Pressure Tunable Electronic Bistability in Fe(II) Hofmann-like Two-Dimensional Coordination Polymer [Fe(Fpz)2Pt(CN)4]: A Comprehensive Experimental and Theoretical Study.

Ruixin Li,Hennagii Fylymonov,Ana Belén Gaspar,Victor V Ivashko,Bingbing Liu,Francisco Javier Valverde-Muñoz,Mengyun Yuan,Georgiy Levchenko,Jose Antonio Real,Quanjun Li

doi:10.1021/acs.inorgchem.1c02318

Abstract

A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)2Pt(CN)4] is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagram for the compound [Fe(Fpz)2Pt(CN)4] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)2Pt(CN)4] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures higher than 0.4 GPa. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis.

Highlights

Molecular materials based on transition-metal coordination compounds are at the forefront of research in material science since they bear the potential to technically solve modern society concerns, such as air and water pollution, energy storage and transport, and data storage and display as well.[1−3] In this context, the study over decades of the molecular electronic bistability exhibited by Fe(II) pseudo-octahedral coordination compounds, known as the spin crossover phenomenon (SCO) or spin transition (ST),[4−10] has brought a variety of molecular sensors capable of sensing, capturing, and storaging gases[11−15] or organic volatile compounds and water pollutants.[15−19] In addition, prototypes of pressure or temperature sensors, actuators, and switches have been developed for civil applications,[10,20−22] and even a sensor that transduces an electrical voltage variation into an optical output has been reported.[23]
We have focused on the study of the thermal- and pressure-induced ST, hereafter thermal-induced spin transition (TIST) and pressure-induced spin transition (PIST), in Fe(II) Hofmann-like spin crossover two- and threedimensional (2D and 3D, respectively) coordination polymers
It has been demonstrated that this particular behavior of the thermal hysteresis of the ST under pressure is not associated with a variation in the interaction of the Fe(II) centers within the 2D structure, but it is exclusively associated with a jump-like change in the intramolecular elastic energy, that is, geometrical distortion of the octahedral surrounding of the Fe(II) centers

Summary

INTRODUCTION

Molecular materials based on transition-metal coordination compounds are at the forefront of research in material science since they bear the potential to technically solve modern society concerns, such as air and water pollution, energy storage and transport, and data storage and display as well.[1−3] In this context, the study over decades of the molecular electronic bistability exhibited by Fe(II) pseudo-octahedral coordination compounds, known as the spin crossover phenomenon (SCO) or spin transition (ST),[4−10] has brought a variety of molecular sensors capable of sensing, capturing, and storaging gases[11−15] or organic volatile compounds and water pollutants.[15−19] In addition, prototypes of pressure or temperature sensors, actuators, and switches have been developed for civil applications,[10,20−22] and even a sensor that transduces an electrical voltage variation into an optical output has been reported.[23]. It is remarkable that the total volume change upon PIST is comparable to that observed for TIST (ΔV = 39.18 Å3; ∼5%).[37] From the V vs P graph (Figure 9), one can approximately obtain the critical pressures of the ST as well as the width of the piezohysteresis These values are P1/2↑ = 1.45 GPa, P1/2↓ = 1.2 GPa, P1/2 = 1.35 GPa, and ΔPc = 0.25 GPa. A small difference between these values of the transition pressure and the hysteresis width and the corresponding ones derived from optical and Raman spectroscopies is observed (Table 1). −11375 J/mol (1354 K) and of the Γ −3940 J/mol (469 K) in comparison with the values obtained at higher pressures

DISCUSSION

CONCLUSION

Findings

EXPERIMENTAL SECTION

■ REFERENCES