Trace mild acid-catalysedZ→Eisomerization of norbornene-fused stilbene derivatives: intelligent chiral molecular photoswitches with controllable self-recovery

Abstract

Stilbene derivatives have long been known to undergo “acid-catalyzed”Z→Eisomerization, where a strong mineral acid at high concentration is practically necessary. Such severe reaction conditions often cause undesired by-reactions and limit their potential application. Herein, we present a trace mild acid-catalyzedZ→Eisomerization found with stilbene derivatives fused with a norbornene moiety. By-reactions, such as the migration of the C-C double bond and electrophilic addition reactions, were completely inhibited because of the ring strain caused by the fused norbornene component. Direct photolysis of theEisomers at selected wavelengths led to theE→Zphotoisomerization of these stilbene derivatives and thus constituted a unique class of molecular switches orthogonally controllable by light and acid. The catalytic amount of acid could be readily removed, and theZ→Eisomerization could be controlled by turning on/off the irradiation of a photoacid, which allowed repeated isomerization in a non-invasive manner. Moreover, theZisomer produced by photoisomerization could spontaneously self-recover to theEisomer in the presence of a catalytic amount of acid. The kinetics ofZ→Eisomerization were adjustable by manipulating catalytic factors and, therefore, unprecedented molecular photoswitches with adjustable self-recovery were realized.