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Small differences in molecular or solid-state structure can afford significant differences in properties. Here, a diene derivative, 1,3-bis((E)-2-bromostyryl)benzene (2Brm), is synthesized and crystallized into two unique solid-state forms, each exhibiting a different π–π stacking geometry, which imparts distinct reactivity and photoresponsivity. Upon exposure of the two solids to UV–Vis light, a [2 + 2] photocycloaddition occurs to afford regioisomeric products due to the difference in the stacking geometries of the dienes. From a single molecular precursor, we further demonstrate that under different wavelengths of light, the chemical functionality can be programmed into discrete and distinct products containing one, two, or three cyclobutane rings as well as oligomeric/polymeric products. Moreover, the two initial solid forms exhibit wavelength-dependent photomechanical behaviors (i.e., photosalience). This work demonstrates a rare, template-free, self-assembly-based strategy that enables access to a suite of discrete and oligomeric/polymeric products via regiocontrolled solid-state photocycloadditions and further presents potential routes toward the design of photoactuating materials. 

The thermal and thermomechanical behavior of multi-component organic solids is tuned at the molecular levelviapreparation of mixed cocrystals. 

Abstract Achieving substantial anisotropic thermal expansion (TE) in solid‐state materials is challenging as most materials undergo volumetric expansion upon heating. Here, we describe colossal, anisotropic TE in crystals of an organic compound functionalized with two azo groups. Interestingly, the material exhibits distinct and switchable TE behaviors within different temperature regions. At high temperature, two‐dimensional, area zero TE and colossal, positive linear TE (α=211 MK⁻¹) are attained due to dynamic motion, while at low temperature, moderate positive TE occurs in all directions. Investigation of the solid‐state motion showed the change in enthalpy and entropy are quite different in the two temperature regions and solid‐state NMR experiments support motion in the solid. Cycling experiments demonstrate that the solid‐state motions and TE behaviors are completely reversible. These results reveal strategies for designing significant anisotropic and switchable behaviors in solid‐state materials.