Binding Geometry and Photophysical Properties of DNA-Threading Binuclear Ruthenium Complexes

The DNA binding conformation and the photophysical properties of the semiflexible binuclear ruthenium complex [μ-bidppz(phen)<sub>4</sub>Ru<sub>2</sub>]<sup>4+</sup> (<b>2</b>) were studied with optical spectroscopy and compared to the rigid, planar homologue in syn conformation [μ-dtpf(phen)<sub>4</sub>Ru<sub>2</sub>]<sup>4+</sup> (<b>3</b>) and the parent “light-switch” complex [Ru(phen)<sub>2</sub>dppz]<sup>2+</sup> (<b>1</b>). Comparison of calculated and observed absorption bands of the bridging ligand, bidppz, confirm earlier suggestions that <b>2</b> is significantly nonplanar, both free in solution and when intercalated into poly(dAdT)<sub>2</sub>, but the conclusion that the intercalated conformation is an anti rotamer is not substantiated by comparison of linear and circular dichroism spectra of <b>2</b> and <b>3</b>. The behavior of the emission quantum yield as a function of temperature is similar for the two binuclear complexes <b>2</b> and <b>3</b> in different protic solvents, and a quantitative analysis suggests that, in solution, the solvent is more strongly hydrogen bonded to the excited state of <b>2</b> than to <b>1</b>. However, the observation that for <b>2</b> the radiative rate constant increases to a value similar to <b>1</b> upon intercalation into DNA suggests that the difference between <b>1</b> and <b>2</b> in accepting hydrogen bonds is less pronounced when intercalated.