Os(phen)_2dppz^(2+) in Photoinduced DNA-Mediated Electron Transfer Reactions

Abstract

The photoinduced electron transfer chemistry between Os(phen)_2dppz^(2+) and Rh(phi)_2bpy^(3+) bound to DNA has been characterized. Os(phen)_2dppz^(2+) serves as an isostructural analogue for Ru(phen)_2dppz^(2+) with a red-shifted emission spectrum, access to a 3+ oxidation state which is stabilized by ∼500 mV relative to the ruthenium complex, and excited-state lifetimes below 10 ns in the presence of DNA. Emission from Δ-Os(phen)_2dppz^(2+) bound to calf thymus DNA is efficiently quenched by Δ-Rh(phi)_2bpy^(3+), and a lower limit for the quenching constant is set at 7 × 10^9 s^(-1). The quenching profile over a range of quencher concentrations is found to be remarkably similar to that of the ruthenium analogue, despite an increase of ∼200 mV in ΔG for the photoinduced, forward electron transfer reaction. Such an observation may indicate the importance of the HOMO energy in the donor excited state, which is similar for both donors. Owing to the lack of spectral overlap between Os(phen)_2dppz^(2+) emission and Rh(phi)_2bpy^(3+) absorption, energy transfer does not contribute to the observed quenching, and therefore, on the basis of the similarity in quenching profiles for the osmium and ruthenium donors, we can also rule out energy transfer in the photoinduced quenching of intercalated Ru(phen)_2dppz^(2+) by Rh(phi)_2bpy^(3+). Moreover, diffusional processes are found not to contribute to quenching, since the faster intrinsic excited state of the osmium complex compared to ruthenium does not lead to a reduction in quenching efficiency. Transient absorption measurements on the microsecond time scale furthermore reveal a transient signal for this electron transfer process, and this transient intermediate has been assigned to the oxidized donor (Os(III)) on the basis of full spectral characterization and comparison to chemical oxidation of Os(II).