Closing the Ring to Bring Up the Light: Synthesis of a Hexacyclic Acridinium Cyanine Dye

The synthesis of a geometrically constrained and near‐planar hexacyclic acridinium cyanine dye 9 is reported. When compared to its unlocked and non‐fluorescent monomethine cyanine dye analogue 3 , this photostable dye emits in the green area of the spectrum with a remarkable quantum yield close to unity in organic solvents and above 0.5 in water. A detailed steady‐state and time‐resolved spectroscopic study revealed that dye 9 forms emissive aggregates in water, which are responsible for a red‐shifted and broadened emission band and longer emission lifetime, τ≈33 compared to 6.5–7.0 ns for the monomeric dye. Dye 9 also binds strongly to DNA (both duplex and quadruplex) in its monomeric form and is very efficiently taken up by cells, in which it accumulates primarily into the nucleus.