Physics – Condensed Matter – Materials Science
Scientific paper
2002-06-03
Physics
Condensed Matter
Materials Science
For IJQC-Sanibel Quantum Chemistry Symposium, 2003
Scientific paper
Electron-hole (e-h) capture in luminescent conjugated polymers (LCPs) is modeled by the dissipative dynamics of a multilevel electronic system coupled to a phonon bath. Electroinjected e-h pairs are simulated by a mixed quantum state, which relaxes via phonon-driven internal conversions to low-lying charge-transfer (CT) and excitonic (XT) states. The underlying two-band polymer model reflects PPV and spans monoexcited configuration interaction singlets (S) and triplets (T), coupled to Franck-Condon active C=C stretches and ring-torsions. Focusing entirely upon long PPV chains, we consider the recombination kinetics of an initially separated CT pair. Our model calculations indicated that S and T recombination proceeds according to a branched, two-step mechanism dictated by near e-h symmetry. The initial relaxation occurs rapidly with nearly half of the population going into excitons ($S_{XT}$ or $T_{XT}$), while the remaining portion remains locked in metastable CT states. While formation rates of $S_{CT}$ and $T_{CT}$ are nearly equal, $S_{XT}$ is formed about twice as fast $T_{XT}$ in concurrence with experimental observations of these systems. Furthermore, breaking e-h symmetry suppresses the XT to CT branching ratio for triplets and opens a slow CT$\to$ XT conversion channel exclusively for singlets due to dipole-dipole interactions between geminate and non-geminate configurations. Finally, our calculations yield a remarkable linear relation between chain length and singlet/triplet branching ratio which can be explained in terms of the binding energies of the respective final excitonic states and the scaling of singlet-triplet energy gap with chain length.
Bittner Eric R.
Karabunarliev Stoyan
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