The molecular light switch effect in ruthenium(II)-dppz molecules

The transition metal coordination complex, [Ru(bpy)2dppz] 2+, has been identified as a luminescent probe of many nonpolar environments, most importantly for DNA. Its usefulness arises from its distinctive luminescent properties. In aqueous solution, [Ru(bpy)2dppz]2+ is essentially nonluminescent due to hydrogen bond formation with water, which quenches the excited-state luminescence and reduces its emission by 2--3 orders of magnitude. In the presence of DNA, this molecule becomes brightly luminescent, because the planar dppz ligand intercalates between adjacent base pairs in the double helix of DNA, thereby restoring its luminescence. This behavior has been called the "light-switch" effect. The stark contrast between the dark and bright states of [Ru(bpy)2dppz] 2+ makes this complex useful for the quantitative study of DNA and other nonpolar microenvironments.; This work has focused on the mechanism responsible for the light-switch behavior. It has been demonstrated that the light-switch property does not require DNA for activation, and luminescence is observed in a number of both protic and aprotic environments. We present temperature-dependent excited state lifetime measurements on dppz-containing Ru(II) complexes in both alcohol and nitrile solvents. These experiments yield a unifying picture of the excited-state photophysics, which appear to be governed by an excited state equilibrium. Our measurements support an excited state equilibrium between two fragments of the dppz ligand corresponding to the bright and dark states of this molecule. They suggest that the bright state is photophysically similar to the emissive state in [Ru(bpy)3]2+.; The experiments presented here suggest that the light-switch behavior is not driven by a state reversal, but is governed by a competition between energetic factors that favor the dark state (even in aprotic solvents) and entropic factors that favor the bright state. Thus the equilibrium corresponds to the transfer of the photoexcited electron between the inner and outer portions of the dppz ligand. Qualitatively speaking, our model describes the [Ru(bpy)2dppz]2+ complex as consisting of a [Ru(bpy)3]2+ core and a phenazine charge-acceptor. This equilibrium description of the light-switch mechanism accounts for a wide range of observations across multiple dppz-containing Ru(II) compounds in a series of solvents with differing chemical character and polarity.