Spin crossover based functional devices: Molecular switches and sensors

Abstract

Molecular switches and sensors are useful for their ability to monitor change and respond by delivering an output that is recognisable. SCO materials have the ability to exist in either the high spin or low spin state, this makes them an obvious choice for the application as a switch. Varying a stimulus, such as temperature or pressure, can initiate a spin state change in the complex causing it to switch. The output of this switch is often an optical output which can be detected and translated to the user (in some cases, it can be suitably detected with the naked eye). If the spin state change isn’t as abrupt, then the system can no longer be used as a switch as reaching a given temperature or pressure does not cause most of the sample to undergo the spin state change. This, however, is essential for a sensor. A sensor is able to monitor a change and give a different response at a variety of different temperatures or pressures.

Silica based data storage is physically restricted by the size of the silica particles that can be synthesised and deposited onto a surface for use in electronic devices. It is therefore essential to find alternate methods for data storage. Molecular level data storage is one of the proposed solutions.

The nature of the SCO transition is dependent on the ability of the molecules within the sample to interact with each other to propagate the change across the sample. With no cooperativity, the metal complexes do no interact, so the change in one complex has no effect on nearby metal complexes leading to a slow spin state transition in the bulk, which is useful for sensors. However, with ligands designed to strengthen interactions between metal complexes either covalently or non-covalently, it is possible for the spin state change in one complex to cause the spin state change in neighbouring complexes etc. across the sample leading to much more abrupt transitions, which is useful in swiches.

In order for these types of molecules to be useful in terms of data storage, there needs to be a memory component. Should the complexes not “remember” their immediate past, the information stored is lost. Hysteresis loops within the transition profile overcome this problem. Hysteresis simply refers to there being a difference in the forward and reverse transition. In thermally induced SCO for example, this means that the SCO is different depending on from which temperature the process is begun, i.e. increasing the temperature from low to high is different than decreasing the temperature from high to low. Once the molecules enter the loop, the temperature can be returned to ambient temperature and the spin state information is retained.

The final requirement for all of these applications is that the complexes need to be processable so they can be incorporated into functional devices. This can be done via components in the ligands, similar to those used for cooperativity, or via associated counter ions.

In this work, the full chemical synthesis of multiple families of Fe(II) and Fe(III) metal complexes was performed in an attempt to vary the ligand field around the metals to be able to access the SCO potential of the metals and to investigate the cooperativity within the systems. Some magnetic susceptibility measurements are included with promising results, however they haven’t not, so far, shown any SCO capabilities.

Complexes were made with both functional ligands and functional counter ions so that the processability of the systems could be investigated – primarily using the Langmuir-Blodgett technique on amphiphilic systems. It was found that introducing amphiphilicity into the complexes enabled the desired deposition which was confirmed via UV-Visible spectroscopy.

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Identifiers

ORCID for Stephen Goldup: orcid.org/0000-0003-3781-0464

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