Tuning the excited state properties of ruthenium(II) complexes with a 4-substituted pyridine ligand

• Four new ruthenium(II) polypyridyl complexes were synthesized.
• 4-substituted pyridine has effect on photochemical and photophysical properties.
• Increased emission or photoinduced ligand dissociation was observed.

A series of complexes of the type [Ru(tpy)(bpy)(4-R-py)]2+, where tpy = 2,2′;6′,2″-terpyridine, bpy = 2,2′-bipyridine, and 4-R-py = pyridine (py, 1), 4-methoxypyridine (4-CH3O-py, 2), 4-aminopyridine (4-NH2-py, 3), 4-acetylpyridine (4-Ac-py, 4) and 4-trifluoromethylpyridine (4-CF3-py, 5), were synthesized and the effect of the pyridine substitution on the ground and excited state properties was examined. Cyclic voltammetry shows that an electron-donating substituent on the pyridine ligand results in complexes that are more easily oxidized, as is the case in 2 and 3, whereas the electron-withdrawing substituents on the pyridine in 4 and 5 result in more positive oxidation potential. The electronic absorption spectra of 2 and 3 feature a detectable red shift of the metal-to-ligand charge transfer (1MLCT) as compared to 1. Additionally, the 1MLCT maxima of 4 and 5 are blue shifted relative to 1. Complexes 1–3 exhibit phosphorescence with luminescence quantum yields of 2.0 × 10–4, 4.3 × 10–4, and 3.1 × 10–4, respectively. The emission quantum yields of 4 and 5 are approximately an order of magnitude lower, 5 × 10–5 and 4 × 10–5. Upon irradiation in CH3CN (λirr ⩾ 395 nm) all five complexes undergo photoinduced ligand exchange with the solvent to form [Ru(tpy)(bpy)(CH3CN)]2+. This process is completed more rapidly in 4 (30 min) and 5 (10 min) as compared to 1 (90 min), 2 (240 min) and 3 (>24 h). Theoretical calculations show that the electron-donating groups increase the energy difference between the ligand field state (LF) and MLCT while the electron-withdrawing groups decrease this value, giving rise to the different excited state properties. These findings demonstrate the synthetic tuning of the series of complexes, which can be useful to vary the photochemistry.

Investigation of [Ru(tpy)(bpy)(4-R-py)]2+, where tpy = 2,2′;6′,2″-terpyridine, bpy = 2,2′-bipyridine, and 4-R-py = pyridine (py, 1), 4-methoxypyridine (4-CH3O-py, 2), 4-aminopyridine (4-NH2-py, 3), 4-acetylpyridine (4-Ac-py, 4) and 4-trifluoromethylpyridine (4-CF3-py, 5) shows that complexes containing electron-donating R-groups have larger emission quantum yields, whereas complexes with electron-withdrawing R-groups have more efficient photoinduced ligand exchange.Figure optionsDownload as PowerPoint slide