On–off photoswitching of MRI contrast in water and gels with spatial resolution.

Molecules or ions are either paramagnetic (unpaired electrons) or diamagnetic (all electrons are paired). Switching between the two states under ambient conditions was considered a typical solid state phenomenon and has been termed spin crossover. The first single-molecule spin state switches operated with light in solution were developed a decade ago and offer a number of technical applications that are not accessible to solid state systems. Magnetic switching in biological environments, however, requires water solubility, and for in vivo applications, switching wavelengths within the bio-optical window (650−950 nm) are needed. We now present molecular spin state switches that are water-soluble and switchable in the far-red and nearinfrared region. At the same time, they are photochromic compounds with excellent photophysical properties. trans−cis isomerization is induced with 505 nm radiation, and cis−trans conversion with 620 or 720 nm radiation. The metastable cis isomers are stable at room temperature for at least several weeks. The detailed mechanism of this surprising and unprecedented long wavelength photoisomerization of azobenzenes is still under investigation.

Temperature can be used as clinical marker for tissue metabolism and the detection of inflammations or tumors.The use of magnetic resonance imaging (MRI) for monitoring physiological parameters like the temperature noninvasively is steadily increasing.I nt his study,w ep resent ap roof-ofprinciple study of MRI contrast agents (CA) for absolute and concentration independent temperature imaging. These CAs are based on azoimidazoles ubstituted Ni II porphyrins,w hich can undergo Light-Driven Coordination-Induced Spin State Switching (LD-CISSS) in solution. Monitoring the fast first order kinetic of backisomerisation (cis to trans) with standard clinical MR imaging sequences allows the determination of half-lives,t hat can be directly translated into absolute temperatures.Different temperature responsive CAswere successfully tested as prototypes in methanol-based gels and created temperature maps of gradient phantoms with high spatial resolution (0.13 0.13 1.1 mm) and low temperature errors (< 0.22 8 8C). The method is sufficiently fast to recordt he temperature flowf rom aheat source as afilm.

Temperature can be used as clinical marker for tissue metabolism and the detection of inflammations or tumors.The use of magnetic resonance imaging (MRI) for monitoring physiological parameters like the temperature noninvasively is steadily increasing.I nt his study,w ep resent ap roof-ofprinciple study of MRI contrast agents (CA) for absolute and concentration independent temperature imaging. These CAs are based on azoimidazoles ubstituted Ni II porphyrins,w hich can undergo Light-Driven Coordination-Induced Spin State Switching (LD-CISSS) in solution. Monitoring the fast first order kinetic of backisomerisation (cis to trans) with standard clinical MR imaging sequences allows the determination of half-lives,t hat can be directly translated into absolute temperatures.Different temperature responsive CAswere successfully tested as prototypes in methanol-based gels and created temperature maps of gradient phantoms with high spatial resolution (0.13 0.13 1.1 mm) and low temperature errors (< 0.22 8 8C). The method is sufficiently fast to recordt he temperature flowf rom aheat source as afilm.