Amplified electron–spin thermal sensitivity in Mn( <scp>ii</scp> ) complexes

Understanding the temperature sensitivity of magnetic resonance properties is an essential step toward any spin application, whether for novel molecular thermometers or quantum sensing platforms. Hence, demonstrations that molecular tuning is effective at modifying the temperature dependence of the electron paramagnetic resonance spectra of open-shell molecules are vital. Herein we show that ligand choice offers one handle for controlling the temperature dependence of the EPR spectrum, ostensibly through modifying the temperature sensitivity of the zero-field splitting parameter, <i>D</i>. For this demonstration, we prepared and analyzed three different encapsulated Mn(II) complexes. High-field, high-frequency EPR spectroscopy reveals EPR spectra for all complexes that vary in width as a function of temperature. At lower temperatures, these temperature sensitivities change starkly with the ligand shell, yielding thermal sensitivities of 2.2 to 9.8 MHz K^-1 for <i>D</i>. These results suggest the ability to modify the variable temperature nature of <i>D</i> by ligand selection, the first time for the Mn(II) metal ion, which shows significant enhancement over the nitrogen vacancy center of a diamond lattice (<i>ca.</i> 74 kHz K^-1).