Alkyl Chain Length‐ and Polymorph‐Dependent photomechanochromic fluorescence of anthracene photodimerization in molecular crystals: roles of the lattice stiffness

An anthracene-pentiptycene hybrid system ( 1-C n , where n refers to the number of carbon atom in the linear alkyl chain) crystalizes in three different polymorphs, called Y (yellow), G (green), and B (blue) forms in terms of the fluorescence color. While all the Y-form crystals perform the same yellow-to-blue fluorescence color responses to the photomechanical stress generated by the antrhacene [4+4] photodimerization reaction, the four G forms display distinct photomechanofluorochromism (PMFC): namely, from green to blue for G-1-C4 , to orange for G-1-C7 , to red for G-1-C8 , and to red then blue for G-1-C9 , and there is no photochromic activity for the B forms. The intriguing RGB three-color PMFC and the abnormal topochemical reactivity of G-1-C9 are attributed to an inherent softness of the crystal lattice.