Ultrafast Light-Switch Properties of G-Quadruplex Binder: [Ru(phen)2(tpphz-DC3)]4+

Light switch probes, i.e., those that emit only on binding their target, are highly attractive in bioimaging as they provide for outstanding contrast, especially in interrogation of discrete nanoscale and dynamic distributed structures like G-quadruplex DNA. Here, we examine the photophysical properties of a G-Quadruplex (G4) selective probe comprising light switch Ru(II) dipyridylphenazine complex co-assembled with the well-known G4 selective ligand PDC3 [Ru(phen)₂PDC3]⁴⁺ (RuPDC3, using femtosecond and nanosecond transient absorption spectroscopy complemented by steady-state spectroscopy and spectro-electrochemistry. We compare the photophysics to a well-known photoswitch [Ru(bpy]₂(dppz)]²⁺Rudppz, and observe marked differences in behaviour. Whereas in Rudppz, the ³MLCT state is either localized on the phenanthroline or the phenazine unit, we show that in RuPDC3, this state has charge delocalised over the entire PDC3 unit. We then compare the ultrafast dynamics of this complex with Rudppz when associated with duplex and G4 DNA of different topologies and observe notable differences in trends in the exited-state dynamics in both RuPDC3 and Rudppz with G4 for the first time RuPDC3 shows greater variation in excited state dynamics with G4 topology and offers the prospect of elucidation of topology by ultrafast imaging.