Effect of Coordination Environment and Electronic Coupling on Redox Entropy in a Family of Dinuclear Complexes

The elucidation of factors that govern the temperature sensitivity of the electrochemical potential is essential to the development of electrochemical systems with target properties. Toward this end, we report a series of isostructural homo- and heterometallic M2 (M = FeII, FeIII, ZnII) complexes supported by a phenoxo-centered tetrapyridyl ligand and ancillary carboxylate ligands that enables independent change in (i) charge, (ii) coordination environment of the redox-active center(s), and (iii) electronic coupling strength between redox centers. Variable-temperature electrochemical analysis of the series reveals the temperature coefficient for Fe-based redox couples to be highly dependent on the coordination environment of the redox-active center(s), with Fe centers in a pseudo-octahedral [FeN3O3] coordination environment affording a 2-fold greater temperature coefficient for the FeIII/FeII redox couple than those in ancillary ferrocenyl groups. In contrast, identical temperature coefficients for the FeIII/FeII redox event in Fe2 and FeZn complexes establish electronic coupling strength to have a minimal impact on the temperature dependence of the Fe-based redox couple. Taken together, these results provide important insights for the design of molecular compounds with target redox properties, and they provide the first examination of how electronic coupling influences the temperature dependence of the redox potential and the associated redox entropy in molecular compounds.

Noor NL, Abdullah N. Ultra-high Seebeck coefficient of a thermal sensor through entropic optimisation of ligand length of Fe(ii) spin-crossover (SCO) materials

In this work, we present a spin-crossover (SCO) complex molecular formulation [Fe(Lⁿ)₂](BF₄)₂ in an electrochemical single couple solution. A Seebeck voltage arises when an electrochemical single couple solution is subjected to a temperature difference, resulting in a single couple reaction at either terminal of the electrochemical cell. The ultrahigh Seebeck coefficients were obtained due to a number of molecular optimisation strategies. The [Fe(L¹⁶)₂](BF₄)₂ complex demonstrated a maximum Seebeck coefficient of 8.67 mV K⁻¹, achieved through a six-pronged approach to maximise entropy during the transition from low spin (LS) to high spin (HS) through: (i) a change in spin state, (ii) a change in physical liquid crystalline state, (iii) the spin Seebeck effect, (iv) the kosmotropic and chaotropic effect, (v) the fastener effect and (vi) thermal heat absorbance. A reduction of the Seebeck coefficient to 1.68 mV K⁻¹ during the HS–LS transition at higher temperatures is related to the single spin state transition entropy change. In summary, this paper presents a systematic study to identify the contributing factors in the production of a sensor with an ultrahigh Seebeck coefficient for energy harvesting through the optimisation of its molecular entropy elements.

The molecular optimisation strategies exhibit ultrahigh Seebeck coefficient through a six-pronged approach to maximise entropy during the transition from low spin (LS) to high spin (HS).

Optimised thermally driven molecular stability of an SCO metal complex for TEC Seebeck generation enhancement

The thermoelectricity effect allows the generation of electrical potential in an electrolyte upon application of a thermal gradient. In the previous work, the spin crossover effect in metal complexes was shown to be beneficial for generating a high Seebeck coefficient due to the high entropy associated with the conformational change accompanying the spin state change. In this study, we examine the diamagnetic stability of a spin crossover material through optimisation of the ligand chain length. We show that the diamagnetic stability of the spin crossover material can enhance the thermoelectrochemical Seebeck effect through ligand optimisation of the octahedral structure. The increase of carbon chain length from C₁₄ to C₁₆ in the long alkyl chain of the N-donor ligand increased Seebeck generation in a Co(iii)L16 complex to 1.94-fold that of a previously studied paramagnetic Co complex, and in a Fe(iii)L16 complex to 3.43-fold that of a less diamagnetic Fe complex. We show with DSC studies of an Fe based octahedral complex that an endothermic absorption accompanies the spin crossover transition, which enhances the Seebeck coefficient of this metal complex. Thus, we can correlate the diamagnetic stabilisation with temperature. We therefore indicate a molecular design strategy for optimisation of a spin crossover metal complex.

Highly stable reversible spin crossover electrolyte transition of Fe²⁺ to Fe³⁺ for high Seebeck generation.