A liquid crystal force to reckon with

May 2, 2013
Figure 1: Disrupting the natural packing tendencies of disc-shaped liquid crystals (light blue cylinders) produces stacked columns with significant dipole moments (top left). These columns self-assemble into hierarchical triangular (top right) and hexagonal films (bottom) perfectly aligned for organic electronic applications. Credit: 2013 WILEY-VCH

A need for fast, solution-based processing of organic electronic devices has sparked increased interest in 'discotic' or disc-shaped liquid crystals. These molecules, which contain a flat aromatic core surrounded by hydrocarbon side chains, can spontaneously pile into column-like structures that could be ideal for one-way charge transport. Research led by Takashi Kajitani and Takanori Fukushima from the RIKEN Advanced Science Institute has now revealed a way to turn individual discotic columns into liquid crystal films with unprecedented hierarchical order in two dimensions.

Chemists recently discovered that the formation of columns from discotic molecules is enhanced when the molecules contain bond dipoles—electrochemical imbalances that exist between atoms such as carbon and nitrogen. These dipoles increase the polarization of the 'pi'-bonds that stack aromatic rings together, leading to better column alignment and better electronic pathways. However, discotic columns self-assembled in this manner have no net dipole; instead, the discs pack with alternate 'head-to-tail' layering to minimize free energy, which cancels out the columns' electronic activity.

Kajitani, Fukushima and their colleagues tried a different approach in order to produce 'head-to-head' stacked discotic columns with significant . Starting with dibenzophenazine, an containing carbon–nitrogen bond dipoles, they attached two types of : one consisting of hydrophobic paraffin chains and another of hydrophilic triethylene glycol. The team anticipated that microscopic phase separation between the different side-chain regions could force the discs into a single orientation during aqueous self-assembly.

Synchrotron x-ray experiments revealed that the modified discotic molecules generated unlike any seen before—a 'superlattice' of columns packed into triangle shapes that press together into a two-dimensional hexagonal film with extended lateral order. According to the researchers, this is clear evidence that head-to-head discotic stacking was successful. The striking film patterns are a consequence of the self-assembly neutralizing the large column dipoles over an extended space. "Liquid crystals normally occupy a space between states of matter, so finding a stable structure with wide-ranging two-dimensional lattice correlation is remarkable," says Kajitani.

Intriguingly, they also found that the superlattice columns stacked almost perfectly perpendicular to a glass surface, whereas most other discotic techniques generate a mix of horizontal and vertical orientations. Conductivity measurements showed that this spontaneous 'homeotropic' alignment yields directional properties suitable for high-efficiency organic electronics. Kajitani notes that the unique assemblies created using this technique may be suitable for applications such as organic thin film solar cells and fuel cell assemblies.

Explore further: Reserchers find new method for manipulating liquid crystals

More information: Yeh, M. et al. Amphiphilic design of a discotic liquid-crystalline molecule for dipole manipulation: Hierarchical columnar assemblies with a 2D superlattice structure. Angewandte Chemie International Edition 52, 1031–1034 (2013). dx.doi.org/10.1002/anie.201207708

Related Stories

Reserchers find new method for manipulating liquid crystals

February 11, 2010

(PhysOrg.com) -- A new method for manipulating the molecules of liquid crystals in ways previously unachieved could result in more effective industrial sealants, improved food packaging and even enhanced electronic displays, ...

New family of liquid crystals created

October 5, 2010

(PhysOrg.com) -- Chemists at Vanderbilt University have created a new class of liquid crystals with unique electrical properties that could improve the performance of digital displays used on everything from digital watches ...

Scientists perfect new nanowire technique

October 14, 2010

Scientists at the University of Leeds have perfected a new technique that allows them to make molecular nanowires out of thin strips of ring-shaped molecules known as discotic liquid crystals (DLCs).

Making liquid crystals stand tall

November 21, 2011

Most liquid-crystalline displays contain rod-like molecules that quickly switch from one orientation to another when subjected to electric fields. This movement creates a shutter effect that turns light on and off at high ...

Recommended for you

New polymer able to store energy at higher temperatures

July 30, 2015

(Phys.org)—A team of researchers at the Pennsylvania State University has created a new polymer that is able to store energy at higher temperatures than conventional polymers without breaking down. In their paper published ...

How to look for a few good catalysts

July 30, 2015

Two key physical phenomena take place at the surfaces of materials: catalysis and wetting. A catalyst enhances the rate of chemical reactions; wetting refers to how liquids spread across a surface.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.