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

Abstract

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.

Keywords: azobenzenes; functional systems; molecular recognition; out-of-the equilibrium; supramolecular chemistry.

Scheme 1

General concept of a dual‐stimuli configurational selection process of an azobenzene molecular switch. A) Energy level diagram showing the dual‐stimuli model building on a fast light‐induced switching between the two azobenzene states and the chemical perturbation of the product distributions caused by a molecular recognition event; B) schematic representation of the equilibria involving an azobenzene architecture undergoing molecular recognition through non‐covalent interactions with a given chemical guest (red sphere).

Figure 1

a) Butterfly crown ether reported by Shinkai and b) binding mode for Na⁺ (left) and K⁺ (right) cations.36

Figure 2

Examples of photo‐controlled crown ethers reported by Shinkai and co‐workers.49, 50, 51, 52

Figure 3

Isomerization reaction in the Ag⁺ receptor described by Tamaoki and Oka.55

Figure 4

Calix[4]arene‐based receptors incorporating an azobenzene moiety.56, 58

Figure 5

Triply stimuli responsive crown ether reported by Shinkai.59

Figure 6

Light and redox control of the PSS composition in copper complexes.64

Figure 7

Structure of the E isomer of bis‐terpyridine‐azobenzene ligand 10 along with the isomerization reactions involving the Fe²⁺‐complexes of ligands E‐10 and Z‐10. Adapted with permission from reference [66]. Copyright 2014 John Wiley and Sons.

Figure 8

Photoswitchable anion receptor reported by the Jiang and co‐workers.77

Figure 9

Photoswitchable aryl‐triazole foldamer reported by Flood and co‐workers (top) and proposed cycle of photodriven binding and release of Cl⁻ ions (bottom). Adapted with permission from reference [79]. Copyright 2010 American Chemical Society.

Figure 10

Tweezer‐like receptors reported by the groups of Jeong 82 (above) and Wang 83 (below).

Figure 11

Urea‐based photoswitchable receptors reported by the group of Jurczak.84, 87

Figure 12

Photoswitchable anion receptor reported by the group of Jurczak.88

Figure 13

Bis‐calix[4]pyrrole receptor reported by the group of Kohnke.89

Figure 14

Examples of intramolecular H‐bonded azobenzene architectures.91, 92, 93

Figure 15

Photoswitchable catalysts in which the off‐state is characterized by intramolecular H‐bonding.95, 96

Figure 16

Isomerization of divalent azobenzene and stilbene axles 23 and 24, divalent host 25, monovalent host 26 and pseudo[2]rotaxanes E‐23@25. Adapted with permission from reference [98]. Copyright 2015 The Royal Society of Chemistry.

Figure 17

Hydrogen‐bonded self‐assembled architectures triggered by the isomerization of an azobenzene.100, 101

Figure 18

Light‐induced folding and unfolding of peptide 29.102

Figure 19

Photoswitchable host‐guest complexes synthesized by Rotello and co‐workers.104

Figure 20

H‐bonding receptors developed by Wisner and co‐workers.105, 106

Figure 21

Isomerization reaction of the uracil‐azobenzene derivative 33 in the free form and in the H‐bonded complex with DAP. Adapted with permission from reference [107]. Copyright 2017 American Chemical Society.