A Transient Phenolate Enables Relaxation of Z-Tetra-ortho-fluoroazobenzene to the Stable State in Seconds

Tetra-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.