Photoswitchable Magnetic Resonance Imaging Contrast by Improved Light-Driven Coordination-Induced Spin State Switch

Abstract

We present a fully reversible and highly efficient on-off photoswitching of magnetic resonance imaging (MRI) contrast with green (500 nm) and violet-blue (435 nm) light. The contrast change is based on intramolecular light-driven coordination-induced spin state switch (LD-CISSS), performed with azopyridine-substituted Ni-porphyrins. The relaxation time of the solvent protons in 3 mM solutions of the azoporphyrins in DMSO was switched between 3.5 and 1.7 s. The relaxivity of the contrast agent changes by a factor of 6.7. No fatigue or side reaction was observed, even after >100,000 switching cycles in air at room temperature. Electron-donating substituents at the pyridine improve the LD-CISSS in two ways: better photostationary states are achieved, and intramolecular binding is enhanced.

Switching Magnetic Properties by a Mechanical Motion

“…This work demonstrates the interest of using switchable ligands to achieve hysteresis without the need of cooperative effects, such as magnetic coupling or lattice interactions. This approach was successfully applied to obtain a photoswitchable Magnetic Resonance Imaging (MRI) contrast agent [55]. The relaxation time of the solvent protons was switched between 3.5 and 1.7 s improving the relaxivity of the contrast agent by a factor of 6.7.…”

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

“…[6][7][8][9][10] However, spin switching of single molecules in solution or on surfaces at ambient temperatures is still challenging and only a few examples are known in the literature. spintronics, data storage, 3,4 switchable diamagnetic levitation 5 or responsive ("smart") contrast agents for magnetic resonance imaging (MRI).…”

“…[6][7][8][9][10] However, spin switching of single molecules in solution or on surfaces at ambient temperatures is still challenging and only a few examples are known in the literature. 10,[25][26][27] In both concepts the photoresponsive axial ligands are designed in such a way that only one photo isomer is capable of coordinating to the Ni 2+ . [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). 10 Thus, the "record player" concept is a promising approach for the development of "smart" MRI contrast agents that can be switched on and off with light for interventional radiology. 10,[25][26][27] In both concepts the photoresponsive axial ligands are designed in such a way that only one photo isomer is capable of coordinating to the Ni 2+ .…”

Arylazoindazole Photoswitches: Facile Synthesis and Functionalization via SNAr Substitution

“…Some examples of azopyridines 7 and azoimidazoles 8 were reported to behave as photodissociable ligands, leading to changes in spin state upon photoisomerisation of metal porphyrins bearing these ligands. 9 Fuchter et al recently reported arylazopyrrole and arylazopyrazole photoswitches, which in some cases show long thermal half-lives and quantitative Z → E photoswitching. 10 Related arylazopyrazoles were used by Ravoo et al in switchable supramolecular host–guest systems.…”