Theoretical Study of Bistable Systems

This thesis deals with the theoretical study of spin-crossover (SCO) complexes, archetype of bistable molecules, with two accessible spin states that can be tuned by external stimuli such as temperature or light. Spin-crossover complexes are potential candidates for applications in information storage, spintronics and molecular electronics. Their implementation in real devices requires immobilization by deposition or interaction with a substrate. Once immobilized, the spin transition process can experience significant changes, such as shifts in the transition temperature, stabilization of one spin state over the other, or even complete suppression. Although experimental research on SCO systems on surfaces is becoming more common, theoretical investigations remain limited. The main objective of this thesis is to clarify the influence of surfaces and external stimuli on the behavior of SCO molecules, with the aim of establishing general predictive rules and supplying tools for the interpretation of experimental data. Using quantum chemistry methods, mainly density functional theory-based approaches, three specific objectives have been addressed. Firstly, a prototypical SCO complex, [Fe(phen)?(NCS)?], deposited on metallic surfaces Cu(111), Au(111) and Ag(111), is investigated to understand how the nature of the surface affects the spin state change. The interaction with the surface is spin-dependent, with the low-spin (LS) state always more stabilized than the high-spin (HS) one, increasing the transition enthalpy with respect to the free molecule. A clear correlation is found between this effect and the strength of the complex–surface interaction, following the trend Cu(111) > Au(111) > Ag(111). Stronger surface interaction leads to a larger energy difference between spin states, making the transition less probable. These calculations also suggest that the spin state of deposited molecules can be distinguished from STM images. The predictive character of theoretical calculations is further explored through the study of a dinuclear Fe(II) complex interacting with a metallic surface. This complex exhibits a two-step spin transition in the bulk, switching from the [LSLS] state to the [LSHS] state and gradually to the [HSHS] state. The results confirm the same behavior for a single molecule without packing effects. Different adsorption orientations on gold are examined, with the vertical orientation being the most stable. In this configuration, the mixed-spin [LSHS] state is favored over the pure [LSLS] and [HSHS] states, blocking the transition. A second conformer is identified where the [LSLS] state is the ground state, and the transition to the mixed-spin state could be thermally activated at relatively low temperature. In all cases, the [HSHS] state is energetically unfavorable, excluding a complete transition at room temperature. Finally, the impact of external electric fields and applied voltages on the spin transition is explored. The study focuses on heteroleptic [FeII(tpy)?]²? complexes in mechanically controlled break-junctions showing voltage-dependent bistabilities. The proposed model explains the hysteretic I–V behavior through voltage-induced distortion of the Fe coordination sphere and interaction with the electric field. In summary, this thesis contributes to the understanding of spin-crossover persistence in molecule-based nanodevices and provides guidelines for their rational design. The thesis is organized as follows: Chapter 1 introduces SCO complexes and applications, Chapter 2 presents the objectives, Chapter 3 describes the computational methodology, Chapter 4 summarizes the main results with published papers, and Chapter 5 presents the conclusions.

Author dc:creator

• Jaber El Lala, Iman

Advisors dc:contributor.advisor

• Jiménez Calzado, Carmen
• Sánchez de Armas, María Rocío

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/11441/187841

OAI identifier oai:identifier

oai:idus.us.es:11441/187841