pH-Dependent Spin State Population and 19F NMR Chemical Shift Via Remote Ligand Protonation in An Iron(II) Complex

An Fe^II complex that features a pH-dependent spin state population demonstrates potential as a ¹⁹F chemical shift-based pH sensor.

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

The development of transition metal-based molecules and materials that can be switched between low-spin and high-spin electronic states has constituted a highly active area of research over the past several decades. Indeed, the magnetic bistability of such spin-crossover compounds make them potential candidates for molecular switches and chemical sensors, as the spin transition can be controlled by a number of external stimuli, such as temperature, pressure, and light.

Recently, researchers have begun to explore the potential for spin-switchable molecules as bioresponsive probes for temperature, anions, and enzyme activity. In addition, given the relationship between tissue acidosis and diseases, including cancer and ischemia, a compound that undergoes a spin state transition as a function of pH could serve as a valuable tool for pH sensing. Nevertheless, pH-induced spin state switching is rare, and compounds that exhibit such behavior are unsuitable for most biological sensing applications due to pKa values far from biological pH or poor stability in water.