Applications of small-molecule chirality
additives in otherwise achiral polymers, enabling existing OLED materials to be adapted without redesign. For example, Fuchter and co-workers (2013) [19] showed that adding an enantiopure helicene ( Figure 5b ) to the common polymer F8BT induced CP emission with |𝑔 𝐸𝐿 | up to 0.2, which could result in a 10% higher efficiency relative to filtered displays. However, fluorescence limits efficiency because only singlet excitons (25% of total states) emit light [20]. Phosphorescent OLEDs (PHOLEDs), which exploit heavy-metal dopants like iridium or platinum to harvest triplet states, offer higher efficiencies. Chiral metal complexes can also emit CP light. Di Bari and colleagues reported the first CP-PHOLED using a europium (III) complex ( Figure 5c ) [21], achieving exceptionally high |𝑔 𝐸𝐿 | values (up to 1.0 at 595 nm), though weak emission limited device efficiency [22]. Later, Crassous, Autschbach, and co-workers [23] developed a helical platinum (II) complex ( Figure 5d ) that combined display-level brightness with |𝑔 𝐸𝐿 | up to 0.38, showing improved balance between polarization strength and luminance. Molecular machines and unidirectional movement Molecular machines refer to molecular systems designed to exhibit directional motion or perform mechanical work at a molecular level [24]. They are distinguished from simpler molecular switches by their ability to undergo significant structural changes between different stable states, rather than just
small, reversible transitions. The use of chirality has become central in the design of these machines, particularly in unidirectional rotary motors ( Figure 6 )[25, 26]. These motors often employ two stereogenic elements within a molecule—one labile (able to undergo changes) and the other stable. A combination of two stereogenic elements can produce a maximum of four possible stereoisomers, with two diastereoisomers that can exist as two enantiomers each ( Figure 7 ) [27]. Unlike enantiomers, which are
Figure 6 Molecular car
energetically equivalent, diastereoisomers have different physical and chemical properties. Feringa's team frequently employs diastereomeric energy differences in rotary motors to dictate the preferred conformation of a given state, which is then altered by an external stimulus to switch between these states, ultimately yielding unidirectional movement.
Figure 7 Difference between enantiomers and diastereomers
A recent example of this type of motor uses the stable point chirality of a sulfoxide group in conjunction with the flexible axial chirality of a biaryl single bond to achieve a chemical fuel-driven unidirectional rotary motion [28]. Synthetic molecular motors have been driven by a range of stimuli, including electrochemical energy [29]. However, those that rely on chemical fuel consumption are a significant improvement, especially given that the biological motors that often inspire them are also powered by chemical energy. The motor consists of two aryl rings, one acting as a rotor and the other as a stator.
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