Temporary and permanent trapping of the metastable twisted conformer of an overcrowded chromic alkene via encapsulation.

An overcrowded alkene with an anti-folded conformation was converted to its twisted conformer, accompanied by a dramatic color change from yellow to deep purple, by inclusion in a self-assembled T(d)-symmetric coordination cage. The shape of the caged cavity was suitable and desirable for trapping of the twisted conformer. The twisted conformation was temporarily memorized in the alkene even after guest ejection. Permanent trapping of the twisted conformation was achieved by bromination of the twisted conformer formed in situ in the cage.