Symmetry-breaking charge transfer in an excited acridine-dione derivative: Effects of hydrogen bonding, clustering, and cooperativity in mixtures of methanol and dimethylformamide

Abstract

A theory of fluorescence quenching of photoexcited centrosymmetric acridine-dione derivative induced by H-bonding interactions with solvent in mixtures of methanol (MeOH) and N,N-dimethylformamide (DMF) is developed. The theory takes into account both dynamic and static mechanisms of the quenching. An unusual threshold-type dependence of the quenching efficiency on the concentration of methanol in the mixture observed earlier in experiment is analyzed theoretically under the following assumptions: (i) individual MeOH molecules and their short hydrogen-bonded clusters form weak complexes with the dye in solution and these interactions do not affect fluorescence kinetics significantly, (ii) long clusters form strongly bound complexes with the dye and thus induce symmetry-breaking intramolecular charge transfer and efficient quenching of fluorescence. These assumptions are consistent with the cooperative character of H-bonding in alcohols and allow one to explain the observed absence of quenching at MeOH concentrations below ∼9 M. A model of H-bond clustering in mixtures of alcohols and H-bond-accepting solvents is developed. The suggested approach gives a quantitative description of the observed concentration dependence of the fluorescence yield and well reproduces the non-exponential kinetics of the fluorescence decay measured in experiments. Since the degree of the charge transfer dissymmetry in these processes is sensitive to interactions between the dye and solvent, new possibilities emerge for experimental studies on details of H-bond clustering in protic solvents.