NPG Asia Materials, in association with Tokyo Institute of Technology

NPG Asia Materials research highlight | doi:10.1038/asiamat.2011.131

Published online 05 September 2011

Chemical sensing: A flip of the chiral switch

The use of anions to turn on molecules that produce circularly polarized light could lead to better display devices.

A chiral compound that flips in the presence of chloride ions can produce circular dichroism (CD) and circularly polarized luminescence (CPL) optical activity on-demand.

Light-emitting organic compounds are already used to brighten the displays of devices like tablet computers and e-readers. Recently, scientists also found that ‘emissive chiral molecules’ — asymmetric substances that naturally polarize light in a forward rotating mode known as circular polarization — can be used to produce clearer, more reliable backlights by eliminating the need for certain display filters. Hiromitsu Maeda and colleagues from Ritsumeikan University and Nara Institute of Science and Technology in Japan have now found a way to impart improved control over this phenomenon by developing the first complex that can turn circularly polarized luminescence (CPL) on or off using an external chemical stimulus1.

Since 2005, Maeda and his co-workers have been designing compounds called boron–dipyrrolyldiketones that respond to anions through rapid changes in molecular geometry. Normally, the complex’s two aromatic–nitrogen rings, known as pyrroles, are oriented with their nitrogen atoms located close to a pair of oxygen-bearing ketone groups. In the presence of certain anions, however, the pyrrole rings flip 180°, altering the molecule’s electronic properties and setting off new optical and fluorescent signals.

To produce a substance with on-demand CPL activity, the team turned to manipulation of the complex’s boron atom. According to Maeda, this element not only augments luminescent emissions, it also serves as a convenient attachment point for chiral aromatic ligands known as binaphthols. X-ray experiments and a theoretical study showed that this novel boron–binaphthol compound was also highly responsive to anions, with its pyrrole rings inverting to produce an M-like conformation when mixed with chloride ions.

After stimulating the boron–binaphthol complex using a laser, optical measurements revealed a dramatic enhancement of CPL after anion binding (see image). The researchers attribute these strong emissions to interactions between the complex’s aromatic rings, which prevent intermolecular electron transfer and force the molecule to emit light. Movement of this light through the chiral ligand yields the desired circular polarization.

Maeda notes that their receptor–anion compounds can also act as planar building blocks for the assembly of supramolecular charge-based materials — emissive chiral ionic substances that may have unusual and extremely strong CPL behavior.

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Reference

  1. Maeda, H.1, Bando, Y.1, Shimomura, K.1, Yamada, I.1, Naito, M.2,3, Nobusawa, K.2, Tsumatori, H.2 & Kawai, T.2 Chemical-stimuli-controllable circularly polarized luminescence from anion-responsive π-conjugated molecules. J. Am. Chem. Soc. 133, 9266–9269 (2011) | article

Author affiliation

1. College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan

2. Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma 630-0192, Japan

3. PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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