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

Abstract

Azo-N-methylimidazole functionalized Ni(ii)porphyrins were rationally designed and synthesized and their performance as molecular spin switches was investigated. They perform intramolecular light-driven coordination-induced spin state switching (LD-CISSS) in the presence of water and therefore are an important step towards spin switches for medicinal applications, particularly functional MRI contrast agents.

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].…”

Spin State Switching in Solution with an Azoimidazole‐Functionalized Nickel(II)‐Porphyrin

“…Irradiation with UV light gives significantly higher amounts of cis isomer (ca. 90 %) in all solvents compared with the previously reported pyr- idine [20] and imidazole [31] "record player" derivatives. Unfortunately, the back-isomerization by irradiation with blue light is somewhat impeded (27-49 % residual cis).…”

“…1-Methylimidazole biphenyl ether "record player" 12 was synthesized in 13 synthetic steps (eight pots) and its switching behavior in several solvents was investigated. In comparison with previously reported pyridine [20] and imidazole [31] derivatives, "record player" 12 provides exceptionally high trans→cis photoconversion in all solvents (up to 94 %). In toluene, the amount of cis species can be photochemically switched between 27 % (PSS at 435 nm) and 94 % (PSS at 365 nm).…”

“…Syntheses So far, we mainly used mixed-aldehyde porphyrin syntheses to build up "record player" molecules [18,20,22,31] and we decided to pursue this strategy for the biphenyl ether "record player" as well. This so-called "tone arm" approach requires the synthesis of the biphenyl ether azoimidazole moiety as benzaldehyde derivative.…”

“…We recently presented a design study in which we optimized the molecular "record player" design for 1-methylimidazole as the photoresponsive axial ligand. [31] Simply replacing pyridine with 1-methylimidazole gave rise to a rather strained geometry of the coordinated cis species and consequently a somewhat lowered spin switching performance. However, an extension of the biphenyl tether (which connects the porphyrin with the switching unit) by a single atom is sufficient for obtaining a relaxed binding situation.…”