  Singlet fission is a process whereby two triplet excitons can be produced
from one photon, potentially increasing the efficiency of photovoltaic devices.
Endothermic singlet fission is desired for maximum energy conversion
efficiency, and such systems have been shown to form an excimer-like state with
multi-excitonic character prior to the appearance of triplets. However, the
role of the excimer as an intermediate has, until now, been unclear. Here we
show, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as a
prototypical example, that, rather than acting as an intermediate, the excimer
serves to trap excited states, to the detriment of singlet fission yield. We
clearly demonstrate that singlet fission and its conjugate process,
triplet-triplet annihilation, occur at a longer intermolecular distance than an
excimer intermediate would impute. These results establish that an endothermic
singlet fission material must be designed that avoids excimer formation, thus
allowing singlet fission to reach its full potential in enhancing photovoltaic
energy conversion.
