Orbital-resolved Observation of Singlet Fission

TL;DR

This study uses advanced spectroscopy to observe the initial step of singlet fission in pentacene, revealing a charge-transfer mediated mechanism involving hybridized excitonic states, which enhances understanding of molecular exciton dynamics.

Contribution

It provides the first orbital- and localization-resolved observation of the primary singlet fission step in crystalline pentacene, clarifying the underlying mechanism.

Findings

Charge-transfer mediated mechanism identified

Hybridization of Frenkel and charge-transfer states observed

Localization and orbital character of excitons mapped

Abstract

Singlet fission may boost photovoltaic efficiency [by transforming a singlet exciton into two triplet excitons and thereby doubling the number of excited charge carriers. The primary step of singlet fission is the ultrafast creation of the correlated triplet pair. While several mechanisms have been proposed to explain this step, none has emerged as a consensus. The challenge lies in tracking the transient excitonic states. Here we use time- and angle-resolved photoemission spectroscopy to observe the primary step of singlet fission in crystalline pentacene. Our results suggest a charge-transfer mediated mechanism with a hybridization of Frenkel and charge-transfer states in the lowest bright singlet exciton. We gained intimate knowledge about the localization and the orbital character of the exciton wave functions recorded in momentum maps. This allowed us to directly compare the localization of singlet and bitriplet excitons and decompose energetically overlapping states based on their orbital character. Orbital- and localization- resolved many-body dynamics promise deep insights into the mechanics governing molecular systems and topological materials.