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First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular pi Stacking

Abstract : Singlet excitons in pi-stacked molecular crystals can split into two triplet excitons in a process called singlet fission that opens a route to carrier multiplication in photovoltaics. To resolve controversies about the mechanism of singlet fission, we have developed a first principles nonadiabatic quantum dynamical model that reveals the critical role of molecular stacking symmetry and provides a unified picture of coherent versus thermally activated singlet fission mechanisms in different acenes. The slip-stacked equilibrium packing structure of pentacene derivatives is found to enhance ultrafast singlet fission mediated by a coherent superexchange mechanism via higher-lying charge transfer states. By contrast, the electronic couplings for singlet fission strictly vanish at the C-2h symmetric equilibrium pi stacking of rubrene. In this case, singlet fission is driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence. Design rules for optimal singlet fission materials therefore need to account for the interplay of molecular pi-stacking symmetry and phonon-induced coherent or thermally activated mechanisms.
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https://hal-amu.archives-ouvertes.fr/hal-01415152
Contributor : Didier Siri <>
Submitted on : Monday, December 12, 2016 - 6:49:05 PM
Last modification on : Thursday, December 5, 2019 - 10:40:03 AM

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Hiroyuki Tamura, Miquel Huix-Rotllant, Irene Burghardt, Yoann Olivier, David Beljonne. First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular pi Stacking. Physical Review Letters, American Physical Society, 2015, 115 (10), pp.107401. ⟨10.1103/PhysRevLett.115.107401⟩. ⟨hal-01415152⟩

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