Rotational dynamics of pyrrolopyrrole derivatives in alcohols: Does solute–solvent hydrogen bonding really hinder molecular rotation?

Abstract

Rotational reorientation times of two structurally similar nonpolar molecules, 2,5dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) have been measured in n-alcohols using steady-state fluorescence depolarization technique. While both DMDPP and DPP contain two C=O groups, the latter also has two NH groups. As these groups are known to form hydrogen bonds with alcohol solvents, the present work is aimed at finding out whether or not such solute–solvent hydrogen bonding is effecting the rotation of the probe molecules. The rotational dynamics of DMDPP is explained reasonably well by the Stokes–Einstein–Debye (SED) hydrodynamic theory with slip boundary condition. It is also found that the hydrogen bonding between the two C=O groups of the probe and the solvent molecules is not influencing the rotation of DMDPP. However, the reorientation times of DPP are found to be longer by a factor of 2.2 to 3.3 compared to that of DMDPP, and followed a super-stick behavior which has been observed for the first time for a nonpolar solute molecule. This is due to the strong hydrogen bonding between the two NH groups of the probe, and the alcoholic solvent molecules.