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

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 Co^II 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 Co^II 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.

Keywords: Diazocine; Isomerization; Photochemistry; Self-Assembly; Supramolecular Chemistry.

Figure 1

Reversible light‐controlled assembly and disassembly of Co₂(1‐Z)₃ and Co₂(2‐E)₃. Self‐assembly is directed with violet and green light. Ligands 1 and 2 are represented as blue and red “sausages”. Co²⁺ ions are grey spheres. Z isomers (cis azo group) have a C shape and E isomers (trans) an S shape. For an assignment to the corresponding chemical structures and DFT optimized geometries, see Figure 2. For the 2D chemical structures of 1 and 2 see Figure 3.

Figure 2

a) Photoswitchable diazocine‐based, regioisomeric ligands 1 and 2 in their Z and E configurations. b) Co₂(1‐Z)₃ and Co₂(2‐E)₃ are the only defined self‐assemblies formed from the four possible ligands (1‐Z, 1‐E, 2‐Z, 2‐E) and Co^II ions. Ligands 1‐E and 2‐Z formed ill‐defined complexes with Co^II ions. Note in (b) double bonds are omitted for clarity and the ligand “sausage” representations highlight the ligand configuration rather than the stereochemistry of the metal complex.

Figure 3

¹H NMR spectra (CD₃CN, 298 K) of the reversible transformation between: a) Co₂(1‐Z)₃ (bottom and top) and the ill‐defined species (oligomers) formed with Co^II and 1‐E (middle) following irradiation with light at 385 nm and 520 nm, respectively; b) the ill‐defined mixture formed with Co^II and 2‐Z (top and bottom) and Co₂(2‐E)₃ (middle) following irradiation with light at 385 nm and 520 nm, respectively. Paramagnetic shifts and line broadening are due to the paramagnetic Co^II ions.

Figure 4

Competition experiments investigating the relative stabilities of Co₂(1‐Z)₃ and Co₂(2‐E)₃ upon addition of the regioisomeric ligand with the opposite configuration.

Figure 5

Reversible light‐controlled assembly and disassembly of Co₂(1‐Z)₃ and Co₂(2‐E)₃. A solution of Co₂(1‐Z)₃ (a: 6 mM, b: 0.33 mM) was mixed with 3 equivalents of 2‐Z and the solution was irradiated with light at 385 and 520 nm in an alternating sequence. a) Bottom spectrum: mainly signals of Co₂(1‐Z)₃ are observed (blue shaded). Second spectrum from the bottom: upon irradiation at 385 nm, the signals of Co₂(1‐Z)₃ disappear and the signals of Co₂(2‐E)₃ appear (red shaded). Third spectrum from the bottom: irradiation at 520 nm restores the signals of Co₂(1‐Z)₃ (blue shaded). Note: chemical shift regions below 30 ppm are omitted for clarity. Figure S65 shows the full spectra. Line broadening and the paramagnetic shifts of the ¹H NMR signals are due to ligand coordination to paramagnetic Co^II ions. b) Switching stability experiment. The absorption at 401 nm (black) and 490 nm (red) is plotted after alternating irradiation at 385 and 520 nm. Switching is reversible without significant fatigue over 20 cycles.