Orientational transition of tin phthalocyanine molecules in the self-assembled monolayer

The molecular orientations and self-assembled arrangements of organic molecules determine the properties of thin films. Here we report the orientational transitions of tin phthalocyanine (SnPc) molecules in the self-assembled structures on Cd(0 0 0 1) surfaces. In the monolayer regime, the low-temperature deposited SnPc molecules form a close-packed phase and ordered arrays of heptamers as well as an ultra-dense-packed monolayer. In particular, all molecules in the close-packed phase are flat-lying, and one seventh of the molecules in the heptamer are tilted, while all molecules in the ultra-dense-packed phase are tilted. These results demonstrate that the specific orientations of SnPc depend on the competition between intermolecular and molecule-substrate interfacial interactions.