Design of Photoswitchable Contrast Agents for Magnetic Resonance Imaging

Magnetically switchable nickel(II)porphyrins were designed, prepared and investigated in view of their application as switchable contrast agents for magnetic resonance imaging (MRI). The results are very promissing. The magnetic switching performance of the applied photochromic nickel(II)porphyrins was investigated and could be drastically improved. Reversible photoswitching of MRI contrasts was achieved with organic solvents. Furthermore two approaches regarding an in vivo application were presented. The first aims at the nickel(II) spin state switching in water. The second aims at a spin switching by irradiation with near infrared light

Coordination-Induced Spin-State-Switch (CISSS) in water

We present a non-ionic water-soluble porphyrin that does not exhibit measurable aggregation even at high concentrations in water. The spin state of the corresponding nickel(II) complex changes from completely diamagnetic (low- spin) to paramagnetic (high-spin) upon addition of a strong axial ligand. This leads to a strongly reduced NMR relaxation time of the water protons even at low concentrations of the complex

Modular Synthetic Route to Monofunctionalized Porphyrin Architectures

The synthesis of a borylated Ni²⁺ porphyrin and its application as a versatile precursor for building up functional <i>ortho</i>-substituted tetraaryl porphyrin architectures is reported. This precursor porphyrin provides the basis for efficient modular syntheses of porphyrin compounds with covalently attached axial ligands which are important as enzyme model complexes, electron transfer dyads, and many other applications. In the present study, the precursor porphyrin was used for the synthesis of molecular spin switches which previously showed high potential as photoresponsive contrast agents for magnetic resonance imaging

Ultrafast dynamics of a bi-stable azopyridine Ni-porphyrin spin switch after photoexcitation in the porphyrin B-bands

Femtosecond time-resolved absorption measurements of a magnetically bi-stable azopyridine Ni-porphyrin in solution at room temperature show that the photo-induced dynamics are dominated by transient low-spin ⇄ high-spin interconversion involving Ni (d2) and (d, d) states

Coordination-Induced Spin-State Switching with Nickel Chlorin and Nickel Isobacteriochlorin

We present the first coordination-induced spin-state switching with nickel chlorin and nickel isobacteriochlorin. The spin-state switching was monitored by UV–vis spectroscopy and NMR titration experiments. The association constants (<i>K</i>₁ and <i>K</i>₂) and thermodynamic parameters (Δ<i>H</i> and Δ<i>S</i>) of the coordination of pyridine were determined. The first X-ray analyses of a paramagnetic nickel chlorin and a nickel isobacteriochlorin provide further information about the structure of the octahedral complexes. Nickel chlorin and even more pronounced nickel isobacteriochlorin exhibit stronger coordination of axial ligands compared to the corresponding nickel porphyrin and thus provide the basis for more efficient spin-switching systems

Switched “On” Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site and then dethreading), with the on-set of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes

Photoswitching in Two-Component Surface-Mounted Metal–Organic Frameworks: Optically Triggered Release from a Molecular Container

The remote control of surface properties is one of the key challenges in interfacial systems chemistry. Here, we report the realization of a SURMOF (surface-mounted metal–organic framework)-based hybrid system in which a crucial component can be switched by light. The realization of this two-component system is made possible by installing vertical compositional gradients <i>via</i> liquid-phase epitaxy. After loading the porous coating with guest molecules, its release is initiated by illumination with visible light and monitored by a quartz crystal microbalance

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

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