# Vibronic exciton theory of singlet fission. III. How vibronic coupling and thermodynamics promote rapid triplet generation in pentacene crystals

## TL;DR

This paper extends vibronic exciton theory to explain rapid singlet fission in pentacene crystals, highlighting the roles of vibronic coupling, triplet state multiplicity, and thermodynamics in promoting efficient triplet generation.

## Contribution

It introduces a non-perturbative, microscopic approach to modeling singlet fission dynamics, emphasizing the impact of vibronic coupling and triplet multiplicity in pentacene.

## Key findings

- Strong vibronic coupling creates resonances facilitating fission.
- Large triplet state multiplicity enhances fission efficiency.
- Peierls coupling can both promote and inhibit fission depending on conditions.

## Abstract

We extend the vibronic exciton theory introduced in our previous work to study singlet fission dynamics, in particular addressing recent indications of the importance of vibronic coupling in this process. A microscopic and non-perturbative treatment of electronic and selected vibrational degrees of freedom in combination with Redfield theory allows us to dynamically consider clusters of molecules under conditions close to those in molecular crystals that exhibit fission. Using bulk pentacene as a concrete example, our results identify a number of factors that render fission rapid and effective. Strong coupling to high-frequency Holstein modes generates resonances between the photo-prepared singlet and product triplet states. We furthermore find the large number of triplet combinations associated with bulk periodic systems to be critical to the fission process under such vibronically resonant conditions. In addition, we present results including, in an approximate manner, the effects of Peierls coupling, indicating that this factor can both enhance and suppress fission depending on its interplay with vibronic resonance and thermodynamics.