Photochromic Crystals: Toward an Understanding of Color Development in the Solid State

Intermolecular interactions between fluorene groups of propargylallenes define the solid state
photochromic response of these materials. Examination of crystal structures of both photoresponsive and
nonresponsive forms (including solvates) indicates that all compounds that are photochromic in the solid state exhibit
donor/acceptor π···π interactions of fluorene aromatic systems that lead to infinite 1-D ribbons. Such interactions
are absent in nonphotochromic crystalline forms.