The photoinduced E-Z isomerization of photochromic moieties such as diarylethenes and stilbenes can lead to a change in the steric environment of the active site, which can block/unblock substrate access or bring substrates closer/farther thus changing the catalytic activity. In 1995, Rebek and coworkers reported that an azobenzene was used as a photochromic functionality in a template molecule to control the rate of an amidation reaction (FIG. ). For the amidation between aminoadenosine and adenosine-derived p-nitrophenol ester, they designed a template molecule that contains two adenine receptors linked by an azobenzene spacer. When the template molecule is in the E configuration, substrates bound to each receptor are far apart, resulting in a slow coupling rate. Upon UV irradiation (λ = 366 nm), the template molecule undergoes a photo-induced isomerization, resulting in a photostationary state ratio of E:Z = 1:1. The Z configuration brings the two substrates in close proximity and thus accelerates the amidation (kfast/kslow = 8.8).