Effect of DNA Scaffolding on Intramolecular Electron Transfer Quenching of a Photoexcited Ruthenium(II) Polypyridine Naphthalene Diimide

Intramolecular emission quenching of a photoexcited ruthenium(II) polypyridine by a covalently linked naphthalene diimide (NDI) has been measured in aqueous buffer both without and with calf thymus DNA. The complex consists of a Ru(2,2‘-bipyridine)2(2,2‘-bipyridine-5-carboxamide)2+ electron donor covalently attached by way of a −CH2CH2CH2− linker to a 1,4,5,8-naphthalene diimide acceptor (Ru−NDI, 1). The NDI portion of the complex intercalates in calf thymus DNA, as indicated by the hypochromism of its optical absorbance bands and observation of an induced circular dichroism spectrum in the same region. Emission quenching in Ru−NDI has been measured relative to a Ru tris-bpy model lacking the NDI moiety by both lifetime and emission quantum yield techniques. Using lifetime averages, the relative emission quenching is, respectively, 99.1% and 97.9% in aqueous buffer solutions without and with DNA. The emission quenching is ascribed to intramolecular electron transfer within the Ru−NDI complex with an estimated driving force (−ΔG°) of 0.33 eV. In buffer, the emission decays of Ru−NDI alone are fit well with a triexponential model with lifetimes of 0.34 (0.88), 1.99 (0.11), and 12.6 (0.008) ns (relative amplitude). The emission decays of the DNA-intercalated Ru−NDI complex are also fit well with a triexponential model with lifetimes of 0.31 (0.79), 2.00 (0.13), and 11.8 (0.08) ns. Thus, the fractional amplitudes of the lifetimes change upon DNA intercalation of the complex, while the lifetimes themselves remain essentially the same. The average rates of electron transfer in aqueous buffer without and with DNA are, respectively, 1.6 × 109 and 6.8 × 108 s-1. The striking result of this study is that the overall character of electron transfer quenching in Ru−NDI is very similar whether or not it is bound to DNA. Intercalation of the NDI in DNA apparently has negligible consequences for electron transfer, implying either that the activation energy and electronic coupling in Ru−NDI are largely unaffected by this, at first glance, seemingly significant environmental change or that changes in these parameters on DNA binding cancel fortuitously.