2011 Volume 36 Issue 2 Pages 141-144
We investigated the two-dimensional polymorphisms in organized molecular films of dialkylammonium-M bis(1, 3-dithiole-2-thione-4, 5-dithiolate) (abbrev. (dmit)2, M = Ni) charge-transfer complexes using the surface pressure-area (π-A) isotherm, in-plane and out-of plane X-ray diffractions (XRD). Out-of plane and in-plane XRD measurements revealed that transition of molecular arrangement in layer structure along the c-axis and packing mode of molecules in ab-plane were occurred in organized film, respectively. In addition, arrangement of metal(dmit)2 unit in complexes were changed from a high density packing to a low density packing in two-dimensional plane with decrease of subphase temperature. These structural changes appear to be caused by weakening of π-π interaction between metal(dmit)2 units based on decrease of mobility. These results were associated with expansion behavior of monolayer on the water surface at low temperature. On the other hand, metal(dmit)2 units of complex having longer alkyl chain form the low density packing at almost all temperature region. As a reason of this phenomenon, enhancement of van der Waals interaction between alkyl chains with increase of number of carbons is suggested. Consequently, these two-dimensional polymorphic behaviors are based on the competition between van der Waals interaction of alkyl chain and π-π interaction between functional groups.
