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

Abstract

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.

Keywords: Metal-Template; Molecular Cage; Non-Equilibrium; Photoswitch; Self-Assembly.

Figure 1

Visible‐light responsive photoswitchable ligand 1 and self‐assembled products [Pd₂(E‐1)₄]⁴⁺ and [Pd(Z‐1)₂]²⁺.

Figure 2

The UV/Visible absorption (DMSO, 298 K) spectra of photoswitch 1 at PSS generated using LEDs with emission centered at various wavelengths. The spectrum for Z‐1 was calculated using the ¹H and ¹⁹F NMR data together with absorption spectra solutions at PSS when irradiated with 405, 470, 530 and 617 nm. See Supporting Information S3 for details.

Figure 3

a) Single crystal X‐ray structures of E‐1. The N=N bond length is 1.243(2) Å and C_Ar−N is 1.414(2) Å. The asymmetric unit contains half the molecule and the two phenyl rings are perfectly co‐planar, with C−N=N bond angles of 115.24°, and C(F)−C−N=N torsion angle of 177.6(1)°. The parent ortho‐tetrafluoroazobenzene ^[31n] is less planar, with C(F)−C−N=N torsion angles of 139.29 and 139.41°. b) Major positional conformer of [Pd₂(E‐1)₄]⁴⁺ in the single crystal X‐ray structure of [Pd₂(E‐1)₄](BF₄)₄ ⋅ 0.5H₂O ⋅ 4CH₃CN. The ligands adopt a range of conformations, with one ligand almost planar and others twisted with the two phenyl rings demonstrating no clear conformational preference in the solid state. c) Major positional disorder of [Pd(Z‐1)₂]²⁺ in the single crystal X‐ray structure of [Pd(Z‐1)₂](BF₄)₂⋅2 CH₃CN⋅0.8 (MePh). For crystallography details, see Supporting Information S11.

Figure 4

A) Partial ¹H (DMSO‐d ₆, 600 MHz, 298 K) and ¹⁹F (DMSO‐d ₆, 565 MHz, 298 K) NMR spectra of i) photoswitch E‐1 ([1]=10 mM, 2 equiv.); ii) the same sample after adding [Pd(CH₃CN)₄](BF₄)₂ (5.2 mM, 1 equiv.), iii) the same sample immediately after irradiating with 530 nm light for 15 min, and iv) the same sample after heating in the dark for 16 h at 90 °C and equilibrating at room temperature for 30 min. B) Zoom scans of select high resolution ESI‐MS peaks for a sample of i) [Pd₂(E‐1)₄](BF₄)₄ ([1]=16 mM), corresponding to [Pd₂(1)₄]⁴⁺, and ii) the same sample after irradiating with 530 nm light for 20 min, corresponding to [Pd(1)₂]²⁺. See Supporting Information S6 for more details.

Figure 5

Partial ¹H NMR (DMSO‐d ₆, 600 MHz, 298 K) spectra of i) A 1 : 1 mixture of E‐1 and Z‐1 generated using 470 nm light ([1]=1.6 mM, 2 equiv.), ii) the same sample 2 hours after adding [Pd(CH₃CN)₄](BF₄)₂ (0.82 mM, 1 equiv.), and iii) the same sample after irradiating with 470 nm light for 20 min, see Supporting Information S8 for details. iv) a mixture of [Pd₂(E‐1)₄](BF₄)₄ and [Pd(Z‐1)₂](BF₄)₂ ([1]=7.1 mM) after irradiating with 405 nm light for 20 min, and v) the sample after irradiating with 530 nm light for 20 min, for more details see Supporting Information S8.

Figure 6

Comparison of thermal isomerization of Z‐1 ([1]=8.7 mM) and [Pd(Z‐1)₂](BF₄)₂ ([1]=7.9 mM), using ¹H NMR (500 MHz, DMSO‐d ₆, 363 K) integrals of photoswitch Z‐1 (9.14 ppm) and [Pd(Z‐1)₂](BF₄)₂ (10.07 ppm) normalized to initial integrals. The barrier for the thermal isomerization of Z‐1 at 363 K was measured at variable temperature (see Supporting Information S3.3 for details). The apparent barrier for the relaxation of [Pd(Z‐1)₂](BF₄)₂ was determined by monitoring its depletion at 363 K.