Molecular confinement is a viable method to fine‐tune the reactivity of chemical species. Metal–organic cages, which have large void spaces within, are especially attractive entities for encapsulating various guests. Here, a water‐soluble flexible metal–organic cage was used to confine two guest molecules, as either homo‐ or heterodimers with a high uptake efficiency. Photo irradiation of the inclusion complexes results in a smooth photo 2 + 2 dimerization reaction in aqueous solution despite the flexibility of this metal–organic cage. The preorganization of two molecules within the cage was responsible for the smooth photo‐dimerization, both for the homodimer and heterodimer. Kinetic study of the photoreaction revealed a pseudo‐first‐order reaction, implying a preformed dimer of guests within the cage was responsible for the reaction.
Wang et al. (2026) studied this question.