Photophysics of a Ruthenium Complex with a π-Extended Dipyridophenazine Ligand for DNA Quadruplex Labeling

The light-switch mechanism of the complex [Ru(bpy)(2)(Br-dpqp)](PF6)(2) (1, bpy = 2,2'-bipyridine, Br-dpqp = 12-bromo-14-ethoxydipyrido[3,2-a:2',3'-c]quinolino[3,2-h]phenazine), i.e., a light-up probe for the selective labeling of Gquadruplexes, is investigated by time-resolved transient absorption and emission spectroscopy. We show that, in contrast to the prototypical light-switch complex [Ru(bpy)(2)(dppz)](PF6)(2) (2, dppz = dipyrido[3,2-a:2',3'-c]phenazine), a (3)pi pi* state localized on the pi-extended ligand is the state determining the excited-state properties in both protic and aprotic environments. In aprotic environments, emission originates from a bright (MLCTphen)-M-3 state, which is thermally accessible from the (3)pi pi* state at ambient temperature. In the presence of water, i.e., in environments resembling in cellulo situations, the thermally accessible (MLCT)-M-3 state is altered and becomes close in energy to the (3)pi pi* state, which induces a rapid excited-state deactivation of the (3)pi pi* state and a comparably weak emission.