Halogen bonding to the azulene π-system: cocrystal design of pleochroism.

We describe the rational development of a design for pleochroic molecular solids, by using C-Iπ halogen bonds to pre-organise different chromophores in a cocrystal. Using the structure of a known naphthalene cocrystal as a blueprint, we demonstrate how a highly robust C-Iπ motif permits the systematic exchange of original cocrystal components with azobenzene and azulene, resulting in optically interesting dichroic or pleochroic materials.