Rotational dynamics of pyrrolopyrrole derivatives in glycerol: A comparative study with alcohols

The rotational dynamics of two structurally similar nonpolar molecules, 2,5-dimethyl-1,4dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) has been studied in glycerol in the temperature range of 300-380 K using both time-resolved and steady-state fluorescence depolarization techniques. While the reorientation times of both the probes are varying linearly as a function of viscosity over temperature, the rotational dynamics of DMDPP is described by the Stokes-Einstein-Debye hydrodynamic theory with slip boundary condition, whereas the reorientation times of DPP are in between slip and stick limits and are about a factor of 1.5 longer than that of DMDPP. This is due to the hydrogen bonding between the two NH groups of the probe molecule and the oxygen atoms of the hydroxyl groups in glycerol. It has also been observed that the rotational dynamics of a nonpolar and noninteracting molecule like DMDPP is essentially the same, both in glycerol and in n-alcohols. (C) 1999 American Institute of Physics. [S0021-9606(99)51445-1].