A Theoretical Investigation into the Photophysical Properties of Ruthenium Polypyridine-Type Complexes

Excited states of ruthenium polypyridine‐type complexes have always attracted the interest of chemists. We have recently found evidence of a remarkable long‐lived excited state (30 μs) for a RuII complex containing a heteroditopic ligand that can be viewed as a fused phenanthroline and salophen ligand.1 To unravel this intriguing electronic property, we have used density functional theory (DFT) calculations to understand the ground‐state properties of [(bpy)2Ru(LH2)]2+, where LH2 represents N,N′‐bis(salicylidene)‐(1,10‐phenanthroline)diamine. Excited singlet and triplet states have been examined by the time‐dependent DFT (TDDFT) formalism and the theoretical findings have been compared with those for the parent complex [Ru(bpy)3]2+. The outstanding result is the presence of excited states lower in energy than the metal‐to‐ligand charge‐transfer states, originating from intraligand charge transfer (ILCT) from the phenolic rings to the phenanthroline part of the coordinated LH2. The spin density distribution for the lowest triplet state provides evidence that it is in fact the lowest triplet state of the free ligand. Correlation between the energy level diagram of orbitals for the ground state and that for the 3ILCT state clearly establishes that the ruthenium retains its formal RuII oxidation state. The quenching of the luminescence and the evidence of the long‐lived excited state observed for [(bpy)2Ru(LH2)]2+ are discussed in the light of the computational results. Intraligand charge transfer: A detailed theoretical analysis of the singlet and triplet excited states (see picture) of a ruthenium complex containing a heteroditopic ligand (a fused phenanthroline and a Schiff base) in its coordination sphere indicate that low‐energy intraligand charge transfer (ILCT) in the modified phenanthroline‐based ligand plays important roles both in the quenching of the luminescence and in the observed long‐lived excited state (30 μs).