Investigating the influence of oriented external electric fields on modulating spin-transition temperatures in Fe(ii) SCO complexes: a theoretical perspective

Abstract

Spin-crossover complexes, valued for their bistability, are extensively studied due to their numerous potential applications. A primary challenge in this molecular class is to identify effective methods to adjust the spin-transition temperature, which frequently falls outside the desired temperature range. This typically necessitates intricate chemical design and synthesis or the use of stimuli such as light or pressure, each introducing its own set of challenges for integrating these molecules into end-user applications. In this work, we aim to address this challenge using an oriented external electric field (OEEF) as one stimulus to modulate the spin-transition temperatures. For this purpose, we have employed both periodic and non-periodic calculations on three well-characterised Fe(II) SCO complexes, namely [Fe(phen)₂(NCS)₂] (1, phen = 1,10-phenanthroline), [Fe(bt)₂(NCS)₂] (2, bt = 2,2′-bi-2-thiazoline) and [Fe(py)₂phen(NCS)₂] (3, py = pyridine) possessing a similar structural motif of {FeN₄N′₂}. To begin with, DFT calculations employing the TPSSh functional were performed on complexes 1 to 3, and the estimated low-spin (LS) and high-spin (HS) gaps are 24.6, 15.3 and 15.4 kJ mol⁻¹, and these are in the range expected for Fe(II) SCO complexes. In the next step, an OEEF was applied in the molecule along the pseudo-C₂ axis that bisects two coordinated –NCS groups. Application of an OEEF was found to increase the Fe-ligand bond length and found to affect the spin-transition at the particular applied OEEF. While the HS state of 1 becomes the ground state at an applied field of 0.514 V Å⁻¹, the LS state lies at a higher energy of 1.3 kJ mol⁻¹. Similarly, complexes 2 and 3 also show the HS ground state at an applied field of 0.514 V Å⁻¹, where the LS state stays at higher energies of 6.13 and 11.62 kJ mol⁻¹, respectively. It is found that the overall change in enthalpy (ΔH_HL) and entropy (ΔS_HL) for the spin transition in the presence of OEEFs decreases upon increasing the strength of the applied field. The computed spin-transition temperature (T_1/2) using DFT was found to be in close agreement with the experimentally reported values. It is estimated that on increasing the strength of the applied electric field, the T_1/2 increases significantly. While the DFT computed T_1/2 values for the optimised geometry of 1, 2 and 3 were found to be 134.6 K, 159.9 K and 111.4 K respectively, at the applied field of 0.6425 V Å⁻¹T_1/2 increases up to 187.3 K, 211.0 K and 184.4 K respectively, unveiling an hitherto unknown strategy to tune the T_1/2 values. A limited benchmarking was performed with five additional exchange–correlation functionals: PBE, BLYP, B3LYP*, B3LYP, and PBE0. These functionals were found to be unsuitable for predicting the correct SCO behaviour for complex 2, and their behaviour under various electric fields did not improve. This emphasises the importance of choosing the correct functional at zero OEEF prior to testing them under various electric fields. Furthermore, calculations were performed with complex 1 adsorbed on the Au(111) surface. The formation of an Au–S bond during adsorption significantly stabilises the low-spin (LS) state, hindering the observation of spin-crossover (SCO) behaviour. Nonetheless, the application of an OEEF reduces this gap and brings the T_1/2 value closer to the desired temperature. This offers a novel post-fabrication strategy for attaining SCO properties at the interface.