Intramolecular Fluorescence Quenching of Crowned 7-Aminocoumarins as Potential Fluorescent Chemosensors

The effects of the nature of solvent, temperature and complex formation with alkali and alkaline-earth metal cations, as well as protonation, on the efficiency and the kinetics of fluorescence of 3-azacrowned 7-diethylaminocoumarins have been studied. For the crown-ethers under investigation, the ratio of a dipole moment to the radius of Onsager cavity delta micro/rho is a constant value, and a macrocycle does not affect delta micro, and rho. The fluorescence of coumarin 1 in acetonitrile is quenched by an electron donor, triethylamine, with the Stern-Volmer constant being equal to (0.474+/-0.009) M(-1). The decrease in coumarin 1 fluorescence quantum yield upon the introduction of N-alkylazacrown moiety into position 3 is caused by an intramolecular photoinduced electron transfer from the nitrogen atom of macroheterocycle to the coumarin moiety, where the excitation is localized. The fluorescence quenching has an activation energy 2.32+/-0.05 kcal/mol in various hydrocarbons, and does not depend on the solvent viscosity. The fluorescence kinetics of free crowned coumarins in methanol is not monoexponential because of the existence of macrocycle conformers, or because of the hydrogen bond complex formation between the solvent and the nitrogen atom of macrocycle, in which the efficiency of intramolecular electron transfer is different. Upon complex formation with alkali and alkaline-earth metal cations and upon protonation, the fluorescence quantum yield increases and fluorescence decay becomes monoexponential.