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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 S^bright_1 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 S^bright_1 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. The overall energy balance E(S^bright_1 ) -(E(T_1)+E(T_1)) remains positive at all investigated nuclear arrangements. Charge-transfer states and the ferromagnetically coupled ^5(TT) state lie energetically too high for taking part in the SF mechanism.
Supplementary materials
Title
Further computational details and results
Description
Computational methods and technical details, definition of state descriptors, validation of the applied methods, results on pentacene and TIPS-pentacene monomer units (molecular orbitals, transition densities, simulated FC-spectra), screening of suitable models, effects of crystal embedding, energies and state descriptors of optically bright dimer states, wavefunction composition of states
with substantial double excitation character, xyz-coordinates of all optimized structures.
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