Acene:spacer blends show exothermic singlet fission to proceed coherently, while endothermic fission proceeds incoherently

TL;DR

This study investigates how singlet fission in organic materials can be either coherent or incoherent depending on energy alignment, using spacer molecules to tune and understand the underlying photophysical mechanisms.

Contribution

It introduces a method to control and distinguish between coherent and incoherent singlet fission pathways in organic thin films through spacer molecule-induced vacancy tuning.

Findings

Exothermic fission in pentacene proceeds coherently driven by vibronic resonances.

Endothermic tetracene exhibits incoherent fission unaffected by vibronic coupling.

Vacancy concentration influences the rate of incoherent fission in tetracene.

Abstract

The fission of singlet excitons into triplet pairs in organic materials holds great technological promise, but the rational application of this phenomenon is hampered by a lack of understanding of its complex photophysics. Here, we use the controlled introduction of vacancies by means of spacer molecules in tetracene and pentacene thin films as a tuning parameter complementing experimental observables to identify the operating principles of different singlet fission pathways. Time-resolved spectroscopic measurements in combination with microscopic modelling enables us to demonstrate distinct scenarios, resulting from different singlet-to-triplet energy alignments. For exothermic fission, as found for pentacene, coherent mixing between the photoexcited singlet and triplet pair states is promoted by vibronic resonances, which drives the fission process with little sensitivity to the vacancy concentration. For endothermic materials, such as tetracene, the impossibility of such vibronic resonances renders fission fully incoherent; a process that is shown to slow down with vacancy concentration.