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−I π 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−I π motif permits the systematic exchange of original cocrystal components with azobenzene and azulene, resulting in optically interesting dichroic or pleochroic materials. A robust architecture of C-I π halogen bonds enabled cocrystallisation of azulene as a functional component in the design of pleochroic behavior.