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

Abstract

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.

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

“…In this work, we use parallel tempering REMD simulations at the DFTB3 level − to investigate the FES and the corresponding configurational and conformational behavior of an early example of photoreversible steric shielding, namely the azobenzene-tethered N-alkylated piperidine base catalysts developed by Hecht and co-workers (Scheme ). − This elegantly designed system consists of a piperidine Brønsted base and a photochromic azobenzene moiety that are rigidly and orthogonally positioned through the spiro junction of an isobenzofuranone ring. In the thermodynamically more stable ( E )-isomer, the 3,5-substituents on the phenylazo unit shields the basic site; UV irradiation affords the metastable ( Z )-isomer and exposes the nitrogen lone pair, enhancing the basicity by almost one order of magnitude and the rate of the Henry reaction , between nitroethane and p -nitrobenzaldehyde.…”

Twist and Return−Induced Ring Strain Triggers Quick Relaxation of a (Z)-Stabilized Cyclobisazobenzene

“…Azobenzene (AB) derivatives represent an important family of molecular photoswitches that are extensively used for spatiotemporally precise and reversible control of a variety of nanostructures − and reactions , of interest to chemistry and biology. Furthermore, ABs are used as building blocks in larger aggregates to form photoresponsive materials. − One of the main application advantages of AB photoswitches is the substantial geometrical change (molecular length change of ∼3.4 Å) associated with the E ↔ Z photoinduced isomerization of the central NN bond.…”

Photoswitchable catalysis based on the isomerisation of double bonds

“…In a more recent study, the hydrogen bond formation ability of 16 was monitored in the infrared spectral range using MeOH as the hydrogen donor, which was used as a probe of the accessibility to the active site of the catalyst. 27 The force generated by the azobenzene in the isomerisation process is enough to extrude MeOH from the active site within just a few picoseconds, thereby preventing access to the binding site. A related switchable piperidine base was also immobilised on silica gel, 28 although its use as a catalyst has not yet been described.…”