Synthesis, crystal structure, electrochemical properties, and photophysical characterization of ruthenium(II) 4,4′-dimethoxy-2,2′-bipyridine polypyridine complexes

Abstract

A series of ruthenium(II) polypyridine complexes of the type [Ru(tpy)((CH3O)2bpy)(4-R-py)]2+, where tpy = 2,2′;6′,2″-terpyridine, (CH3O)2bpy = 4,4′-dimethoxy-2,2′-bipyridine, and 4-R-py = pyridine (py, 2), 4-methoxypyridine (4-CH3O-py, 3), 4-aminopyridine (4-NH2-py, 4), were synthesized and their crystal structures, electronic absorption, luminescence, and electrochemical properties were investigated. The effect of adding electron-donating groups to the bidentate and monodentate ligand was investigated and compared with [Ru(tpy)(bpy)(py)]2+ (1) where bpy = 2,2′-bipyridine. While anticipated trends were not observed for the Ru-N(6) bond length as 4-R-py was varied, noticeable modifications of the measured photophysical properties were observed. A red-shift of the metal-to-ligand charge transfer (1MLCT) is observed from 466 nm in 1 to 474 nm, 478 nm, and 485 nm for 2–4, respectively. Additionally, a red-shift in the luminescence maxima is observed in 2–4 as compared to 1, with 4 exhibiting the greatest shift of more than 100 nm. Complexes 2–4 exhibited luminescence quantum yields of 2.7 × 10−4, 7.2 × 10−4, and 7.4 × 10−4, respectively, which are increased compared to the quantum yield of 2.0 × 10−4 in 1. These findings demonstrate systematic tuning of absorbance and luminescence properties of ruthenium polypyridine complexes by addition of π-donating substituents to the monodentate and bidentate ligand.