Molecular stacking in epitaxial crystals of oxometal phthalocyanines

Molecular stacking structures in epitaxial crystals of oxotitanium and oxovanadium phthalocyanines (TiOPc and VOPc) prepared by physical vapor deposition (PVD) and molecular beam epitaxy (MBE), respectively, onto the (001) face of a KBr substrate were investigated by means of x-ray and electron diffraction studies. In thin epitaxial film of VOPc deposited by MBE at a substrate temperature of 20 °C, the flat-lying molecules oriented in the $\text{(3×3)}$ commensurate square lattice $\text{(a=1.40\hspace{0.17em}}\mathrm{nm})$ were alternately stacked with body-centered head-to-head and $\text{a/4}$ -slipped face-to-face dimeric bilayers. The crystal structure determined was the triclinic space group $\text{P1̄}$ with $\text{a=b=1.40,}$ $\text{c=1.06\hspace{0.17em}}\mathrm{nm},$ $\text{α=109.2,}$ $\text{β=133.9,}$ $\text{γ=90.0°,}$ $\text{Z=2.}$ When TiOPc was thickly deposited by PVD at a substrate temperature of 200 °C, islandlike crystals epitaxially grew in a truncated pyramidal morphology and assumed the same orientation. In the pyramidal crystals the thick molecular layer stacking separate from the KBr surface was relaxed and had a square lattice with a shorter constant $\text{(a=1.33\hspace{0.17em}}\mathrm{nm}\mathrm{).}$ The three-dimensional stacking structure explained the observed grain boundary formation and asymmetric pyramidal morphology.