Nuclear dynamics of singlet exciton fission: a direct observation in pentacene single crystals

TL;DR

This study directly observes nuclear dynamics during singlet exciton fission in pentacene crystals, revealing how atomic motions influence the ultrafast process crucial for optoelectronic efficiency.

Contribution

It provides the first direct measurement of nuclear motions involved in singlet exciton fission using femtosecond electron diffraction combined with simulations.

Findings

Coherent atomic motions at 1 THz detected

Long-range molecular sliding motions influence excitonic coupling

Nuclear dynamics are key to ultrafast triplet pair disintegration

Abstract

Singlet exciton fission (SEF) is a key process in the development of efficient opto-electronic devices. An aspect that is rarely probed directly, and yet has a tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density functional theory and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the disintegration of the electronically correlated triplet pairs, and shed light on why SEF occurs on ultrafast timescales.