Programmable Solid-State [2 + 2] Photocycloadditions of Dienes Directed by Structural Control and Wavelength Selection.

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.