Design and Synthesis of Photodissociable Ligands Based on Azoimidazoles for Light-Driven Coordination-Induced Spin State Switching in Homogeneous Solution

Abstract

Light-switchable azoimidazoles were rationally designed and synthesized, and their performance was investigated as photodissociable ligands (PDL) and for spin state switching of Ni porphyrins. The rationally designed ligands exhibit a high photochemical conversion rate (trans → cis > 98%) and no measurable fatigue over a large number of switching cycles at room temperature under air. As compared to the known phenylazopyridines, the phenylazoimidazoles exhibit a much stronger affinity as axial ligands to Ni porphyrin in the binding trans configuration and a low affinity in their cis form. This affinity switching was used to control the coordination number of Ni(2+). Concomitant with the change in coordination number is the change of the spin state from triplet (high spin) to singlet state (low spin). We report on phenylazoimidazole-based PDLs that switch the paramagnetic ratio of the investigated nickel species by up to 70%. Consequently, azoimidazoles exhibit considerably higher switching efficiencies than previously described pyridine-based PDLs.

Azoimidazole functionalized Ni-porphyrins for molecular spin switching and light responsive MRI contrast agents

“…placing stents). The same trends were also observed with the photoswitchable 3-phenylazo-substituted pyridines 24 and 5-phenylazo-substituted imidazoles 22 (PDLs, see the ESI †). 28 However, for MRI guided interventions switchable contrast agents are needed to continuously check the dynamic blood flow.…”

“…So, the tether remains to be optimized. Inspection of structure 4 reveals 22 with Ni(II)TPPF 20 (in toluene) as a function of their pK a value. This level of theory was chosen because, according to an empirical study, it performed best in predicting the coordination energy of axial ligands to nickel porphyrins.…”

“…spintronics, data storage, 3,4 switchable diamagnetic levitation 5 or responsive ("smart") contrast agents for magnetic resonance imaging (MRI). [15][16][17][18][19][20][21] We have demonstrated highly efficient and fatigue resistant light-driven coordinationinduced spin state switching (LD-CISSS) by adding photodissociable ligands [22][23][24] to a solution of a Ni 2+ -porphyrin (PDL approach) as well as by tethering azopyridines directly to the porphyrin ("record player" approach). [11][12][13][14] Ni 2+ complexes change their spin state from diamagnetic (S = 0) to paramagnetic (S = 1) when the coordination sphere is changed from square planar (CN = 4) to square pyramidal (CN = 5) or square bipyramidal (CN = 6).…”

“…A number of diseases such as stroke and heart attack are increasingly treated with minimal invasive, catheter-based interventions under X-ray imaging control (e.g. 22 We attribute this fact to a reduced steric hindrance of the hydrogen atoms in ortho position with respect to the coordinating nitrogen in the 5-membered ring. Replacing X-ray imaging by MRI would have several advantages: avoidance of ionizing radiation, better soft tissue contrast, and 3D control.…”

Switching Magnetic Properties by a Mechanical Motion

“…No fatigue or side reactions were observed, even after >100,000 switching cycles in air at room temperature showing the robustness of the system. In an extension of this work, azoimidazoles were found to exhibit higher switching efficiencies than previously described phenylazopyridine ligands [56], and were used in the tethered form for spin state switching in solution [57], as well as light responsive MRI contrast agent [58]. In order to improve the effective concentration of the ligand, a tethered azopyridine ligand to the Ni(II)-porphyrin complex was used (Figure 8) [53].…”