A Transient Phenolate Enables Relaxation of <i>Z‐</i>Tetra‐<i>ortho</i>‐fluoroazobenzene to the Stable State in Seconds

Abstract

AbstractTetra‐ortho‐fluoroazobenzenes (TFAs) exhibit several unique properties, including a large separation in the n→π* absorption bands, which allows for efficient photoisomerization using green light, and exceptional thermal stability of their metastable Z‐isomer. These features make them attractive for light‐responsive applications, but controlling the thermal Z‐to‐E isomerization has remained a challenge. In this work, we present a hybrid molecular system that couples a TFA subunit with a spiropyran photoswitch. The TFA subunit retains its ability to undergo selective photoisomerization with green light. Z‐to‐E isomerization in the hybrid occurs approximately 1000 times faster than in a TFA model compound. This acceleration arises from a rate‐determining step involving the formation of merocyanine—the ring‐opened form of spiropyran. The transient phenolate form of merocyanine promotes rapid Z‐to‐E isomerization of the TFA, consistent with the known behavior of azobenzene phenolates. The thermodynamics of TFA isomerization are coupled with those of the spiropyran ring‐opening, allowing the isomerization process to be regulated by external stimuli that influence the spiropyran–merocyanine equilibrium. In addition to observing a 100–1000‐fold solvent‐dependent rate increase, we demonstrate a 106‐fold acceleration in TFA isomerization via either photochromism or adsorptiochromism.