Manipulating electron redistribution to achieve electronic pyroelectricity in molecular [FeCo] crystals

Abstract

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

“…We reasoned the same steric influence of the peripheral ‘R’ substituents could stabilise heteroleptic SCO complexes [Fe(L 1 R)L] 2+ or [Fe(L 2 R)L] 2+ (L = another meridional tridentate ligand). 31 The chirality of such compounds is another attractive feature, since chiral-at-molecule SCO complexes are still quite rare. 28–30,32 Such compounds have potential for switchable ferroelectric or chiroptical properties in the solid state.…”

Electronic pyroelectricity: the interplay of valence tautomerism and spin transition

“…20,21 Among the array of valence tautomeric compounds reported so far, the pyroelectric properties reported by Sato and coworkers can be observed in the absence of an electric field because the molecular orientation in the molecular crystals is fixed. 18,19 Moreover, the advanced design strategy can improve the predictability and precision during the incorporation of polar molecular crystals into energy-efficient materials. Notably, the pyroelectric coefficient remains at B3.4 nC cm À2 K À1 at 300 K, largely comparable to that of polyvinylidene difluoride (PVDF).…”

“…The heteronuclear [FeCo] complex (Scheme 1) was obtained by mixing enantiopure mononuclear complexes of [Fe(AcO)( RR -cth)](PF 6 ) and [Co(AcO)( SS -cth)](PF 6 ) in a methanolic solution of dhbq in equimolar ratio, followed by oxidation with AgPF 6 . 19 To determine the direction of the current, pyroelectric measurements were performed on a plate-like single-crystal indexed by silver paste attached to one side and carbon paste to the other to distinguish the faces of the crystal (010 and 010, respectively). The observed sharp pyroelectric current peak (∼30 nC cm −2 K −1 ), followed by a continuous pyroelectric current (Fig.…”

Thermally induced charge transfer in a quinoid-bridged linear Cu3compound

“…1 Metal-metal or metalligand charge transfer always occurs in molecular valence tautomers, leading to the redistribution of electron density and exhibiting electrical, optical, and magnetic properties, and can be further controlled by multiple external stimuli such as temperature, light, and electric field. [2][3][4][5][6][7][8] Therefore, molecular valence tautomers have potential application in molecular switches, displays, memory devices, and sensors. [9][10][11] The redox-active quinone ligand is one of the typical ligands for building metal-ligand charge transfer valence tautomerism.…”

“…Therefore, quinone-bridged compounds possess outstanding physical properties such as unique magnetic, conductivity and pyroelectric properties. 7,15,16 Charge transfer in multi-center donor-acceptor architectures is expected to achieve more novel properties while embodying the above functions. However, as is well known, this charge transfer is always driven by entropy gain, which is very sensitive to intermolecular interactions, nearby solvate molecules and guest molecules.…”