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 (ST) 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 ST 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 similar structural motif of {FeN4N’2}. To begin with, calculations 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, and these are in the range expected for Fe(II) SCO complexes. In the next step, OEEF was applied in the molecule along the pseudo-C2 axis that bisects two coordinated -NCS groups. Application of OEEF was found to increase the Fe-ligand bond length and found to affect the spin-transition at particular applied OEEF. While the HS state of 1 becomes the ground state at an applied field of 0.514 V/Å, the LS state lies at a higher energy of 1.3 kJmol-1. Similarly, complexes 2 and 3 also show the HS ground state at an applied field of 0.514 V/Å where the LS state stays at higher energies of 6.13 and 11.62 kJmol-1 respectively. It is estimated that on increasing the strength of the applied electric field the T1/2 increases significantly. Further, calculations were performed with complex 1 adsorping 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 SCO behaviour. Nonetheless, the application OEEF reduces this gap and brings the T1/2 value closer to the desired temperature. This offers a novel post-fabrication strategy for attaining SCO properties at the interface.