Observing the Multiexciton State in Singlet Fission and Ensuing Ultrafast Multielectron Transfer

TLDR

Multiple exciton generation produces two or more electron‑hole pairs from a single photon, yet its implementation remains difficult and the underlying photo‑physical dynamics in nanocrystalline and molecular systems are still unclear. By employing pentacene/fullerene bilayers and femtosecond nonlinear spectroscopy, the authors directly observed the multiexciton state that arises from singlet fission in pentacene. The data reveal that this multiexciton state coherently co‑exists with the optically populated singlet and that electron transfer to fullerene occurs within sub‑picoseconds—an order of magnitude faster than transfer from the triplet exciton.

Abstract

Multiple exciton generation (MEG) refers to the creation of two or more electron-hole pairs from the absorption of one photon. Although MEG holds great promise, it has proven challenging to implement, and questions remain about the underlying photo-physical dynamics in nanocrystalline as well as molecular media. Using the model system of pentacene/fullerene bilayers and femtosecond nonlinear spectroscopies, we directly observed the multiexciton (ME) state ensuing from singlet fission (a molecular manifestation of MEG) in pentacene. The data suggest that the state exists in coherent superposition with the singlet populated by optical excitation. We also found that multiple electron transfer from the ME state to the fullerene occurs on a subpicosecond time scale, which is one order of magnitude faster than that from the triplet exciton state.