Ion-Responsive Fluorescent Compounds, 2. Cation-Steered Intramolecular Charge Transfer in a Crowned Merocyanine

Article abstract of DOI:10.1021/j100346a099

The dimethylamine group of DCM (4-dicyanomethylene-2-methyl-6--4H-pyran), a well-known merocyanine often used as a laser dye, has been replaced by a macrocycle (monoaza-15-crown-5) which acts also as an electron donating substituent.The resulting fluoroionophore (DCM-crown) has almost identical photophysical properties as DCM, but upon complexation by alkaline-earth-metal-cations, it undergoes drastic changes in absorption spectrum (hypsochromic shift and hypochromic effect) and fluorescence quantum yield (quenching), whereas the emission spectrum is only slightly blue-shifted and the fluorescence lifetime is almost unchanged.These effects which strongly depend on the charge density of the cation can be interpreted in terms of cation-steered reduction of the efficiency of tranfer from a nonemissive locally excited state to an emissive relaxed intramolecular charge-transfer state (RICT).