Photomechanochemical control over stereoselectivity in the [2 + 2] photodimerization of acenaphthylene.

Tuning solubility and mechanical activation alters the stereoselectivity of the [2 + 2] photochemical cycloaddition of acenaphthylene. Photomechanochemical conditions produce the cyclobutane, whereas the solid-state reaction in the absence of mechanical activation provides the . When the photochemical dimerization occurs in a solubilizing organic solvent, there is no selectivity. Dimerization in HO, in which acenaphthylene is insoluble, provides the product. DFT calculations reveal that insoluble and solid-state reactions proceed a covalently bonded excimer, which drives selectivity. Alternatively, the noncovalently bound conformer is more mechanosusceptible than the , meaning it experiences greater destabilization, thereby producing the product under photomechanochemical conditions. Cyclobutanes are important components of biologically active natural products and organic materials, and we demonstrate stereoselective methods for obtaining or cyclobutanes under mild conditions and without organic solvents. With this work, we validate photomechanochemistry as a viable new direction for the preparation of complex organic scaffolds.