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.