Multicolor Fluorescence of a Styrylquinoline Dye Tuned by Metal Cations

A styrylquinoline dye with a dipicolylamine (DPA) moiety (1) has been synthesized. The dye 1 in acetonitrile demonstrates multicolor fluorescence upon addition of different metal cations. Compound 1 shows a green fluorescence without cations. Coordination of 1 with Cd2+ shows a blue emission, while with Hg2+ and Pb2+ exhibits yellow and orange emissions, respectively. The different fluorescence spectra are due to the change in intramolecular charge transfer (ICT) properties of 1 upon coordination with different cations. The DPA and quinoline moieties of 1 behave as the electron donor and acceptor units, respectively, and both units act as the coordination site for metal cations. Cd2+ coordinates with the DPA unit. This reduces the donor ability of the unit and decreases the energy level of HOMO. This results in an increase in HOMO-LUMO gap and blue shifts the emission. Hg2+ or Pb2+ coordinate with both DPA and quinoline units. The coordination with the quinoline unit decreases the energy level of LUMO. This results in a decrease in HOMO-LUMO gap and red shifts the emission. Addition of two different metal cations successfully creates intermediate colors; in particular, the addition of Cd2+ and Pb2+ at once creates a bright white fluorescence.