Luminescence quenching by photoinduced charge transfer between metal complexes in peptide nucleic acids

A new scaffold for studying photoinduced charge transfer has been constructed by connecting a [Ru(Bpy)3](2+) donor to a bis(8-hydroxyquinolinate)2 copper [CuQ2] acceptor through a peptide nucleic acid (PNA) bridge. The luminescence of the [Ru(Bpy)3](2+*) donor is quenched by electron transfer to the [CuQ2] acceptor. Photoluminescence studies of these donor-bridge-acceptor systems reveal a dependence of the charge transfer on the length and sequence of the PNA bridge and on the position of the donor and acceptor in the PNA. In cases where the [Ru(Bpy)3](2+) can access the π base stack at the terminus of the duplex, the luminescence decay is described well by a single exponential; but if the donor is sterically hindered from accessing the π base stack of the PNA duplex, a distribution of luminescence lifetimes for the donor [Ru(Bpy)3](2+*) is observed. Molecular dynamics simulations are used to explore the donor-PNA-acceptor structure and the resulting conformational distribution provides a possible explanation for the distribution of electron transfer rates.