Assessment of the electronic structure of a triruthenium acetate-pyridylnaphthalimide cluster

Abstract A synthetic route for attaining the combination of an electroactive unit with the chromophore N-(4-pyridyl)-1,8-naphthalimide (NI-py) was developed, yielding the trinuclear ruthenium cluster [Ru3O(CH3COO)6(NI-py)3]PF6 (1). Due to the presence of one unpaired electron in the [Ru3O(CH3COO)6]+ unit, 1H NMR analysis showed paramagnetic shifts in the NI-py hydrogens by displacing, for example, hydrogen ortho to the N-heteroatom in the free ligand from 8.83 ppm to 0.73 ppm in the coordinated ligand. The electronic spectrum of 1 showed absorption at 696 nm (metal-to-metal transitions of the [Ru3O(CH3COO)6] unit), and a band at 334 nm, assigned to cluster-NI-py charge transfer superimposed with the low energy π → π * NI transitions. The latter band did not show any shifts upon coordination or change in the formal charge of the [Ru3O(CH3COO)6] core, suggesting that the electronic states involved in these transitions are not perturbed significantly upon complexation. However, the comparison of both NI-py and complex luminescence spectra indicated that the metal unit can quench NI-py fluorescence. The computational calculations confirmed the electronic delocalization over the [Ru3O(CH3COO)6]+ core in the ground state and the superposition of both NI-py π → π * and the CLCT bands. A film, assembled on a FTO electrode surface by adsorption, showed the redox pair [Ru3O(CH3COO)6]+1/0 at 0.365 V vs. SHE, which is in accordance with the NI-py strong π -acid character. This film was electrochromic, being blue for [Ru3O(CH3COO)6(NI-py)3]+ and yellow for [Ru3O(CH3COO)6(NI-py)3]0.