Rotational diffusion of dihydroxy coumarins: Effect of OH groups and their relative position on solute-solvent interactions.

The rotational dynamics of 6,7- and 5,7-dihydroxy-4-methylcoumarin in a series of linear alcohols have been studied by time-resolving their fluorescence anisotropy decay with the frequency up-conversion method. Through estimations of their rotational diffusion coefficients in a series of linear alcohols, it was verified that these two coumarins keep nearly the same hydrodynamic contributions to friction, which accounts for only about 35% of the observed reorientational times. Whereas the former compound has the two -OH groups bonded to adjacent carbon atoms in the aromatic frame, in the latter compound, the two hydroxyl groups are separated by enough space to develop more stable interactions involving a network of several solvent molecules. These findings show that this structural difference results in significantly slower rotational relaxation for the 5,7-dihydroxylated coumarin as a result of specific hydrogen-bonding networks as determined at B3LYP/6-311G(d,p) level of theory.