[2 + 2] Cycloaddition of Halogen-Substituted Phenyl Cinnamate Derivatives: Single Crystal Structures, Stereospecificity, and Photomechanical Behaviors

A series of new halogen-substituted phenyl cinnamate derivatives were synthesized, and their single crystal structures were obtained. Most microcrystals of these compounds were able to undergo [2 + 2] cycloaddition reactions under UV light irradiation in the solid state based on ¹H NMR experimental data before and after the irradiation. The single crystal structure, Hirshfeld surface, energy decomposition analysis, energy framework, and molecular electrostatic potential surface study of these compounds disclosed that the hydrogen bond of the halogen···H, π···π interaction played an important role in the packing arrangement. In particular, the single crystal structure of the dimers D-FPCCl, D-FPCBr, D-BrPCBr, D-IPCCl, and D-IPCBr was also obtained, and IPCBr was the highest yield of monomer conversion to dimer after 10 min of irradiation, reaching 63%. It was interesting that the distance between the double bonds in the crystal of IPCCl was 5.872 Å, which was greater than 4.2 Å, but the [2 + 2] cycloaddition still occurred. Most importantly, none of these phenyl cinnamate derivatives have been found to have undergone [2 + 2] cycloaddition before, and these molecules are also reported for the first time. Furthermore, the needle-like crystal of IPCBr and BrPCCl exhibited photomechanical behaviors of swing under irradiation UV light of 365 nm.