A Naked-Eye On–Off–On Molecular “Light Switch” Based on a Reversible “Conformational Switch” of G‑Quadruplex DNA

Herein, we report a new strategy for developing an on–off–on molecular “light switch” by utilizing the pH value to control the “conformational switch” of G-quadruplex DNA. A novel ruthenium(II) complex with an emission enhancement factor of 150 was synthesized and introduced to detect the switch by the naked eye. The “light switch” can be repeatedly cycled off and on through the addition of H+ and OH–, respectively. The conformational transitions of G-quadruplex DNA in K+ solution at different pH values in the acidic region were evidenced by circular dichroism and fluorescence titrations. Computational calculations by applying density functional theory (DFT)/time-dependent DFT and molecular docking were also carried out to gain insight into the “light-switch” mechanism.