A novel water-soluble quinoline-indole derivative as a three-photon fluorescent probe for identifying nucleolus RNA and mitochondrial DNA.

We present the design and synthesis of water-soluble quinoline-indole-based derivatives (IM-1, IM-2, and IM-3) with three-photon absorption activity. IM-3 can specifically target DNA and RNA accompanied by an obvious three-photon fluorescence enhancement in the second near-infrared window (1000-1700 nm). The in vitro experiments demonstrate that IM-3 can simultaneously stain mitochondria and the nucleolus both in living and fixed cells. The organelle-specific targeting behaviour is successfully visualized under stimulated emission depletion (STED) nanoscopy.