Ultrafast Light-Driven Substrate Expulsion from the Active Site of a Photoswitchable Catalyst

The azobenzene-containing 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 compound's basicity and hydrogen-bonding affinity. The catalyst's reversible switching dynamics is probed in the infrared spectral range by monitoring hydrogen-bond formation between its active site and methanol (MeOH) as hydrogen-bond 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 azobenzene even in the final phase of its isomerisation is sufficient to break a strong hydrogen bond within 3 ps and to shut down access to the active site.