Influence of intermolecular interactions on the structure of phthalocyanine layers in molecular thin film heterostructures

Free-base phthalocyanine $\left({\mathrm{H}}_2\mathrm{Pc}\right)$ molecules have been shown to stack layer by layer when deposited on a plane perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) layer, a structure very different from the α-herringbone structure formed in the bulk or when ${\mathrm{H}}_2\mathrm{Pc}$ is deposited at room temperature on noninteracting substrates such as glass. In this paper theoretical studies have been carried out to rationalize this structural modification using van der Waals intermolecular interaction energy calculations. In the case of bulk ${\mathrm{H}}_2\mathrm{Pc}$ unit cells, the α-herringbone structure is more stable (by ∼6%) than the planar layered structure, consistent with the existence of the α-herringbone structure when ${\mathrm{H}}_2\mathrm{Pc}$ is grown on noninteracting substrates. For ${\mathrm{H}}_2\mathrm{Pc}$ unit cells on a PTCDA layer, however, the planar layered structure is more stable (by ∼9%) in agreement with the experimental observations of a modified ${\mathrm{H}}_2\mathrm{Pc}$ structure due to templating. At the energy minimum, the interplanar stacking distance of the planar layered ${\mathrm{H}}_2\mathrm{Pc}$ is calculated to be 3.29 Å, in good agreement with the experimentally determined value of 3.33 Å. The calculations indicate that the structural modification in the double layer heterostructure is due to the strong intermolecular interactions between the two layers at the heterointerface.