Indications for a Direct Singlet Fission Mechanism in TIPS-Pentacene Crystals from Hybrid DFT/MRCI and Molecular
Timo Schulz1, Simon Hédé1, Christel M Marian1
1Institute of Theoretical and Computational Chemistry, Faculty of Mathematics and Natural Sciences, Heinrich Heine University Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany.
Abstract:
A hybrid quantum mechanics/molecular mechanics setup was used to model the singlet fission (SF) of electronically excited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) in the crystalline phase. The optically bright S1 state possesses nearly identical excitation energies and oscillator strengths in the two nonequivalent dimer units with large and small structural overlap, respectively. A shearing/tilting motion of the two slip-stacked TIPS-pentacene building blocks is the key for stabilizing the singlet-coupled triplet-pair state, 1(TT), in the large/small overlap situation. In both dimer models, the S1 and 1(TT) states swap energetic order upon geometry relaxation, indicative of strong nonadiabatic coupling between these states and a direct SF mechanism in TIPS-pentacene crystals, at variance with unsubstituted pentacene where charge-transfer contributions are substantial in V-shaped substructures. The overall energy balance E(S1) - (E(T1) + E(T1)) remains positive at all investigated nuclear arrangements. The ferromagnetically coupled 5(TT) state lies energetically too high to take part in the SF mechanism.
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