Light-driven reversible ring contraction and expansion to modulate strain, conformation, and reactivity

Chemists utilize cyclic constraints in molecules to control conformation, shape, and reactivity. The strain introduced in the (macro)cycles is released during reactions and drives transformations ranging from strain-promoted in vivo ligation to ring-opening metathesis polymerization. However, in each case, the ring size needs careful optimization and cannot be (re)adjusted. For remote optical modulation of strain, we designed looped diarylethene photoswitches that undergo reversible ring contraction/expansion upon electrocyclic ring closure/opening. Investigating a homologous series, we discovered that the long-wavelength absorption of the closed isomer serves as a diagnostic tool for stored molecular strain. By incorporating an internal allene reactive group in the loop, we enhanced its reactivity in a [3+2] dipolar cycloaddition with ethyl diazoacetate under visible light. Quantum chemical calculations helped rationalize the experimentally observed size-dependent photochemistry and reactivity of the macrocycles. Our approach opens opportunities for optical spatiotemporal reactivity control in life and materials science applications.