Applications of small-molecule chirality
Naaman al. demonstrated that the chiral-induced spin selectivity (CISS) effect can be used to control et electrochemical reactions [35, 36], such as hydrogen production through water
Figure 10 Molecular orbital diagrams of singlet and triplet state oxygen
Figure 9
splitting—a crucial process for energy storage and liquid fuel manufacturing. The efficiency of this process is restricted by the oxidation half-reaction, which frequently requires a high overpotential, thus decreasing its overall productivity. Naaman's team demonstrated that by regulating the electron's spin alignment, the overpotential in the photoelectrochemical oxidation of water could be substantially reduced. Electrodes coated with achiral organic molecules displayed overpotentials ranging from 0.5 to 0.7 V, whereas those coated with chiral molecules showed significantly lower overpotentials between 0.0 and 0.2 V. The authors suggested two mechanisms to achieve this reduction. First, in normal water splitting, two types of oxygen can be produced: triplet state oxygen ( 3 O 2 ) ( Figure 10 ) [37], which is more stable and requires less energy to form, and singlet state oxygen ( 1 O 2 ), which is less stable and contributes to the overpotential. The formation of the triplet state is preferred over the singlet state in spin-selection processes with chiral coatings.
Second, spin control inhibits the formation of hydrogen peroxide, which can corrode the electrode and decrease the efficiency of water splitting. In achiral semiconductor- coated anodes lacking spin control, the consequence is the formation of •OH radicals with opposite spin, which can recombine to produce hydrogen peroxide. In contrast, chiral semiconductor coatings enable the selective capture of electrons with the same spin orientation, resulting in spin- aligned •OH radicals that cannot recombine unless their spin is reversed [35]. ( Figure 11 ) [2]
Figure 11
Evaluation and conclusion Small-molecule chirality has shown great potential in various technological applications, such as circularly polarized light emission, controlling molecular motion, and managing electron spin. While the concept is promising, many applications are still at an early stage [2], and more research is needed to move from theoretical models to practical, real-world implementations. The choice of chiral structure plays a crucial role in the performance of these materials, influencing properties like optical behaviour, charge transport, and crystallinity. Further exploration of how different chiral compositions impact material performance is essential. Clearly, the identification of any privileged chiral structure is not restricted to small molecules; numerous alternative chiral materials, including liquid crystals [38], polymers [39, 40], and plasmonic nanostructures [41], are also being investigated. To fully harness the benefits of chirality in technology, increased collaboration between scientists from diverse fields, including chemistry and engineering, will be key in advancing these applications in the future.
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