จากการที่ข้าพเจ้าได้เข้าร่วมงานงานประชุมวิชาการระดับนานาชาติด้านเคมีในงาน PACCON 2019 7 – 8 กุมภาพันธ์ 2562 สิ่งที่ได้รับ เพื่อนำมาแชร์ประสบการณ์ คือ งานวิจัยด้าน MOLECULAR SEITHCES โดย Prof. Dr. Bernard L. Feringa University of Groningen, The Netherlands 2016 Nobel Laureate in Chemistry MOLECULAR SWITCHES Chiroptical molecular switches and information storage In our initial attempts to design molecules with the intrinsic dynamic functions that ultimately evolved into molecular rotary motors, we took inspiration from the process of vision. ?is amazing natural responsive process is based on an elementary chemical step, the photochemical cis-trans isomerization around a carbon–carbon double bond in the retinal chromophore (Figure 1). We envisioned the exploration of this simple switching process in the design of molecular information storage units and responsive elements in dynamic molecular systems and materials. Although molecular bi-stability can be induced by various input signals including light, redox reactions, pH changes, metal ion binding, temperature, and chemical stimuli, the use of photochemical switching has distinct advantages as it is a non-invasive process with high spatial-temporal precision. Building on seminal work by Hirshberg on azobenzenes, Heller on fulgides, Irie on diarylethylenes and others, numerous photochromic molecules have been explored in recent years in our group to achieve responsive function, including control of optical and electronic properties of materials, supramolecular assembly processes and biological function. In our journey towards bistable molecules with excellent photoreversibility and high fatigue resistance, we focused on the synthesis of chiral overcrowded alkenes (Figure 1). Non-destructive read-out of state is a central aspect of any potential molecular information storage system, and was addressed by taking advantage of the distinct right (P)- and le (M)-handed helicities in this system, enabling read-out by chiroptical techniques far outside the switching Fig. 1 regime. ?e interconversion between two isomers with distinct chirality, i.e. a chiroptical molecular switch, denes a zero-one digital optical information storage system at the molecular level. Although high-density optical information storage materials based on this approach are promising, the fundamental challenge of addressing individual molecules at the nanoscale in a closely packed assembly in an all-optical device remains to be solved, despite the spectacular advances in single molecule detection techniques seen over the last decades [22]. At this point it is appropriate to emphasize two aspects of these studies. Firstly, the chiral overcrowded alkenes that formed the basis for the chiroptical molecular switches have their genesis in my PhD studies under the guidance of Hans Wijnberg on biaryl atropisomers. ?e idea that twisted olens might show atropisomerism was explored, using the then recently invented McMurry coupling reaction, in the synthesis of cis- and trans-isomers of inherently dissymmetric overcrowded alkenes (see Figure 2 for a time line) [23]. ?e realization that these novel structures had an intrinsic chiral stilbene type chromophore that was immune from the notorious photocyclization seen in stilbenes, provided a stepping stone more than a decade later to chiroptical switches and two decades Fig. 2 later to light-driven rotary molecular motors. Secondly, with the photoisomerization of these chiral overcrowded alkenes, reported in 1991 [24], we demonstrated that controlled clockwise or counterclockwise motion in either direction of one half of the molecule with respect to the other half was achieved simply by changing the wavelength of irradiation. Control of directionality of rotary motion was key to the latter development of molecular rotary motors. ?e photoresponsive overcrowded alkenes were used as chiral dopants in mesoscopic materials to achieve chiroptical switching between cholesteric liquid crystal phases [25], as well as control elements for molecular rotors [26]and for photoswitching the handedness in circular polarized luminescence [27]. ?e wavelengths of switching and the stereoselectivity of the isomerization process were tuned, for instance, via donor-acceptor substituents. In a series of studies together with the Harada group at Tohoku University, we established the chiroptical properties, absolute conguration and racemization pathways of biphenanthrylidenes [28]. An important milestone was our discovery of dynamic control and amplication of molecular chirality by circular polarized light (CPL) [25]. Here CPL irradiation shied the equilibrium between P or M helices of chiroptical switches to achieve a tiny chiral imbalance that was amplied through formation of a twisted nematic liquid crystalline phase. -is discovery strengthened the idea that unidirectional rotary motion was in principle possible using CPL irradiation although, on
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