Direct Observation of Ultrafast Singlet Exciton Fission in Three Dimensions

TL;DR

This paper introduces a novel ultrafast microscopy technique capable of tracking photoexcitations in three dimensions with high spatial and temporal resolution, revealing detailed dynamics of singlet exciton fission in pentacene films.

Contribution

The study demonstrates a new interferometric pump-probe microscopy method for 3D tracking of exciton dynamics with sub-10 nm spatial and 15 fs temporal precision, applied to ultrafast singlet fission.

Findings

Observed 25 nm, 70 fs expansion of joint-density-of-states along crystal a,c-axes.

Detected 6 nm, 115 fs change in exciton density along the b-axis.

Proposed mechanism involves exciton expansion along maximal orbital overlap, followed by molecular sliding-induced triplet separation.

Abstract

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub 10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO$_2$) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital $\pi$-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. This methodology opens a new route for the study of three dimensional transport on ultrafast timescales.