Coherent vs Thermally Activated Singlet Exciton Fission in Acene Derivatives From First Principles Quantum Dynamics Simulations: Molecular Packing Makes the Difference

Understand

The mechanisms underlying coherent and thermally activated singlet exciton fission in pi-stacked acene crystals are clarified based on quantum dynamics simulations parameterized against a highly correlated description of the electronic excitations and their couplings to intramolecular and intermolecular vibrations.

• In TIPS-pentacene crystals, the relative longitudinal shift of the molecular backbones yields large electronic couplings of the triplet exciton pair with both the singlet exciton and charge transfer (CT) states.
• CT-mediated superexchange and direct pathways are found to contribute synergetically to the ultrafast (100fs) singlet fission process driven by vibronic coherences.
• By contrast, the electronic couplings for singlet fission strictly vanish at the equilibrium pi-stacking of rubrene that exhibits C2h symmetry.