Programmable photoresponsive materials based on a single molecule via distinct topochemical reactions

Engineering the preorganization of photoactive units remains a big challenge in solid-state photochemistry research. It is of not only theoretical importance in construction of topochemical reactions but also technological significance in fabrication of advanced materials. Here, a cyanostilbene derivative, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(naphthalen-2-yl) acrylo-nitrile (BNA), was crystallized to two polymorphs under different conditions. The two crystals, BNA-α and BNA-β, have totally different intra-π-dimer and inter-π-dimer hierarchical architectures on the basis of a very simple monomer, which endows them with distinct reactivities, functions and photoresponsive properties. Firstly, two different types of solid-state [2+2] photocycloaddition reaction: (i) a typical olefin-olefin cycloaddition within the symmetric π-dimers of BNA-α and (ii) an unusual olefin-aromatic ring cycloaddition within the offset π-dimers of BNA-β have been observed, respectively. Secondly, the crystal of BNA-α can be bent to 90° without any fractures, exhibiting outstanding flexibility upon UV irradiation. While, the reversible photocycloaddition/thermal cleavage process (below 100 °C) accompanies with unique fluorescence changes can be achieved in crystal of BNA-β. Finally, the micro-scale photoactuators and the light-writable anti-counterfeiting materials have been successfully fabricated. This work paves a simple way to construct smart materials through a bottom-up way that is realized by manipulating hierarchical architectures in the solid state.