The photoswitchable piperidine general base catalyst is a prototype structure for light control of catalysis. Its azobenzene moiety moves sterically shielding groups to either protect or expose the active site, thereby changing the basicity and hydrogen-bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor. Steady-state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within a few picoseconds. Thus, the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break a strong HB within 3 ps and to shut down access to the active site.

The photoswitchable piperidine general base catalyst is ap rototype structure for light control of catalysis.I ts azobenzene moiety moves sterically shielding groups to either protect or expose the active site,t herebyc hanging the basicity and hydrogen-bonding affinity of the compound. The reversible switching dynamics of the catalyst is probed in the infrared spectral range by monitoring hydrogen bond (HB) formation between its active site and methanol (MeOH) as HB donor.Steady-state infrared (IR) and ultrafast IR and UV/Vis spectroscopies are used to uncover ultrafast expulsion of MeOH from the active site within afew picoseconds.Thus,the force generated by the azobenzene moiety even in the final phase of its isomerization is sufficient to break as trong HB within 3psand to shut down access to the active site.Precise temporal and spatial control of (bio)chemical processes opens up new possibilities for applications.I n recent years,av ariety of molecular properties have successfully been rendered photoswitchable,f or instance secondary structure formation in peptides, [1] protein function [2] or force generation in molecular motors. [3] Ty pically,photocontrol occurs via irreversible bond cleavage or via reversible switching. [4] Theu se of light to irreversibly photodissociate and thereby activate ac aged compound allows the initiation of areaction at adefined point in time.U nfortunately the process induced by the initial cleavage cannot be reversed or shut down. This limitation is overcome by making the activation reversible and repeatable, utilizing geometric rearrangements of the molecular structure.T his approach has been used to photoswitch biological activity of proteins [4,5] and DNAtranscription. [6] Considerable efforts are being made to reversibly photoregulate catalysis, which is perhaps the most attractive function to control from ac hemistsp oint of view. [7] Ap rominent example is the previously developed azobenzene (AB) based supramolecular catalyst, [8] which brings two metal centers in proximity in the active state.A nother system is al ight-responsive cavitand [9] which binds and thereby activates the catalyst. Osorio-Planes et al. [10] recently reported regulation of catalytic activity via photoswitchable intramolecular hydrogen bonding that inhibits the active site.N ote that photoregulated catalysis (via uncaging or photoswitching) has to be distinguished from photocatalysis, [11] where ap hotoreaction is promoted by electronic excitation.Thep reviously developed catalyst under study (Figure 1A,blue structures), [12,13] shortly called "A zocat" (derived from AB-based catalyst), is aprototype system for conformational control of reactivity.This molecule catalyzes the Henry reaction, ab ase-catalyzed addition of nitroalkanes to aldehydes or ketones,for which reversible active-site accessibility has already been shown. [12,13] Other types of active centers can in principle be controlled in as imilar fashion.Azocat consists of four main structural moieties:t he active center, formed by a t...