Trace mild acid-catalysed Z → E isomerization of norbornene-fused stilbene derivatives: intelligent chiral molecular photoswitches with controllable self-recovery

Stilbene derivatives have long been known to undergo acid-catalyzed Z -> E isomerization, 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-catalyzed Z -> E isomerization 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 the E isomers at selected wavelengths led to the E -> Z photoisomerization 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 the Z -> E isomerization could be controlled by turning on/off the irradiation of a photoacid, which allowed repeated isomerization in a non-invasive manner. Moreover, the Z isomer produced by photoisomerization could spontaneously self-recover to the E isomer in the presence of a catalytic amount of acid. The kinetics of Z -> E isomerization were adjustable by manipulating catalytic factors and, therefore, unprecedented molecular photoswitches with adjustable self-recovery were realized.

Automated summary

Mild acid-catalyzed Z→E isomerization was discovered in stilbene derivatives fused with a norbornene moiety, inhibiting undesired by-reactions and allowing orthogonal control through light and acid. The isomerization process could be controlled by turning on/off photoacid irradiation, and the Z isomer produced by photoisomerization could self-recover to the E isomer in the presence of a catalytic amount of acid. This study realized unprecedented molecular photoswitches with adjustable self-recovery kinetics by manipulating catalytic factors.