Ru is [Ru(bpy)₃]²⁺ and Ir^F4 is [Ir(F₂ppy)₂(4-bpy-C₆H₄-)]⁺ (ppy = 2-phenylpyridyl, bpy = 2,2′-bipyridyl)}. In the dinuclear species, the two metallic components are linked by a biphenylene bridge connected between the 4-positions of the bpy of Ir and one of the three bpy's of Ru. The tetrametallic species comprises the dimeric unit appended with Ir^F4 units at the 4′-positions of the ppy ligands. For both the dinuclear and tetranuclear complexes, the metal centers are held at fixed distances by the interposed phenylene bridges. We have obtained the optical (absorption and emission) properties for the mononuclear species Ir^F4, Ir and Ru and for the polymetallic species Ir–Ru and (Ir^F4)₂–Ir–Ru. For Ir–Ru, the Ir → Ru energy transfer step is exothermic by ca. 0.22 eV, based on the emission energies of the respective mononuclear components at 77 K. In turn, within (Ir^F4)₂–Ir–Ru, the excited state energy of Ir^F4 is ca. 0.26 eV higher than Ir. By using λ_exc = 278 nm, it is possible to predominantly excite the iridium-based units of both Ir–Ru and (Ir^F4)₂–Ir–Ru. For both cases, energy transfer is found to be fast and efficient with k_en > 2 × 10⁸ s⁻¹, leading to detectable emission only from the Ru component both at room temperature and at 77 K. In particular, for (Ir^F4)₂–Ir–Ru, based on the evaluated intermetallic distances (e.g. 28.5 Å for the shortest Ir^F4–Ru distance) and the overlap integrals of donor and acceptor units, the observed energy transfer is too fast to be accounted for by through-space Förster transfer. In this species, energy transfer probably occurs in a two-step process, Ir^F4 → Ir → Ru, both steps involving a through-bond Dexter mechanism.