  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.
