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Multifunctional Spin Crossover Materials

Periodic Reporting for period 2 - SPINSWITCH (Multifunctional Spin Crossover Materials)

Reporting period: 2019-01-01 to 2021-12-31

Molecular multifunctional switching materials attract attention from both scientific and technological points of view due to the possibility of their application in molecular electronics. For example, such materials can be applied as active elements for switching, information storage, signal processing, molecular spintronics, and sensors. An important fact is that molecule-based bistable materials can change their states under the influence of different external stimuli. Spin crossover (SCO) complexes form a group of materials that can change their electronic configuration between the high- (HS) and low-spin (LS) states in response to the change of temperature or pressure, light irradiation, effect of guest molecules or substrates, magnetic field, etc. This effect of molecular bistability is mostly studied for iron(II) complexes. SCO if followed by a strong variation of most of the physical properties of these materials, such as magnetic, optical, electric, mechanical and other properties. Crystal structure of SCO complexes changes as well along the spin transition.
A lot of work in the SCO research that has been done during the last 20 years is directed towards materials which can find some technological applications. The most outstanding examples of SCO application include contrast agents for tomography, thermal displays, routes towards information storage through dielectric bistability, photo-commutation of spin state, microactuators, etc.
Current SCO investigations are devoted to the development of controllable molecular systems, where (i) SCO of the materials can be routed by its another easily accessible property (sorption-desorption, phase behavior etc.) and/or where on contrary (ii) SCO controls some other functions of the material (mechanical, electric, optical, fluorescent properties, etc.). An interesting aspect is the use of SCO units as active elements of sensing materials that can switch in response to change in ambient conditions (pressure, temperature, humidity, chemical guests etc.). In some finely designed systems, the specific parts of SCO molecule can function as a receiver for the external stimuli which makes it an essential element of the multifunctional switching complex. This type of sensitivity allows precise control of the SCO parameters (temperature, abruptness, completeness), which is difficult to be achieved by other methods.
SPINSWITCH has several main goals: Investigation of new multifunctional materials, composites, hybrid mixtures where a controllable macroscopic event controls SCO properties; new metal-organic SCO Hofmann clathrates with sorption-desorption functionality; pressure effect study for prototyping pressure sensors; nanostructuration of SCO materials.
The work performed included full cycle starting from the design and synthesis of new ligands and preparation of new multifunctional SCO compounds based on them to obtain new functional nanomaterials (nanoparticles and thin films). The work included development of synthetic protocols for new SCO complexes with pyridazines, naphtyridine, isothiocarbohydrazide. Protocols towards complexes with isomerizable ligands and those carrying long alkyl chains were as well elaborated. New SCO Fe(II) cyanometallic and triazolic nanoparticles and films were obtained. Additionally, films based upon an Fe(II) triazolylborate complex were elaborated.
The work performed within the project as well included full physico-chemical characterization of new compounds and nanomaterials. Different types of experiments (SQUID magnetometry, single crystal and powder x-ray diffraction, Mössbauer, Raman, IR, UV-vis spectroscopies, optical microscopy and many other techniques) were performed in order to fully characterize physico-chemical parameters of new SCO compounds and materials. Complexes with pyridazines, naphtyridine, isothiocarbohydrazide, series of complexes with isomerizable ligands and those carrying long alkyl chains, new SCO cyanometallic, triazolic and triazolylborate nanoparticles and films were investigated.
The measurements of SCO compounds under pressure and elaboration of SCO pressure sensors were performed. We obtained data regarding SCO characteristics for the nanoparticles of iron-gold complex under pressure. Additionally, pressure effects on the electrical properties of bulk and nanosized samples of iron-gold complex were monitored. Pressure effects were also studied for Fe(II) triazolic nanoparticles and two polymorphs of one Fe(III) SCO complex. Besides, effects of pressure on the Fe(II) triazolic nanoparticles and electric properties of SCO thin films were investigated.
This project displays sophisticated results which can be achieved through fine combination of chemistry, physics and nanoscience. The development of new techniques, strategies, and approaches within these areas is an endless process. A tight contact and collaboration between all the participants (countries, agencies, research groups) that work in the mentioned fields is a priority for ERA, because such a contact contributes to the strength and attractiveness of this research direction. We believe that this project moves in this direction and encourages the exchange of research expertise, know-how, and technologies to create the background for the further innovation activity. The success of this exchange program is the result of a tight collaboration on research and technological development among all participants. This project gave an opportunity to combine efforts of scientists with different fields of expertise for the solution of common tasks. Application of synthetic organic and coordination chemistry, magnetochemistry, photophysics and nanotechnology approaches allowed to develop a common strategy for systematic design of novel materials. A series of new compounds and materials of technological use with predefined properties were obtained. The obtained SCO materials form a strong background needed for the development of efficient, selective and low-cost chemical detectors and pressure sensors.
New multifunctional compounds are of particular interest for academic and industrial partners working in the growing field of molecular materials. These materials can find application in a new generation of molecule-based devices. We have developed compounds with unique characteristics of spin crossover and achieved extraordinary effects of the pressure on different SCO materials. Until the end of the project we plan to extend the set of new SCO materials for potential application and to propose a SCO pressure sensor.
phase diagram in P-T coordinates