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Cleverly designed molecules display high luminescent efficiency even when assembled into aggregates. Many molecules that emit light when dissolved in solution are not so effective when
embedded into a solid. This makes exploiting these ‘luminophores’ in a practical device difficult. Luminescent molecules that are perfectly efficient even in thin solid films have now been
engineered by Ben Zhong Tang and co-workers at the Hong Kong University of Science and Technology and Jilin University in China.1 “Luminophores in a dilute solution are isolated and there is
little interaction between them,” explains Tang. “In the solid state, however, the molecules aggregate: they exist in close vicinity, and this enables strong intermolecular processes.
Excited molecules can therefore relax without emitting light.” For example, a dimeric derivative of the organic compound triphenylamine (TPA) emits approximately 75 photons for every 100
absorbed when in solution. In a thin film, however, the efficiency of this process decreases to just 14%. Previous techniques to circumvent the problem have focused on preventing
aggregation, but this has inevitable side effects, including obstructing the flow of charge carriers, which present an obstacle to the development of electronic devices based on these
materials. Fig. 1: The TPA–TPE system emits blue light in powder (top right) and crystal (bottom) form. It does not emit light in solution (top left), but when water is added to induce
aggregation, the emission increases by 100-fold.© 2010 B. Z. Tang Tang and his co-workers take an entirely new approach that doesn’t try to avoid aggregation but in fact encourages it. They
synthesized a molecule that combines TPA with tetraphenylethene (TPE). TPE has exactly the opposite characteristic of TPA: it only emits light when aggregated, and not when dissolved in
solution. A combination of the two molecules has the advantages of both: the aggregate-induced emission of TPE and the charge-transport properties of TPA. The scientists dissolved the new
molecules in a solvent in a way that prevented aggregation. The solution emitted very little light when illuminated with an ultraviolet source (Fig. 1). However, promoting aggregation by
adding water to the solvent increased the emission intensity by a factor of almost 100. What is more, the luminescence efficiency of thin films of the material was as high as 100%, depending
on the structure of the molecule. The electronic properties of the films were also good enough for use in organic light-emitting diodes. Tang hopes that in the future, the idea can be
generalized and applied to other fluorescent molecules. REFERENCES * Yuan, W.Z., Lu, P., Chen, S., Lam, J.W.Y., Wang, Z., Liu, Y., Kwok, H.S., Ma, Y. & Tang, B.Z. Changing the behavior
of chromophores from aggregation-caused quenching to aggregation-induced emission: development of highly efficient light emitters in the solid state. _Adv. Mater._ Published Online: 8 Mar
2010 doi: 10.1002/adma.200904056 Article CAS Google Scholar Download references ADDITIONAL INFORMATION 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. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Organic optics: Working together.
_NPG Asia Mater_ 2, 86 (2010). https://doi.org/10.1038/asiamat.2010.67 Download citation * Published: 06 May 2010 * Issue Date: July 2010 * DOI: https://doi.org/10.1038/asiamat.2010.67 SHARE
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