Photoresponsive synthetic receptors: binding properties and photocontrol of catalytic activity
The photoresponsive ditopic receptors 1a,b bearing two adenine binding sites linked by an azobenzene moiety were prepared. Photochromism, binding of mono- and bis-adenine derivatives and template effects on the coupling reaction of aminoadenosine 2 and the adenosine-derived active ester 3 were studied. Photoswitching of the azobenzene linker from the extended E to the folded Z isomer enhances the catalytic activity of the template 1a by a factor of 50. The binding studies provide evidence for the formation of termolecular complexes 1·2·3 and allow for mechanistic considerations. The influence of various adenine derivatives on the rate of the thermal Z→E isomerization of (Z)-1a, b was evaluated and a significant stabilization of the Z isomers by the bis-adenine guest 4 was observed.