Chiral isotropic liquids from achiral molecules

Boomerang-shaped liquid crystal phase molecules that don’t exhibit ’handedness’ (chirality) have been found to form unusual chiral structures that spontaneously separate into left- and right-handed domains.

Scientists have observed spontaneous separation of mirror-image (enantiomeric) structures in an achiral isotropic liquid crystal - a phase of liquid crystal where the molecules are believed to be randomly aligned, with no long-range order. The finding has been described by the scientific community as remarkable and counterintuitive.

Spontaneous separation of a mixture of mirror-image assemblies of achiral molecules has been seen before in an anisotropic liquid crystal - in which there is some structural order over a long range - but not in an isotropic phase, where the liquid had previously been believed to be macroscopically without order (like water).

The team,  led by Noel Clark at the University of Colorado, Boulder, in the US, had observed this phenomenon before in achiral bent-core liquid crystals, where the molecules are bent in such a way that they look like bananas or boomerangs. 

Previously they saw this in a system with order over a long range (the anisotropic system), with the left- or right-handed enantiomers lining up next to each other and, because of their bent shape, the layer started to tilt and eventually twist - forming left- or right-handed helixes. This inability to line up in a straight line is just the same as if you tried to pack lots of bananas close together. But why this occurs when the molecules are only packed together over a short-range, 100nm or less, isn’t yet clear. 

The Boulder team observed two different types of these phases - a totally disordered phase¹ and a slightly more ordered system where helical structures formed.² These nanofilaments look like nanosized fusilli pasta, and are  incredibly robust and very, very chiral, says Loren Hough, one of the researchers working on the project.

’Spontaneous resolution in a liquid is not intuitive at all,’ says David Amabilino, who works on chiral liquid crystals at Barcelona institute of materials science in Spain. ’You’d think in a liquid everything would balance out and you’d just have the two enantiomers next to each other.’

John Goodby, an expert in liquid crystals at the University of York, UK, agrees, describing the findings as absolutely remarkable scientifically: ’It’s the most bizarre and fascinating thing that takes chirality to extremes’. These findings have ramifications for both the liquid crystal community and its understanding of nanoscale interactions in chemistry, he adds.

’What is particularly impressive about both of these papers is the way they have figured out the layers’ says Amabilino. ’The [transmission electron] microscopy that they have done is really outstanding as it has allowed them to get structural explanations of very complex things that haven’t been looked at before.’

As well as extending our fundamental understanding of science, this work may also have a practical application says Hough. ’We’ve started to think about using the chiral filaments as a template for chiral synthesis,’ he says. When ’dissolved’ in another solvent the filaments separate from each other and remain intact. ’So you can get the individual filaments by themselves; we are wondering if they could be used in an application that uses some kind of chiral surface,’ he adds. Amabilino also finds the idea of using these filaments for organic synthesis intriguing: ’The nanofilament phase doesn’t have very dense packing, so there are lots of voids. Using those voids to do chemistry in is an interesting thing to do.’ 

Nina Notman