Reversible photoswitching of encapsulated azobenzenes in water

Abstract

Efficient molecular switching in confined spaces is critical for the successful development of artificial molecular machines. However, molecular switching events often entail large structural changes and therefore require conformational freedom, which is typically limited under confinement conditions. Here, we investigated the behavior of azobenzene-the key building block of light-controlled molecular machines-in a confined environment that is flexible and can adapt its shape to that of the bound guest. To this end, we encapsulated several structurally diverse azobenzenes within the cavity of a flexible, water-soluble coordination cage, and investigated their light-responsive behavior. Using UV/Vis absorption spectroscopy and a combination of NMR methods, we showed that each of the encapsulated azobenzenes exhibited distinct switching properties. An azobenzene forming a 1:1 host-guest inclusion complex could be efficiently photoisomerized in a reversible fashion. In contrast, successful switching in inclusion complexes incorporating two azobenzene guests was dependent on the availability of free cages in the system, and it involved reversible trafficking of azobenzene between the cages. In the absence of extra cages, photoswitching was either suppressed or it involved expulsion of azobenzene from the cage and consequently its precipitation from the solution. This finding was utilized to develop an information storage medium in which messages could be written and erased in a reversible fashion using light.

Keywords: azobenzene; confinement; coordination cages; photochromism.

Fig. 1.

Reversible photoswitching of unsubstituted azobenzene in water. (A) Synthesis of octahedral cage 1 from a triimidazole-based ligand and a cis-blocked Pd acceptor (34). (B) UV/blue light-induced isomerization of unsubstituted azobenzene 2. (C) Changes in the NMR spectrum of trans-2 (i) upon encapsulation within 1 (ii) and subsequent irradiation with UV light (iii) and blue light (iv). Spectrum i was recorded in (CD₃)₂SO and spectra ii–iv in D₂O (400 MHz, 298 K). (D) X-ray crystal structure of (trans-2)₂ ⊂ 1 (see also SI Appendix, Fig. S15). Color codes for all X-ray structures: C, gray; N, blue; O, red; F, green; Pd, brown sphere. Hydrogens, counterions, and solvent molecules are not shown for clarity. (E) Changes in the UV/Vis absorption spectrum of (trans-2)₂ ⊂ 1 (black) following exposure to UV light (10 min, purple) and then to blue light (8 min, blue).

Fig. 2.

Reversible photoswitching of tetra-o-methoxyazobenzene 4 in water. (A) Yellow/blue light-induced isomerization of 4. (B) The ¹H NMR spectra of inclusion complex 4 ⊂ 1 obtained by encapsulating cis-4 (Upper) and trans-4 (Lower) (500 MHz, D₂O, 298 K). (C) Changes in the UV/Vis absorption spectrum of cis-4 ⊂ 1 following exposure to blue light (5 min) and then to yellow light (90 min). (D) Eight cycles of reversible photoisomerization of 4 within 4 ⊂ 1, followed by UV/Vis absorption spectroscopy.

Fig. 3.

Reversible photoswitching of tetra-o-fluoroazobenzene 5 in water. (A) Green/blue light-induced isomerization of 5. (B) X-ray crystal structure of (trans-5)₂ ⊂ 1 (see also SI Appendix, Fig. S60). (C) Partial ¹H NMR spectra of cage 1 in the presence of increasing amounts of guest 5 (500 MHz, D₂O, 298 K). The signals highlighted in red correspond to imidazole (N=CH–N) protons of empty 1 and those highlighted in green correspond to 1 filled with 2 eq of 5. (D) Changes in the UV/Vis absorption spectrum of (trans-5)₂ ⊂ 1 following exposure to green light (4 min) and then to blue light (4 min). (E) Ten cycles of reversible photoisomerization of 5 within 5₂ ⊂ 1, followed by UV/Vis absorption spectroscopy. (F) The stepwise mechanism underlying the trans → cis photoisomerization of 5₂ ⊂ 1. (G) Changes in ¹H NMR spectra of (trans-5)₂ ⊂ 1 (bottom spectrum) during irradiation with 520-nm light (indicated in green font) for up to 30 min and subsequently with 420-nm light (blue font) for up to 11 min (500 MHz, D₂O, 298 K). For a complete set of spectra, please refer to SI Appendix, Fig. S62. (H) Changes in the relative concentrations of different isomers of 5 as a function of irradiation. White markers denote encapsulated cis-5; black markers denote trans-5 within (trans-5)₂ ⊂ 1; orange markers denote trans-5 within (trans-5·cis-5) ⊂ 1. The peak indicated with an arrow is due to transient intermediate (trans-5·cis-5) ⊂ 1.

Fig. 4.

Light-induced expulsion of fluorinated azobenzene 5 from coordination cage 1. (A) Thin piece of agarose gel soaked with an aqueous solution of (trans-5)₂ ⊂ 1 subjected to 520-nm irradiation for increasing periods of time (indicated on the top). (Right) X-ray crystal structure of cis-5. (B) Writing and erasing an image in a 5₂ ⊂ 1-soaked agarose gel using green light (with a mask) and blue light, respectively. (C) Four images created consecutively in the same piece of light-sensitive agarose gel. Clockwise from Top Left: the Penrose triangle, the structural formula of cis-5, Plane-Filling Motif with Reptiles (a 1941 woodcut by M. C. Escher; all M.C. Escher works © 2018 The M.C. Escher Company, The Netherlands. All rights reserved. Used by permission. www.mcescher.com), and part of the logo of the Weizmann Institute (reproduced with permission of the Weizmann Institute of Science).