Singlet Fission among two Single Molecules

TL;DR

This study uses single molecule spectroscopy to investigate singlet fission in a TDI dimer, revealing heterogeneities and coherent states that deepen understanding of the process at the molecular level.

Contribution

It introduces single molecule spectroscopy as a novel method to study singlet fission, uncovering heterogeneities and coherent superpositions not observable in ensemble measurements.

Findings

Observed static and dynamic heterogeneities in singlet fission rates.

Detected delayed fluorescence and rate fluctuations during spin evolution.

Identified formation of a coherent multiexciton superposition state.

Abstract

Singlet fission (SF) is a photophysical process where a singlet excitation generates two triplet excited states, enhancing exciton multiplication potentially useful for solar energy conversion. Since SF typically outcompetes radiative decay, single molecule studies of SF have remained elusive. Here, we present single molecule spectroscopy of a terrylenediimide (TDI) dimer at room and cryogenic temperatures. By analysing the stream of photons emitted by single dimers, the rates of formation and decay of SF-born triplet states were determined. We report considerable static and dynamic heterogeneities of the SF process which are reflected in broad rate distributions as well as the occasional occurrence of delayed fluorescence and rate fluctuations during spin evolution. Cryogenic experiments point to the formation of a coherent multiexciton superposition state which decays into the singlet exciton from which a correlated triplet pair evolves. Our results establish single molecule spectroscopy as a new avenue into mechanistic details of the SF process which often are drowned by ensemble av-eraging.