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)](PF ₆ ) 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 G-quadruplexes, 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)](PF ₆ ) ₂ (2, dppz = dipyrido[3,2- a:2',3'- c]phenazine), a ³ ππ* state localized on the π-extended ligand is the state determining the excited-state properties in both protic and aprotic environments. In aprotic environments, emission originates from a bright ³ MLCT _phen state, which is thermally accessible from the ³ ππ* state at ambient temperature. In the presence of water, i.e., in environments resembling in cellulo situations, the thermally accessible ³ MLCT state is altered and becomes close in energy to the ³ ππ* state, which induces a rapid excited-state deactivation of the ³ ππ* state and a comparably weak emission.