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Theory of Primary Photoexcitations in Donor-Acceptor Copolymers
Karan Aryanpour1, Tirthankar Dutta1, Uyen N V Huynh2
1Department of Physics, University of Arizona, Tucson, Arizona 85721, USA.
Abstract:
We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have a stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.
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