Highly Solvent Dependent Luminescence from [Ru(bpy)n(dppp2)3−n]2+ (n = 0−2)

Abstract

The photophysical properties of a series of ruthenium(II) complexes possessing the dppp2 (dppp2 = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline) ligand, [Ru(bpy)(n)(dppp2)(3-n)](2+) (bpy = 2,2'-bipyridine; n = 0-3), were investigated. The dppp2-containing complexes possess a remarkable solvent dependence of the luminescence maximum, lifetime, and quantum yield. For example, the emission maximum of [Ru(bpy)(2)(dppp2)](2+) blue shifts from 752 nm in CH(3)CN to 653 nm in CH(2)Cl(2), concomitant with a 19-fold enhancement in the luminescence quantum yield. Electronic structure calculations, transient absorption spectroscopy, and electrochemistry were also used to gain insight into the photophysical properties of the dppp2 complexes. The pronounced solvent effect of the emission of these complexes is attributed to the changes in the relative stabilities of two low-lying metal-to-ligand charge transfer (MLCT) excited states on the environment, where the lowest energy MLCT state is more sensitive to the polarity of the solvent than that which lies at slightly higher energy. Transient absorption spectroscopy shows that the identity of the lowest energy MLCT state does not change as a function of solvent, however, its emission maximum and lifetime are greatly affected by the polarity of the surrounding medium. In contrast to [Ru(bpy)(2)(dppz)](2+) (dppz = dipyrido[3,2-a:2',3'-c]phenzine), the lowest energy excited state in the dppp2-containing complexes is assigned as arising from a triplet MLCT state where the charge is localized on the portion of the dppp2 ligand distal to the metal, (3)MLCT(dis).