Tunable microwave absorption of switchable complexes operating near room temperature†
Materials that are able to switch microwave radiation are strongly desired for their potential applications in electronic devices. In this paper, we show the spin-dependant interaction of spin-crossover materials with microwave radiation, namely, the ability of coordination compounds [Fe(NH2trz)3]Br2 and [Fe(NH2trz)3](NO3)2 that undergo a cooperative spin transition between low-spin and high-spin states to operate as thermoswitchable microwave absorbers. The characteristics of the microwave reflection and transmission of these spin-crossover complexes were investigated at variable temperatures. The evolution of both the transmission and reflection spectra in the 26–37 GHz frequency band within the temperature range of spin crossover showed significant differences in the interaction of microwave radiation with the high-spin and low-spin forms of the compounds. The microwave transmission coefficient shows a notable decrease upon transition to the high-spin state, while the reflection coefficient can be both increased or decreased on the characteristic frequencies during the spin transition. The different microwave absorbing properties of the low-spin and high-spin forms are found to be associated with a notable microwave permittivity change upon spin crossover. The switchable microwave reflection/transmission correlates well with the transition characteristics found in the optical and differential scanning calorimetry measurements. These results widen the spectroscopic range in which spin-crossover materials can be applied and contribute to the creation of a preliminary database of the microwave absorbing properties of spin-crossover complexes.