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The added value of small-molecule chirality in technological applications

Abstract

Chirality is a fundamental symmetry property; chiral objects, such as chiral small molecules, exist as a pair of non-superimposable mirror images. Although small-molecule chirality is routinely considered in biologically focused application areas (such as drug discovery and chemical biology), other areas of scientific development have not considered small-molecule chirality to be central to their approach. In this Review, we highlight recent research in which chirality has enabled advancement in technological applications. We showcase examples in which the presence of small-molecule chirality is exploited in ways beyond the simple interaction of two different chiral molecules; this can enable the detection and emission of chiral light, help to control molecular motion, or provide a means to control electron spin and bulk charge transport. Thus, we demonstrate that small-molecule chirality is a highly promising avenue for a wide range of technologically oriented scientific endeavours.

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Figure 1: Direct emission and detection of CP light.
Figure 2: Chiroptical switches and molecular machines.
Figure 3: Controlling and exploiting electron spin.
Figure 4: Control of bulk electronic properties by stereochemical composition.

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Acknowledgements

The authors thank the chirality community for the fruitful discussions that inspired this Review, acknowledge their long-standing collaboration with A. Campbell and thank the Engineering and Physical Sciences Research Council (EPSRC) for funding (EP/L014580/1, EP/L016702/1, and EP/P000525/1).

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Glossary

Chirality

The geometric property of an object (or spatial arrangement of points or atoms) of being non-superimposable on its mirror image; such an object has no mirror plane, centre of inversion or rotation–reflection axis. If the object is superimposable on its mirror image the object is achiral (non-chiral).

Enantiomers

A pair of molecular entities that are non-superimposable mirror images of each other.

Stereochemistry

A subdiscipline of chemistry that involves the study and manipulation of the relative spatial arrangement of atoms that form the structure of molecules.

Cahn–Ingold–Prelog (CIP) priority rules

The CIP priority rules are a set of rules to unequivocally name a stereogenic element within a molecule. CIP rules are used to assign an R or S descriptor to stereocentres (and other stereogenic elements) and an E or Z descriptor to double bonds.

Chiral composition

A chiral mixture that differs in the ratio of enantiomers (known as the enantiomeric ratio). Example chiral compositions include racemates, enantiopure substances or scalemic mixtures (see further definitions for each of these terms).

Circularly polarized (CP) light

Circular polarization of an electromagnetic wave is a polarization state in which the electric field of the passing wave does not change strength but only changes direction in a rotary manner. Circularly polarized light is chiral and can be left- or right-handed.

Dissymmetry factor

(g factor). A measure of the dissymmetry between the left- and right-handed components of circularly polarized light; it is defined as g = (lLlR)/½(lL + lR), where lL and lR are the absorption or emission intensities for left- and right-handed polarized light, respectively. The parameter observed is denoted as a subscript: for example, absorption (gabs), photoluminescence (gPL) or electroluminescence (gEL).

Enantiopure

A substance that consists of a single enantiomer, within the limit of detection.

Chiroptical

A term referring to the optical techniques (using refraction, absorption or emission of anisotropic radiation) for investigating chiral substances (for example, measurements of optical rotation at a fixed wavelength, optical rotatory dispersion, electronic circular dichroism and circular polarization of luminescence).

Stereogenic elements

Groupings within molecular entities that give rise to stereoisomerism. Some stereogenic elements can be interconverted between the two possible configurations in a selective (that is, stereospecific) or unselective manner (for example, racemization) upon exposure to stimuli such as light, heat or chemical reagents. For the purpose of this Review, we differentiate between stereogenic elements that can interconvert under the selected conditions (labile) and those that cannot (stable).

Stereoisomers

Isomers that differ in the spatial arrangement of atoms without any differences in connectivity or bond multiplicity. Enantiomers and diastereoisomers are examples of stereoisomers.

Diastereoisomers

Stereoisomers that are not related as mirror images. Diastereoisomers are characterized by differences in physical properties and by some differences in chemical behaviour.

Meso isomers

Achiral members of a series of diastereoisomers in which at least one isomer is chiral.

Racemates

Equimolar mixtures of enantiomer pairs.

Scalemic mixtures

Mixtures of enantiomers at a ratio other than 1:1.

Organic semiconductors

A class of conjugated organic materials that combine the electronic advantages of semiconducting materials with the chemical and mechanical benefits of organic compounds such as plastics. Their charge transport properties can be induced to lie between an insulator and a semiconductor.

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Brandt, J., Salerno, F. & Fuchter, M. The added value of small-molecule chirality in technological applications. Nat Rev Chem 1, 0045 (2017). https://doi.org/10.1038/s41570-017-0045

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