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Author(s):Lin, Zhou
Contributor(s):Van Voorhis, Troy; Iwasaki, Hikari
Subject(s):Mini-symposium: Multiple Potential Energy Surfaces
Abstract:Singlet fission (SF) converts a singlet exciton (S$_1$) into a pair of triplet ones (T$_1$) via a ``multi-exciton'' (ME) intermediate: S$_1$ $longleftrightarrow$ $^1$ME $longleftrightarrow$ $^1$(T$_1$T$_1$) $longrightarrow$ 2T$_1$.footnote{M. B. Smith and J. Michl, {it Chem. Rev.} {bf 110}, 6891 (2010).} In exothermic cases, {it e.g.}, crystalline pentacene or its derivatives, the quantum yield of SF can reach 200%. With SF doubling the electric current generated by an incident high-energy photon, the solar conversion efficiency in pentacene-based organic photovoltaics (OPVs) can exceed the Shockley--Queisser limit of 33.7%.footnote{W. Shockley and H. J. Queisser, {it J. Appl. Phys.} {bf 32}, 510 (1961).} _x000d_ The ME state is popularly considered to be a dimeric state with significant charge transfer (CT) character that is strongly coupled to both S$_1$ and $^1$(T$_1$T$_1$),footnote{T. C. Berkelbach, M. S. Hybertsen, and D. R. Reichman, {it J. Chem. Phys.} {bf 141}, 074705 (2014).} while this local model lacks strong support from full quantum dynamics studies._x000d_ Intramolecular SF (ISF) occurring to covalently-bound dimers in the solution phase is an excellent model for a straightforward dynamics simulation of local excitons. _x000d_ In the present study, we investigate the ISF mechanisms for three covalently-bound dimers of pentacene derivatives, including {it ortho}-, {it meta}-, and {it para}-bis(6,13-bis(triisopropylsilylethynyl)pentacene)benzene, in non-protic solvents. Specifically, we propagate the real-time, non-adiabatic quantum mechanical/molecular mechanical (QM/MM) dynamics on the potential energy surfaces associated with the states of S$_1$, $^1$(T$_1$T$_1$) and CT.footnote{M. G. Mavros, D. Hait, and T. A. Van Voorhis, {it J. Chem. Phys.} {bf 145}, 214105 (2016).}$^,$footnote{V. Vaissier, and T. A. Van Voorhis, {it in preparation}}_x000d_ We explore how the energies of these ISF-relevant states and the non-adiabatic couplings between each other fluctuate with time and the instantaneous molecular configuration ({it e.g.}, intermonomer distance and orientation). We also quantitatively compare Condon and non-Condon ISF dynamics with solution-phase spectroscopic data._x000d_ Our results allow us to understand the roles of CT energy levels in the ISF mechanism and propose a design strategy to maximize ISF efficiency.
Issue Date:6/21/2017
Publisher:International Symposium on Molecular Spectroscopy
Citation Info:APS
Date Available in IDEALS:2017-07-27

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