Mechanisms of fluorescence quenching of aromatic molecules by potassium iodide and potassium bromide in methanol–ethanol solutions

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Abstract
Fluorescence quenching of 1-cyanonaphthalene, 2-cyanonaphthalene, 1-methoxynaphthalene, 2-methoxynaphthalene, and fluoranthene by iodide and bromide ions in methanol–ethanol solutions was measured between 293 and 333 K. The dependences of the quenching rate coefficients on the Gibbs free energy (ΔG) of the electron-transfer reaction and temperature are analysed on the basis of: (i) Rips–Jortner and Onuchic theories of electron-transfer reactions in fluid solutions, (ii) numerical evaluation of diffusion limitation of the rate coefficient. The results show clearly that the mechanisms of the fluorescence quenching of aromatic molecules by halide ions are different for negative ΔG(electron-transfer mechanism) and positive ΔG(heavy atom effect). For ΔG < – 0.25 eV, the activation energies of the quenching process show dependences that can be modelled using the Onuchic formula for the electron-transfer rate and the diffusion model for molecular transport.