Solvatochromism and solution π-stacking of N-(4-pyridyl)-1,8-naphthalimide and its corresponding triruthenium...

Due to the scarcity of spectroscopic studies on metal-coordinated naphthalimides, and aiming to investigate fundamental spectroscopic aspects, we have described here the aggregates of N-(4-pyridyl)-1,8-naphthalimide (NI-py) in solution as well as solvatochromism displayed by it and by the coordination compounds [Ru₃O(CH₃COO)₆(NI-py)₃]ⁿ, n = +1 or 0. Based both on theoretical calculations and luminescence spectra, we demonstrated that in aqueous media, the NI-py π-stacking is thermodynamically favored, suggesting a preferable conformation where the pyridine and naphthalene moieties of two NI-py molecules are parallel to each other, but are not co-planar within an individual molecule, due to steric hindrance. The NI-py ππ* band displayed positive solvatochromism, to which the major contribution was the Catalan's SP parameter (solvent polarizability). This observation is fully consistent with the extended π-electron cloud of the NI-py naphthalene ring. However, a secondary contribution of the SA (solvent acidity) was also observed, owing to the electron pairs available at the N-heteroatom of the pyridine rings and at the carbonyl-group oxygen atoms. Finally, the multiparametric solvent effect analysis indicated that the electronic coupling between coordinated NI-py and the metallic core is modulated by the charge of the [Ru₃O(CH₃COO)₆] core, being higher for the reduced species [Ru₃O(CH₃COO)₆(NI-py)]⁰. In addition, in this reduced species, there is no overlap between NI-py ππ* and the [Ru₃O(CH₃COO)₆] charge transfer (CT) transitions, leading to the observation of the dependence of the CT energy with the SdP parameter (solvent dipolarity) since the CT transition implies in a charge-separation state.