Intermolecular Vibrations Drive Ultrafast Singlet Fission

TL;DR

This study reveals that intermolecular vibrational modes, especially rocking motions, facilitate ultrafast singlet fission in pentacene by acting as an electronic bridge, advancing understanding of the primary process in organic semiconductors.

Contribution

The paper combines advanced spectroscopy and quantum calculations to identify specific intermolecular vibrational modes that drive ultrafast singlet fission, providing new mechanistic insights.

Findings

Vibrational coherence mediates singlet to triplet-pair transition.

Intermolecular rocking modes connect singlet and triplet-pair states.

Vibrations drive the process at a multidimensional conical intersection.

Abstract

Singlet fission is a spin-allowed exciton multiplication process in organic semiconductors that converts one spin-singlet exciton to two triplet excitons. It offers the potential to enhance solar energy conversion by circumventing the Shockley-Queisser limit on efficiency. Recently, the mechanism of the primary singlet fission process in pentacene and its derivatives have been extensively investigated, however, the nature of the primary ultrafast process in singlet fission is still a matter of debate. Here, we study the singlet fission process in a pentacene film by employing a combination of transient-grating (TG) and two-dimensional (2D) electronic spectroscopy complemented by quantum chemical and nonadiabatic dynamics calculations. The high sensitivity of heterodyne detected TG spectroscopy enabled us to capture the vibrational coherence and to show that it mediates the transition from the singlet excited electronic state to the triplet-pair state. This coherent process is further examined by 2D electronic spectroscopy. Detailed analysis of the experimental data reveals that significant vibronic couplings of a few key modes in the low- and high-frequency region connect the excited singlet and triplet-pair states. Based on quantum chemical calculations, we identify these key intermolecular rocking modes along the longitudinal molecular axis between the pentacene molecules. They play the essential role of an electronic bridge between the singlet and triplet-pair states. Along with high-frequency local vibrations acting as tuning modes, these rocking motions drive the ultrafast dynamics at the multidimensional conical intersection in the singlet fission process.