A variety of azobenzenes were synthesized to study the behavior of their E and Z isomers upon electrochemical reduction. Our results show that the radical anion of the Z isomer is able to rapidly isomerize to the corresponding E configured counterpart with a dramatically enhanced rate as compared to the neutral species. Due to a subsequent electron transfer from the formed E radical anion to the neutral Z starting material the overall transformation is catalytic in electrons; i.e., a substoichiometric amount of reduced species can isomerize the entire mixture. This pathway greatly increases the efficiency of (photo)switching while also allowing one to reach photostationary state compositions that are not restricted to the spectral separation of the individual azobenzene isomers and their quantum yields. In addition, activating this radical isomerization pathway with photoelectron transfer agents allows us to override the intrinsic properties of an azobenzene species by triggering the reverse isomerization direction (Z → E) by the same wavelength of light, which normally triggers E → Z isomerization. The behavior we report appears to be general, implying that the metastable isomer of a photoswitch can be isomerized to the more stable one catalytically upon reduction, permitting the optimization of azobenzene switching in new as well as indirect ways.

A two‐component core–shell UiO‐68 type metal–organic framework (MOF) with a nonfunctionalized interior for efficient guest uptake and storage and a thin light‐responsive outer shell was prepared by initial solvothermal MOF synthesis followed by solvent‐assisted linker exchange. The bulky shell linker features two tetra‐ortho‐fluorinated azobenzene moieties to exploit their advantageous photoisomerization properties. The obtained perfect octahedral MOF single crystals can be switched repeatedly and with an unprecedented efficiency between E‐ and Z‐rich states using visible light only. Due to the high photoswitch density per pore of the shell layer, its steric demand and thus molecular uptake (and release) can be conveniently modulated upon green and blue light irradiation. Therefore, the “smart” shell acts as a light‐controlled kinetic barrier or “gate” for the diffusion of cargo molecules in and out of the MOF crystals.

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal−organic framework (MOF). The azobenzene undergoes trans-to-cis-photoisomerization upon irradiation with green light and reverts back to trans upon violet light. While each moiety in cis conformation is chiral, we show the trans isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, R-and S-cis as well as R-and S-trans, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites R-cis and R-trans enantiomers, producing a MOF with enriched S-enantiomers, and vice versa. The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, trans and cis, are stabilized in nonplanar, chiral conformations in the MOF, the trans isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.

A sensor array with four identical photoresponsive azobenzene-containing metal–organic framework films is selectively irradiated. By photoprogamming the array, the sensor selectivity is switched and optimized.

Cyanodiarylethene chromophores are able to undergo constitutional exchange via dynamic covalent chemistry (DCC). During this process, the central ethylene bridge of the molecular scaffold can be broken and thereby enables the assembly of a new combination of aryl moieties around the reformed ethylene bridge. The reversible CC double bond exchange has exemplarily been investigated using α-cyanostilbenes. Establishing a dynamic equilibrium reaction from α-cyanodiarylethene with arylacetonitriles under mild conditions has been the basis to access constitutional libraries of new photoswitches with potentially improved properties. When subject to irradiation with light of adequate wavelength, α-cyanodiarylethenes undergo Z/E isomerization followed by ring-closure. By screening the thus accessible dynamic chromophore libraries using a desired detection wavelength, we could identify specific dithienyl analogues that exhibit three-state photochromism. The combination of dynamic constitutional libraries of functional chromophores in combination with the light-guided screening and selection should lead to more rapid exploration of structural diversity dye chemistry.