# Vibronic exciton theory of singlet fission. I. Linear absorption and the anatomy of the correlated triplet pair state

## TL;DR

This paper develops a vibronic exciton theory for singlet fission, analyzing the role of vibronic coupling in the process, and applies it to pentacene to interpret experimental absorption data and characterize key excited states.

## Contribution

It introduces a microscopic, non-perturbative vibronic model for singlet fission and applies it to pentacene, providing insights into excited states and their relation to experimental observations.

## Key findings

- Vibronic coupling is crucial for singlet fission.
- The model accurately characterizes the correlated triplet pair state.
- Optimal basis size for calculations is identified.

## Abstract

Recent time-resolved spectroscopic experiments have indicated that vibronic coupling plays a vital role in facilitating the process of singlet fission. In this work, which forms the first article of a series, we set out to unravel the mechanisms underlying singlet fission through a vibronic exciton theory. We formulate a model in which both electronic and vibrational degrees of freedom are treated microscopically and non-perturbatively. Using pentacene as a prototypical material for singlet fission, we subject our theory to comparison with measurements on polarization-resolved absorption of single crystals, and employ our model to characterize the excited states underlying the absorption band. Special attention is given to convergence of photophysical observables with respect to the basis size employed, through which we determine the optimal basis for more expensive calculations to be presented in subsequent work. We furthermore evaluate the energetic separation between the optically prepared singlet excited state and the correlated triplet pair state, as well as provide a real-space characterization of the latter, both of which are of key importance in the discussion of fission dynamics. We discuss our results in the context of recent experimental studies.