Guest Signaling Compound:  trans-3,3‘-Bis(diphenylhydroxymethyl)azobenzene

A chromogenic azobenzene compound, ( E )-(3,3‘-(diazene-1,2-diyl)bis(3,1-phenylene))bis(diphenylmethanol), forms lattice inclusion compounds with a range of common solvents. It is yellow in the unsolvated crystalline form, but orange or orange-red when crystallized as solvates with guest molecules that contain hydrogen-bond acceptors. Close examination of crystal structures of unsolvated and solvated forms reveals that differences in hydrogen bonding and a change in conformation of the azobenzene core are responsible for the change in λ of absorption of the chromophore and hence color of the crystals. Hirshfeld surfaces and fingerprint plots reveal subtle differences in intermolecular interactions in the solvated and unsolvated crystalline forms.