Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal-organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O-Pd2 L4 cages is driven by coordination between Pd(2+) and a photochromic dithienylethene bispyridine ligand (O-PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O-Pd2 L4 cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three-dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light-induced phase and structural transformations readily occur owing to the reversible photochromic open-ring/closed-ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd2 L4 cage-based gels show multiple reversible gel-solution transitions when thermal-, photo-, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids.

CreatingCoordination-Based Cavities in a Multiresponsive Supramolecular Gel

Abstract A photoswitchable ligand based on azobenzene is self‐assembled with palladium(II) ions to form a [Pd2(E‐L)4]4+ cage. Irradiation with 470 nm light results in the near‐quantitative switching to a monomeric species [Pd(Z‐L)2]2+, which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self‐assembly, and the thermal half‐life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

Abstract Visible light can be used to shift dynamic covalent imine assemblies out of equilibrium. We studied a fluorinated azobenzene building block that reliably undergoes geometric isomerism upon irradiation. The building block was used in combination with two different amines, ethylenediamine and R,R‐1,2‐diaminocyclohexane, to create a library of imine macrocycles. Whereas the simple amine can be used to access a polymeric state and a defined bowl‐shaped macrocycle, the chiral amine gives access to a rich network of macrocycles that undergo both isomerisation as well as interconversion between different macrocyclic species, thereby allowing for control over the number of monomers involved in the cyclo‐oligomerization; 1H‐ and 19F‐DOSY NMR, MALDI‐MS measurements, and UV/Vis spectroscopy were used to study the processes.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/anie.202212745

Photoresponsive Dissipative Macrocycles Using Visible‐Light‐Switchable Azobenzenes

Abstract We report on two regioisomeric, diazocine ligands 1 and 2 that can both be photoswitched between the E‐ and Z‐configurations with violet and green light. The self‐assembly of the four species (1‐Z, 1‐E, 2‐Z, 2‐E) with CoII ions was investigated upon changing the coordination vectors as a function of the ligand configuration (E vs Z) and regioisomer (1 vs 2). With 1‐Z, Co2(1‐Z)3 was self‐assembled, while a mixture of ill‐defined species (oligomers) was observed with 2‐Z. Upon photoswitching with 385 nm to the E configurations, the opposite was observed with 1‐E forming oligomers and 2‐E forming Co2(2‐E)3. Light‐controlled dis/assembly was demonstrated in a ligand competition experiment with sub‐stoichiometric amounts of CoII ions; alternating irradiation with violet and green light resulted in the reversible transformation between Co2(1‐Z)3 and Co2(2‐E)3 over multiple cycles without significant fatigue by photoswitching.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/anie.202212571

Light‐Controlled Destruction and Assembly: Switching between Two Differently Composed Cage‐Type Complexes

We derive design principles for the assembly of rectangular tetramines into Zn8L6 pseudo-cubic coordination cages. Because of the rectangular, as opposed to square, geometry of the ligand panels, and the possibility of either Δ or Λ handedness of each metal center at the eight corners of the pseudo-cube, many different cage diastereomers are possible. Each of the six tetra-aniline subcomponents investigated in this work assembled with zinc(II) and 2-formylpyridine in acetonitrile into a single Zn8L6 pseudo-cube diastereomer, however. Each product corresponded to one of four diastereomeric configurations, with T, Th, S6 or D3 symmetry. The preferred diastereomer for a given tetra-aniline subcomponent was shown to be dependent on its aspect ratio and conformational flexibility. Analysis of computationally modeled individual faces or whole pseudo-cubes provided insight as to why the observed diastereomers were favored.

/pdf/tetramine-aspect-ratio-and-flexibility-determine-framework-i6t83yot.pdf

Tetramine Aspect Ratio and Flexibility Determine Framework Symmetry for Zn8L6 Self-Assembled Structures.

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. The E-isomer is combined with palladium(II) salts affords a double-walled triangle with composition [Pd3L6]6+ and a distorted tetrahedron [Pd4L8]8+(1:2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with ~80% of the E-isomer of the ligand which results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the kinetic product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.

/pdf/visible-light-switching-of-metallosupramolecular-assemblies-1nthr1f53e.pdf

Ray DiNardi

Papers

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

Visible light switching of metallosupramolecular assemblies