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The fission of singlet excitons into triplet pairs in organic materials holdsgreat technological promise, but the rational application of this phenomenon ishampered by a lack of understanding of its complex photophysics. Here, we usethe controlled introduction of vacancies by means of spacer molecules intetracene and pentacene thin films as a tuning parameter complementingexperimental observables to identify the operating principles of differentsinglet fission pathways. Time-resolved spectroscopic measurements incombination with microscopic modelling enables us to demonstrate distinctscenarios, resulting from different singlet-to-triplet energy alignments. Forexothermic fission, as found for pentacene, coherent mixing between thephotoexcited singlet and triplet pair states is promoted by vibronicresonances, which drives the fission process with little sensitivity to thevacancy concentration. For endothermic materials, such as tetracene, theimpossibility of such vibronic resonances renders fission fully incoherent; aprocess that is shown to slow down with vacancy concentration.

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