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

TL;DR

This study uses quantum dynamics simulations to compare coherent and thermally activated singlet exciton fission in acene crystals, revealing how molecular packing influences the mechanism and efficiency of the process.

Contribution

It provides a detailed first-principles analysis of exciton fission mechanisms, highlighting the role of molecular packing and vibronic couplings in different acene derivatives.

Findings

TIPS-pentacene exhibits ultrafast singlet fission driven by vibronic coherences.

Rubrene's singlet fission is incoherent and temperature-dependent due to symmetry constraints.

Molecular packing critically determines whether fission is coherent or thermally activated.

Abstract

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. In this case, the process is incoherent and driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence.