Applications of small-molecule chirality
( Figure 8 ) [2] The rotor contains a hydrogen atom in one ortho position and a bromine atom in the other. In contrast, the stator features a chiral sulfoxide group and a fluorine atom. Initially, the rotor and stator are positioned perpendicularly to each other. The motor begins to operate upon the introduction of a palladium (II) reagent, which induces C-H cyclometallation [30], allowing one ring to pass over the
other. Following this, the palladacycle undergoes reductive cleavage, resulting in the formation of palladium (0) and causing the rings to settle into an alternate perpendicular configuration. The incorporation of a phosphine ligand then prompts the oxidative addition of Pd (0) to the C-Br bond. This
Figure 8
enables an additional 90° rotation. The concluding 90° rotation is accomplished through reductive elimination of the C-Br bond, thereby finalizing the complete cycle of the motor’s unidirectional rotation. The unidirectional rotation is governed by selective binding of palladium, which gives rise to diastereomeric metal complexes. These diastereomers have different energy states, which favour a specific rotational direction, ensuring the motor's motion is unidirectional. Furthermore, chirality can help align the rotations of multiple motors in the same direction, facilitating the translation of molecular rotary motion into observable macroscopic work. For example, in systems like the rotation of a micrometre-sized glass rod, chirality ensures that the motors' rotational directions are coordinated, allowing them to perform larger-scale tasks [31]. Without chirality, achieving this kind of directional alignment would be challenging or impossible with non-chiral materials. Spin filters and water splitting Magnetism and chirality are closely related. The Chiral-Induced Spin Selectivity (CISS) effect [32], first explored by Ron Naaman and his colleagues, is a phenomenon where chiral molecules can significantly filter electron spins, much more effectively than expected—by over two orders of magnitude. Creating this effect is similar to generating a pseudo-magnetic field, equivalent to hundreds of Tesla. An electron moving through a chiral electrostatic potential encounters an effective magnetic field that aligns along its path, thereby favouring one particular electron spin orientation. If electrons with parallel spin moving in one direction are preferred, those moving in the opposite direction are preferred with antiparallel spin [33, 34]. This coupling of spin and momentum results in spin-polarized electron transport. The energy diagram illustrates the underlying principle of the chiral-induced spin selectivity (CISS) effect: the spin and linear momentum of an electron are coupled in a chiral environment. Starting from an energetically favoured state ( Figure 9a ) [2], inverting either the spin ( Figure 9c ) or the linear momentum ( Figure 9d ) requires energy; however, the other low-energy state ( Figure 9b ) is produced by inverting both spin and linear momentum simultaneously. Opposite enantiomers lead to opposite spins being favoured.
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