Photoswitching of Co(ii)-based coordination cages containing azobenzene backbones

Abstract

Inclusion of photoswitchable azobenzene units as spacers into ditopic bridging ligands L^m and L^p, containing two chelating pyrazolyl-pyridine termini, allows formation of metal complex assemblies with Co(ii) that undergo a range of light-induced structural transformations. One notable result is the light-induced conversion of a Co₂(L^p)₃ dinuclear triple helicate (based on the E ligand isomer) to a C ₃-symmetric Co₄(L^p)₆ assembly, assumed to be an edge-bridged tetrahedral cage, based on the Z ligand isomer. Another is the preparation of a series of Co₄(L^m)₆ complexes, of which Co₄(E-L^m)₆ was crystallographically characterised and consists of a pair of Co₂(L^m)₂ double helicates connected by an additional two bridging ligands which span the pair of helicate units, giving a cyclic Co₄ array in which one and then two bridging ligands alternate around the periphery. A set of Co₄(L^m)₆ complexes could be prepared containing different ratios of Z : E ligand isomers (0 : 6, 2 : 4, 4 : 2 and 6 : 0) of which Co₄(Z-L^m)₂(E-L^m)₄ was particularly stable and dominated the speciation behaviour, either during light-induced switching of the ligand geometry in pre-formed complexes, or when ligand isomers were combined in different proportions during the preparation. These examples of (i) interconversion between Co₂L₃ (helicate) and (ii) Co₄L₆ (cage) assemblies with L^p, and the interconversion between a series of Co₄L₆ assemblies Co₄(Z-L^m)_n(E-L^m)_6-n with L^m, constitute significant advances in the field of photoswitchable supramolecular assemblies.

Scheme 1. Ligands used in this paper and their isomeric forms.

Fig. 1. Molecular structure of L^m from crystallographic data, including the atomic labelling scheme and the packing arrangement.

Fig. 2. (a) Absorption spectra of 10 μM solutions of L^m and L^p in MeCN, pre (E form) and post (Z form) irradiation with 340 nm light. Arrows indicate changes in absorbance maxima. (b) Switching reversibility experiment monitoring absorbance of each ligand (10 μM) at 325 nm, after alternating irradiation with 340 nm and white light.

Fig. 3. Stacked ¹H NMR spectra (300 MHz, MeCN-d₃, 10 μM, 298 K) of (a) L^m and (b) L^p showing changes between E forms (bottom) and predominantly Z forms (top), following photoirradiation at 340 nm, which generates a Z : E mixtures of 95 : 5 (L^m) and 94 : 6 (L^p).

Fig. 4. Changes in composition of Z/E mixtures of L^p (blue and pink for E and Z isomers, respectively) and L^m (orange and green for E and Z isomers, respectively) measured by in situ illumination ¹H NMR (700 MHz, CD₃CN, 298 K). A sample of E-L^m (2.0 mM) was irradiated with 325 nm light for ca. 17 minutes to induce the E to Z isomerisation; followed by illumination with 450 nm light for ca. 17 minutes, to switch the ligands back to the E configuration. A sample of E-L^p (2.0 mM) was irradiated with 365 nm light for ca. 20 minutes, with subsequent irradiation with 450 nm light for ca. 20 minutes. In both cases the slow change to the Z isomer during the first 17–20 minutes with the UV irradiation, and then the very fast change back to the E isomer following visible light irradiation, are clear.

Fig. 5. (a) ¹H NMR spectra (400 MHz, CD₃CN, 338 K) indicating the change in the methylene proton signals of (predominantly) Z-L^m peaks following heating at 65 °C in CD₃CN to effect thermal relaxation back to E-L^m; (b) proportion of E-L^m and E-L^p with time during this heating process.

Fig. 6. (Top) ¹H NMR spectrum (300 MHz, CD₃CN, 298 K) of Z·Co·L^p, prepared by irradiation at 340 nm of a solution of E·Co·L^p. (Bottom) ¹H NMR spectrum (300 MHz, CD₃CN, 298 K) of E·Co·L^p. An expansion of the 47–54 ppm region makes clear the presence of two closely-spaced signals: this is also apparent in the signal at −14 ppm.

Fig. 7. MM2-optimised model of E·Co·L^p showing a dinuclear, triple-helicate with D₃ symmetry.

Fig. 8. ¹⁹F NMR spectra (400 MHz, CD₃CN, 298 K) of (bottom) Zn(BF₄)₂ showing the presence of only one anion environment (the two separate signals arise from the ¹⁰B and ¹¹B isotopomers which give slightly different chemical shifts for attached ¹⁹F atoms); (middle) E·Zn·L^p showing by the extra splitting the presence of two BF₄⁻ anion environments in the complex; and (top) Z·Zn·L^p showing the presence of only one anion environment.

Fig. 9. GFN-xTB generated structure of the C₃ symmetric M₄L₆ tetrahedral cage Z·Co·L^p. The structure contains two distinct ligand environments, with one ligand environment spanning fac (top metal ion in Figure) and mer (other three metal ions at base of Figure) metal vertices, and the other spanning two mer vertices but with no internal symmetry due to the cage chirality.

Fig. 10. (a) UV/Vis spectra of E·Co·L^p and Z·Co·L^p (MeCN, 13 μg ml⁻¹), as well as E-L^p and Z-L^p (MeCN, 10 μM). (b) Switching stability experiment monitoring the absorbance at 340 nm [black arrow in part (a)] of a 13 μg ml⁻¹ sample after alternating irradiation with 340 nm for 30 minutes and then white light for 30 minutes, to switch between E·Co·L^p and Z·Co·L^p complexes.

Fig. 11. Photoswitching of a solution of E·Co·L^p to Z·Co·L^p and back, using 365 nm and then 405 nm irradiation respectively, monitored by in situ¹H NMR spectroscopy (700 MHz, CD₃CN, 298 K); see main text. The light blue trace shows the initial solution containing 3 : 97 Z : E complexes converting over several minutes to a PSS containing ca. 90 : 10 Z : E forms under UV irradiation, followed by rapid conversion back to the starting composition under 405 nm light excitation.

Fig. 12. Series of ¹H NMR spectra (400 MHz, CD₃CN, RT) showing the slow thermal conversion of Z·Co·L^p (bottom spectrum) back to E·Co·L^p (top spectrum) over 5 weeks. The top and bottom spectra match those in Fig. 6.

Fig. 13. Two views of the molecular structure of the M₄L₆ complex cation of E·Co·L^m from an X-ray crystal structure determination, with different types of ligand environment highlighted in green or orange (within the two M₂L₂ helicate units), or grey (cross-linking the two helicate units).

Fig. 14. Structure of the M₂L₂ double-helicate unit coloured green in Fig. 13, with approximate (non-crystallographic) C₂ internal symmetry in which the two ligands are essentially identical but conformational details two ends of each ligand are different (i.e. both adopt the same ‘head to tail’ arrangement within the M₂L₂ unit).

Fig. 15. Structure of the M₂L₂ double-helicate unit coloured orange in Fig. 14, with each ligand having approximate (non-crystallographic) C₂ internal symmetry with each end the same: the two ligands are essentially identical, giving local D₂ symmetry for the M₂L₂ unit.

Fig. 16. ¹H NMR spectra (300 MHz, CD₃CN, 298 K) of (a) redissolved crystals of E·Co·L^m; (b) Z·Co·L^m prepared by combination of Z-L^m with Co(BF₄)₂; and (c) Z·Co·L^m prepared by irradiation of a sample of E·Co·L^m at 340 nm. Comparison of signals labelled * between parts (b) and (c) show additional complexity in the spectrum of the sample generated by photoswitching.

Fig. 17. GFN-xTB generated structure of the M₄L₆ complex cation of Z·Co·L^m comprising two M₂L₂ helicate units joined by two cross-linking ligands, with no internal symmetry.

Fig. 18. (a) ¹H NMR spectra [300 MHz, MeCN-d₃/CDCl₃ (85 : 15 v/v), 298 K] recorded during addition of 0–6 equivalents (from bottom up) of Z-L^m to a solution of E·Co·L^m. The spectrum labelled * (after addition of two equivalents of Z-L^m) signals the point at which Co·L^m·2,4 is the dominant species present with little change thereafter. (b) ¹H-NMR spectrum (300 MHz, MeCN-d₃/CDCl₃ (85 : 15 v/v), 298 K) of Co·L^m·2,4 prepared separately in the same solvent mixture – compare the 50–70 ppm region with the spectrum labelled * in Fig. 18a.

Fig. 19. (a) UV/Vis spectra of Z·Co·L^m and E·Co·L^m in MeCN (20 μg ml⁻¹), as well as E-L^p and Z-L^p (MeCN, 10 μM). (b) Switching stability experiment monitoring the absorbance of Co·L^m (20 μg ml⁻¹) species at 340 nm, after alternating irradiation with 340 nm for 30 minutes and white light for 30 minutes.

Fig. 20. ¹H NMR (700 MHz, CD₃CN, 298 K) spectral changes during illumination of E·Co·L^m (orange) with 325 nm light for 24 minutes, with subsequent illumination with 405 nm light for 24 minutes. Characteristic spectra of Z·Co·L^m (green) and Co·L^m·2,4 (teal blue) prepared by combination of stoichiometric quantities of Z and E-L^m with Co(BF₄)₂, are shown for comparison. A third spectrum prepared by combination of Z-L^m, E-L^m and Co(BF₄)₂ in a 4 : 2 : 4 ratio shows the presence of a fourth species (red) amongst signals associated with Z·Co·L^m and Co·L^m·2,4. For discussion of signals labelled with coloured arrows, see main text.