Cluster structure of alcohols and their mixtures with aprotic H-accepting solvents

Fluorescent properties of symmetric electron-donor-acceptor compouds are currently in focus of extensive experimental and theoretical research, since kinetics and quantum yield of their fluorescence are highly sensitive to interactions with the environment. Of particular interest is the role of the hydrogen-bonding interactions between the fluorophore and solvent in liquids. Hydrogen bonds are known to be able to break the excited-state symmetry, leading to intramolecular charge transfer between two branches of the symmetric molecule and causing subsequent fast nonradiative deactivation of the fluorophore. Recent experimental studies on a centrosymmetric molecule, an acridine-dione derivative, revealed unexpected behavior of its time-dependent fluorescence pro- files in protic and aprotic solvents. The observed fluorescence did not exhibit any spectroscopic evidence of quenching in aprotic solvents (even highly polar), but manifested strong quenching in alcohols. In binary mixtures of protic (MeOH) and aprotic (DMF) solvents, on the other hand, the quenching was detected only at fairly high concentrations of methanol, [MeOH] > 9 mol/L. These results are unusual for this type of the reaction and require detailed analysis and explanation. In this study two models of linear clustering in alcohols and mixtures of alcohols with aprotic H-accepting cosolvents are considered. Analytical expressions for the H-bonding probabilities and the probabilities of including an alcohol molecule in cluster chains of various lengths are obtained. The influence of the mixture composition on the cluster structure is studied. The models considered can be used to analyze experimental data on fluorescence quenching by the mechanism of hydrogen-bond-induced nonradiative deactivation.