Molecular surface structure of 4,4 ',7,7 '-tetrachlorothioindigo crystal observed by atomic force microscopy

Molecular surface structures of the 4,4',7,7'-tetrachlorothioindigo crystal , which possessed the excellent photoconduction property, were studied with atomic force microscopy. On the (001) surface, the two-dimensional lattice was slightly enlarged compared with the bulk data, and two kinds of transl ationally inequivalent molecules could be distinguished in the image. By co mparison of the lattice of the monolayer, it was indicated that the molecul ar planes on the surface were more parallel than those expected from the bu ll;. structure and less parallel than those of the monolayer, and as a resu lt, the HOMO and LUMO that are delocalized in the indigoid part were more l ikely to be exposed on the surface. On the (110) surface, the two-dimension al lattice expected from the bulk structure almost remained intact on the s urface. It was also confirmed on the (110) surface that the molecular plane s in two kinds of the stacking columns were nearly parallel to the (110) an d that the molecules in one of the stacks were further rearranged in such a way that their molecular longer axes were inclined to the (110) surface. B oth the relaxation of the lattice on the (001) surface and the molecular re arrangements on the (110) surface suggested that the intermolecular interac tions with stacking neighbors along the c-axis strongly predominated over t he surface of the tetrachlorothioindigo crystal.