Thermometric Properties of Thio/Selenocyanato-Bridged Spin-Crossover Networks

Thermometry is a well-researched topic in materials science, but recent advances in multifunctional complexes have introduced the idea of highly sensitive and accurate contact or noncontact thermometers, which utilize the temperature-dependent evolution of physical properties. In this context, the {[Fe­(μ-pyrazine)2]­[Fe­(MeOH)2]­[Hg­(μ-ECN)3(ECN)]2}·2H2O (E = S, FeHgS, and E = Se, FeHgSe) three-dimensional networks with spin-transition temperature T1/2 = 191 K were considered promising materials. Temperature-dependent UV–visible–NIR, IR, and Raman spectra displayed a strong dependence on the peak intensities and positions with temperature. Subsequently, first-principles calculations for low- and high-spin states gave a thorough description of the spectroscopic properties and origins. The unique temperature-dependent variability caused by a combination of gradual spin-crossover (SCO)-active Fe centers and SCO-inactive Fe centers as a reference allowed us to prove the feasibility of the thermometric concept via the characterization of UV–vis–NIR, IR, and Raman intensity ratios. In particular, compared to UV–vis–NIR absorption thermometry, IR and Raman thermometry manifested high thermal sensitivity (Sr) with the best values of 3.2% K–1 at 182 K (FeHgS, IR), 3.1% K–1 at 131 K (FeHgSe, IR), 16.3% K–1 at 184 K (FeHgS, Raman), and 12.1% K–1 at 186 K (FeHgSe, Raman). Overall, the realization of optical and vibrational SCO thermometers has significant implications for the practical application of switchable materials and leads to the development of advanced thermometers.