The Singlet–Triplet Exchange Energy in Conjugated Polymers

Authors


  • We thank Z. Shuai for fruitful discussions and J.-P. Calbert for assistance with the analysis software package ZOA [29]. AK thanks Peterhouse, Cambridge, and the Royal Society for funding. The Mons–Cambridge joint collaboration is supported by the European Commission IST programs ‘STEPLED’ and ‘LAMINATE’. The work in Mons is partly supported by the Belgian Federal Services for Scientific, Technical, and Cultural Affairs (InterUniversity Attraction Pole 5/3) and the Belgian National Science Foundation (FNRS). DB is an FNRS Senior Research Associate.

Abstract

Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ΔEST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ΔEST value close to 0.7 eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the π-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.

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