We provide a Cambridge Structural Database (CSD) analysis of the role of halogen bonds and halogen···halogen interactions in the solid-state structures of porphyrins. This survey provides a detailed view of halogens in porphyrin self-assembly, highlighting interaction motifs beyond the often considered iodine-based halogen bonds. Particularly notable are the observations of self-assembly through β-positioned substituents, the role of the pyrrole π-system as a halogen bond acceptor, and an abundance of chlorine-based halogen···halogen and halogen-bonding motifs. This overview enabled the observation and systematic classification of complex, multipoint recognition motifs in halogen-based architectures. Similarly to synthons in hydrogen-bonded crystal engineering, these motifs appear in discrete molecular assemblies and can also underlie the formation of two- and three-dimensional frameworks. The systematic categorization of these robust motifs provides a foundation for the design of complex halogen-based supramolecular architectures.

Carbon, although the central element in organic chemistry has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arenes, has almost exclusively been limited to π-stacking of aromatic systems, or derivatization with heteroatoms as sites for molecular recognition. Here, we demonstrate that C-I···Cπ halogen bonds to carbon-based π-systems can be reliably used as direction-al interactions for the creation of extended structures based on planar, as well as curved aromatic systems, without any need for derivatization or π-stacking. Specifically, we describe the first systematic study of a series of cocrystals containing non-derivatized carbon-only aromatic systems of different sizes and shapes, including polycyclic aromatic hydrocarbons (PAHs) and fullerene C60, which are held together by directional halogen bonds to aromatic carbon atoms. In a large majority (~90%) of structures, the C-I···Cπ halogen bonding with PAHs leads to a supramolecular ladder-like motif, in which the PAHs act as the rungs and halogen bond donors as rails, demonstrating this motif as the first example of a supramolecular synthon based on carbon. These results, supported by novel cocrystal structures, theoretical calculations, and a systematic analysis of the Cambridge Structural Database, offer a new, previously overlooked paradigm for the assembly of carbon-only aromatic systems, not based on π-stacking, but via specific, directional halogen bonding. This new ability to use a car-bon-based supramolecular synthon to direct the assembly aromatic systems provides an exciting opportunity to create materials with new and modified properties based on non-derivatized aromatic systems, as seen from large red and blue shifts in solid-state luminescence for cocrystals of pyrene, coronene and perylene, as well as the appearance of room-temperature phosphorescence upon cocrystal formation.