Toward Fractioning of Isomers through Binding-Induced Acceleration of Azobenzene Switching

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

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.

Configurational Selection in Azobenzene-Based Supramolecular Systems Through Dual-Stimuli Processes

Azobenzene is one of the most studied light-controlled molecular switches and it has been incorporated in a large variety of supramolecular systems to control their structural and functional properties. Given the peculiar isomeric distribution at the photoexcited state (PSS), azobenzene derivatives have been used as photoactive framework to build metastable supramolecular systems that are out of the thermodynamic equilibrium. This could be achieved exploiting the peculiar E/Z photoisomerization process that can lead to isomeric ratios that are unreachable in thermal equilibrium conditions. The challenge in the field is to find molecular architectures that, under given external circumstances, lead to a given isomeric ratio in a reversible and predictable manner, ensuring an ultimate control of the configurational distribution and system composition. By reviewing early and recent works in the field, this review aims at describing photoswitchable systems that, containing an azobenzene dye, display a controlled configurational equilibrium by means of a molecular recognition event. Specifically, examples include programmed photoactive molecular architectures binding cations, anions and H-bonded neutral guests. In these systems the non-covalent molecular recognition adds onto the thermal and light stimuli, equipping the supramolecular architecture with an additional external trigger to select the desired configuration composition.

A phototunable anion receptor for C-HX interactions with benzoate anions

A supramolecular receptor consisting of two anthracene moieties with binding motifs for binding of benzoate anions is reported here. NMR studies indicate that the binding involves π-π interactions and CHX interactions. Upon exposure to >350 nm light, the receptor undergoes a [4 + 4] photoelectrocyclization restricting the access to the binding site for benzoate. The reverse reaction works in the presence of the dual stimuli of 254 nm light and the benzoate anions. The work thus demonstrates a light mediated dynamic control of the binding pocket of a supramolecular anion receptor.

Ultralarge Dielectric Relaxation and Self-Recovery Triggered by Hydrogen-Bonded Polar Components

Subtle integration of rotatable polar components into dielectric crystals can contribute significantly to adjustable switching temperatures ( T_s) and dielectric relaxation behaviors. Currently, one of the biggest challenges lies in the design of optimal polar components with moderate motion resistance in a crystalline system. In this work, we demonstrate that under refrigerator conditions, rotatable hydrogen-bonded one-dimensional (1D) cationic chains, {[C₂H₆N₅]⁺} _n (C₂H₆N₅ = 3,5-diamino-1,2,4-triazolinium), and two-dimensional (2D) anionic layers, {[(H₂O)₂·SO₄]^2-} _n, can be generated in an organic salt, 3 ([C₂H₆N₅]₂·[(H₂O)₂·SO₄]). Compared with the nonhydrated precursor, 2 ([C₂H₇N₅]·[SO₄]), the rotation of these 1D and 2D ionic species triggers a reversible phase transition and dielectric switching in 3. In addition, the significantly sluggish rotation of the 1D cationic chains from parallel to unparallel stacking and the counter-clockwise rotation of the 2D anionic layers, compared with their reverse processes, induce a frequency-dependent dielectric response with a more highly adjustable heating T_s↑ than the cooling T_s↓. More importantly, 3 possesses excellent self-recovery ability attributed to the highly dynamic character of the hydrogen-bonded ionic species. The strategy here can provide a fairly good model for designing dielectric crystals with desired rotatable polar components.