Contrary to a long-used model, electrons moving in polymer semiconductors can retain coherence as they travel along the chain
Introduction
Thanks to their printable nature, conjugated polymers offer great promise as electronic components in future devices that would be smaller and cheaper than traditional ones, and could even form the basis for flexible displays. Electrons — bound to a positive hole in an exciton — are thought to move through these polymers by hopping, during which quantum coherence is lost. Apart from a recent contradictory finding in a single crystal at low temperatures, this model has accounted for all experimental studies.
Now, Elisabetta Collini and Gregory Scholes from the University of Toronto have used ultrafast spectroscopy to show1 that — at least on femtosecond timescales — some coherence is retained. They used two-time anisotropic delay experiments to study the electron energy transfer in a poly(p-phenylenevinylene).
They showed that coherence is preserved when the energy difference between two nearby excited states is similar to the coupling between them. This was only the case when the polymer was in a 'good' solvent that allowed the chains to extend; in aqueous suspensions, the polymers are coiled together and the excitons hop between chains, losing coherence. This discovery may lead to improving the efficiency of electron transfer in polymers, by guiding synthetic strategies that enhance transfer along the conjugated chains.
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Reference
Collini, E. & Scholes, G. D. Coherent intrachain energy migration in a conjugated polymer at room temperature. Science 323, 369–373 (2009). | Article | ChemPort |