Photo SCO LCs · Photoresponsive spin-crossover liquid crystals

Photoresponsive spin-crossover liquid crystals

Since the end of the 20th century, digital information has been permeating every aspect of our lives and society, leading to the exploding growth of information production. A recent study reported the total amount of data created, captured, copied and consumed worldwide in 2018 was 33 zettabytes (ZB), equivalent to 33 trillion gigabytes (GB). In 2020, this number grows to 59 ZB and is expected to reach an unimaginable 175 ZB by 2025. Therefore, scientists face a big challenge to develop more lightweight and high-density information storage materials. With the rapid development of the field of microelectronics, electronic components at the molecular level have attracted more and more attention and become one of the research hotspots in materials science. Professor O. Kahn has defined bistability at the molecular level as follows: molecular bistability means that under certain external conditions, the molecule will exist two stable or metastable electronic states. Spin-crossover (SCO) complex is a kind of typical bistable material, which can change electron configuration between high spin (HS) state and low spin (LS) state in response to light, temperature, pressure and other external stimuli, resulting in a series of physical and chemical properties such as magnetism, color, dielectric constant, density and so on. As a result, the high-low spin state is compared to the binary encoding 0 and 1 in electronics and show attractive application prospect in the fields of information storage. Since the first observation of SCO behavior in a mononuclear Fe(III) complex in 1932, numerous SCO complexes with different SCO characteristic, such as abruptness, step and hysteresis, have be observed. At present, most of the research on SCO is mainly limited to the properties of the crystalline compounds themselves, however, the research on SCO functional devices has been difficult because the mechanical strength and processing properties of SCO materials are limited by the nature of their coordination compounds. Therefore, the developing of soft SCO materials are of particular importance to realize the industrial application. For obtained this goal, the combination of SCO behaviour with liquid crystal (LC) properties in a single material has been preliminarily explored by researchers because of the advantages in potential applications such as process ability in thin films. In 2001, Galyametdinov et al reported the first example of coexistence of thermal spin transition and liquid-crystal (LC) properties. The compound displays a gradual spin conversion between 300 and 80 K, while the smectic phase forms around 400 K. This study first demonstrated the possibility of coexistence of spin crossover and liquid-crystalline (LC) properties in a single compound. This result motivated several groups to decorate the long alkyl chain on the SCO molecules to obtain the SCO LCs and study the synergy between SCO and LC properties. However, so far, the reported examples are still rare. In 2018, the hosting group at Centre de Recherche Paul Pascal (CRPP), in collaboration with Institute for Solid State Chemistry Bordeaux (ICMCB) has used this strategy and synthesized mononuclear Fe(II) based SCO molecules with decyl chains to demonstrate the synergetic behavior between conformational changes of alkyl groups and SCO behavior. Based on the preliminary work, the objective of this fellowship was to modify the decoration position and numbers of alkyl groups to realize the LCs in the selected SCO molecular system. Then, by introducing photochromic azobenzene group in the system, the spin state of SCO molecules could be optically controlled through structural trans-cis photoisomerism.

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