MOLECULAR SELF-ASSEMBLIES .3. QUANTITATIVE PREDICTIONS FOR THE PACKING GEOMETRY OF PERYLENEDICARBOXIMIDE TRANSLATION AGGREGATES AND THE EFFECTS OF FLEXIBLE END-GROUPS - IMPLICATIONS FOR MONOLAYERS AND 3-DIMENSIONAL CRYSTAL-STRUCTURE PREDICTIONS

The one-dimensional aggregate-packing geometry of 22 perylene dicarbox imide pigment molecules consisting of the same rigid chromophore with differing end groups (but taken to be rigid) has been determined using Monte Carlo Cooling techniques to generate local energy minima. The f orce field used was a nonbonded atom-atom potential taken from MM2 plu s a Coulomb electrostatic term with a distance-dependent dielectric co nstant. The results compare very favorably with experimentally known g eometries determined by Klebe et al. In all but four cases there is a local minimum structure which is close to the experimental structure n ot more than 5 kcal above the apparent global minimum. The number of d istinctly different geometries within 5 kcal of the global is never mo re than nine for each molecular aggregate. The electrostatic energy co ntribution to the total energy is less than 4%. The packing geometry i s thus determined by the nonbonded energy term with changes in the sub stitution pattern on the ends of the molecules having considerable inf luence on the overall packing. Inclusion of internal variables (torsio n variables) for the end groups in the Monte Carlo simulation has litt le effect on the local minima determined by the external variables. Th e results suggest an aufbau principle which we call KAP (Kitaigorodski i's aufbau principle) whose significance for the prediction of monolay er geometries and full three-dimensional crystal geometries of organic molecular self-assemblies with flexible groups is discussed.