Effects of bay substituents on the racemization barriers of perylene bisimides: resolution of atropo-enantiomers.

The activation parameters for the interconversion of atropisomers (P- and M-enantiomer) of core-twisted perylene bisimides have been determined by dynamic NMR spectroscopy (DNMR) and time- and temperature-dependent CD spectroscopy. By comparing the activation parameters of a series of perylene bisimides containing halogen or aryloxy substituents in the bay area (1,6,7,12-positions), a clear structure-property relationship has been found that demonstrates that the kinetic and thermodynamic parameters for the inversion of enantiomers are dependent on the apparent overlap parameter Sigmar* of the bay substituents. This study reveals a high stability (DeltaG(368 K) = 118 kJ/mol) for the atropo-enantiomers of tetrabromo-substituted perylene bisimide in solution. Accordingly, the enantiomers of this derivative could be resolved by HPLC on a chiral column. These enantiomers do not racemize in solution at room temperature and, thus, represent the first examples of enantiomerically pure core-twisted perylene bisimides.