Multichromophoric hybrid species made of perylene bisimide derivatives and Ru(ii) and Os(ii) polypyridine subunits

Abstract

Herein, the synthesis and the photophysical and redox properties of a new perylene bisimide (PBI) species (L), bearing two 1,10-phenanthroline (phen) ligands at the two imide positions of the PBI, and its dinuclear Ru(II) and Os(II) complexes, [(bpy)₂Ru(μ-L)Ru(bpy)₂](PF₆)₄ (Ru₂; bpy = 2,2′-bipyridine) and [(Me₂-bpy)₂Os(μ-L)Os(Me₂-bpy)₂](PF₆)₄ (Os₂; Me₂-bpy = (4,4′-dimethyl)-2,2′-bipyridine), are reported. The absorption spectra of the compounds are dominated by the structured bands of the PBI subunit due to the lowest-energy spin-allowed π–π* transition. The spin-allowed MLCT transitions in Ru₂ and Os₂ are inferred by the absorption at 350–470 nm, where the PBI absorption is negligible. The absorption band extends towards the red region for Os₂ due to the spin-forbidden MLCT transitions, intensified by the heavy osmium center. The reduction processes of the compounds are dominated by two successive mono-electronic PBI-based processes, which in the metal complexes are slightly shifted compared to the free ligand. On oxidation, both metal complexes undergo an apparent bi-electronic process (at 1.31 V vs. SCE for Ru₂ and 0.77 V for Os₂), attributed to the simultaneous one-electron oxidation of the two weakly-interacting metal centers. In Ru₂ and Os₂, the intense fluorescence of L subunit (λ_max, 535 nm; τ, 4.3 ns; Φ, 0.91) is fully quenched, mainly by photoinduced electron transfer from the metal centers, on the ps timescale (time constant, 11 ps in Ru₂ and 3 ps in Os₂). Such photoinduced electron transfer leads to the formation of a charge-separated state, which directly decays to the ground state in about 70 ps in Os₂, but produces the triplet π–π* state of the PBI subunit in 35 ps in Ru₂. The results provide information on the excited-state processes of the hybrid species combining two dominant classes of chromophore/luminophore species, the PBI and the metal polypyridine complexes, and can be used for future design on new hybrid species with made-to-order properties.