Published in “Chemical Communications” (Online Publication, January 18, 2022).

Organic thin films consisting of two-dimensional functional molecules are the key to functional materials such as organic thin-film solar cells, and the control of molecular orientation in the films at our will is the key to the free control of physical properties. Thus far, hydrogen bonding and dispersive forces have mainly been used for the spontaneous two-dimensional assembly of molecules. The inherent cohesive forces of molecules alone, however, have limitations in controlling the orientation, and in particular, it has extremely been difficult to achieve edge-on orientation, in which only the short axis of the polycyclic aromatic ring is oriented perpendicular to the substrate. To achieve the edge-on orientation, the π-π interaction has been incorporated into the interaction between the substrate surface and the thin film, and the substrate surface has been coated with graphene etc. However, to realize edge-on orientation in a large area at low cost, it is desirable to be able to fabricate it on an uncoated substrate.

In the present study, we have first realized fabrication of the edge-on orientation of tetrapyridyl porphyrin on an uncoated silicon substrate by using strong intermolecular interaction through the coordination bonds. In this study, we synthesized compounds in which the central metal of the porphyrin was systematically changed among divalent Fe, Co, Ni, and Cu ions, and investigated whether the strength of the coordination bond would be controlled by systematically changing the number of electrons in the orbitals responsible for the coordination bond, thereby controlling the acceptability of the unpaired electrons of the nitrogen of the pyridyl group. As a result, it was quantitatively demonstrated by Grazing Incidence X-ray Diffraction (GIXD) and Multiple-Angle Incidence Resolution Spectrometry (MAIRS) that the desired edge-on orientation was achieved when Fe2+ was at the center, which is the most receptive metal.

Furthermore, we found that the molecules lose their orientation while maintaining their crystalline nature when the central metal is Co2+, and that they become face-on oriented while changing their crystal polymorphs when the central metal is Ni2+ or Cu2+. These changes in crystal polymorphism and orientation can be explained by the electronic structure of the metal, and we have succeeded a new way to control the structure of thin films.

Tomita, K.; Shioya, N.; Shimoaka, T.; Wakioka, M.; Hasegawa, T., Control of Supramolecular Organizations by Coordination Bonding in Tetrapyridylporphyrin Thin Films, Chem. Commun. (doi: 10.1039/d1cc06169k) (2022).