Steady-State and Time-Resolved Spectroscopic Study of [Ru(phen)₂(dpqa)]²⁺ Showing Enantiomer-Dependent DNA...

Ruthenium polypyridyl complexes are becoming increasingly important as probes for cellular investigations due to their luminescent properties and the possibility of exploiting the interaction with biomolecules due to the differing chirality of their enantiomers. In this study, a DNA light-switch metal complex comprising an intercalating polypyridyl ligand with an amide-linked pentyl chain [Ru(phen)₂(dpqa)]²⁺ (1) (dpqa = 2-pentylamidodipyrido[3,2-f:2',3'-h]-quinoxaline) that undergoes cell internalization is reported. The photophysical properties are dramatically impacted by the amide linker such that 1 is nonluminescent in aqueous media but strongly luminescent when bound to natural DNA. Transient visible absorption and time-resolved infrared measurements show that this is caused by the emissive MLCT excited state being rapidly deactivated in water (11 ps) to another nonemissive excited state with a characteristic infrared spectrum. In the presence of DNA, the delta enantiomer is about 9 times more emissive than the lambda. 1 is internalized by live HeLa cells, and luminescence is observed in hydrophobic pockets. In the presence of the polysorbate 80 nonionic surfactant (Tween 80), which is expected to permeabilize the cell and nuclear membranes, a strong enantiomer-dependent signal was seen from the nucleus.