Light‐Fueled Transformations of a Dynamic Cage‐Based Molecular System

Abstract

In a chemical equilibrium, the formation of high‐energy species—in a closed system—is inefficient due to microscopic reversibility. Here, we demonstrate how this restriction can be circumvented by coupling a dynamic equilibrium to a light‐induced E/Z isomerization of an azobenzene imine cage. The stable E‐cage resists intermolecular imine exchange reactions that would “open” it. Upon switching, the strained Z‐cage isomers undergo imine exchange spontaneously, thus opening the cage. Subsequent isomerization of the Z‐open compounds yields a high‐energy, kinetically trapped E‐open species, which cannot be efficiently obtained from the initial E‐cage, thus shifting an imine equilibrium energetically uphill in a closed system. Upon heating, the nucleophile is displaced back into solution and an opening/closing cycle is completed by regenerating the stable all‐E‐cage. Using this principle, a light‐induced cage‐to‐cage transformation is performed by the addition of a ditopic aldehyde.

Photoresponsive Supramolecular Cages and Macrocycles

“…The structurally similar 34 2 36 3 imine cage, which was also initially published by Vögtle and Bauer, [22] was utilised by Feringa and co-workers to generate otherwise inaccessible high-energy species, which will be discussed in a subsequent section. [61] Schmidt and colleagues reported on different halogenbonded boxes based on U-shaped anthracene building blocks functionalised with lutidine units as halogen bond acceptors and fluorinated iodoazobenzenes as halogen bond donors. [62] The formation of the boxes, which were composed of two donors and two acceptors, was observed in the solid-state as well as in solution.…”

“…The open cage could be isomerised by irradiation and transformed back into the initial 34 2 36 3 by heating. [61] Figure 24. Irradiation of E,E,E-56 3 64 3 with red light led to the isomerisation of the trianglimine as well as the formation of the smaller macrocycle Z,Z-56 2 64 2 .…”

“…Back-to-back, the groups of Feringa and Schmidt published azobenzene-based systems that underwent reversible rearrangements upon irradiation with light, utilising the reversible formation of imine bonds. [61,77] Feringa and co-workers utilised the azobenzene-based imine cage 34 2 36 3 [61] originally published by Vögtle and Bauer (Figure 23). [22] While the thermodynamically most stable E,E,E-34 2 36 3 showed nearly no amine/imine exchange upon addition of amine 63, the mixture of isomers (PSS 340 -34 2 36 3 ) that was generated upon irradiation with 340 nm light was opened by the addition of an amine.…”

Shining a Light on Dissipative Supramolecular Assemblies

“…Back-to-back, Feringa et al 10c presented a dissipative photoswitchable imine cage based on the tripodal tris(2-aminoethyl)amine ( Tri ) precursor and ditopic 3,3′-(diazene-1,2-diyl)dibenzaldehyde ( Di ). These building blocks readily form an E , E , E - Tri 2 Di 3 cage, 11 first synthesized by Bauer and Vögtle, 12 which is reversibly convertible into a mixture of different cages PSS 340 - Tri 2 Di 3 (containing the various isomeric cages E , E , Z - Tri 2 Di 3 , E , Z , Z - Tri 2 Di 3 , and Z , Z , Z - Tri 2 Di 3 ) by irradiation with light of the wavelength of 340 nm and vice versa at 420 nm (Figure 4 ).…”

Crystal Engineering of Two Light and Pressure Responsive Physisorbents

“…This has previously been attempted through photochromic guests in framework cavities, [25] linker ligands with pendant AB or BTCP sidearms [19,20] or grafting of such moieties into either cages or framework backbones. [33][34][35][36][37][38][39][40] Although the latter category has the potential to produce the most pronounced structural changes, tethering reduces conformational freedom and mobility, requiring judicious linker selection (BTCP ligands are thus more optimal than AB ligands). Nevertheless, of the 58 ring-closed BTCP structures reported in the Cambridge Structural Database (CSD), [41] only two entries are porous sorbents wherein BTCP was used to modulate gas sorption.…”