Endothermic singlet fission is hindered by excimer formation

Singlet fission is a process whereby two triplet excitons can be producedfrom one photon, potentially increasing the efficiency of photovoltaic devices.Endothermic singlet fission is desired for maximum energy conversionefficiency, and such systems have been shown to form an excimer-like state withmulti-excitonic character prior to the appearance of triplets. However, therole of the excimer as an intermediate has, until now, been unclear. Here weshow, using 5,12-bis((triisopropylsilyl)ethynyl)tetracene in solution as aprototypical example, that, rather than acting as an intermediate, the excimerserves to trap excited states, to the detriment of singlet fission yield. Weclearly demonstrate that singlet fission and its conjugate process,triplet-triplet annihilation, occur at a longer intermolecular distance than anexcimer intermediate would impute. These results establish that an endothermicsinglet fission material must be designed that avoids excimer formation, thusallowing singlet fission to reach its full potential in enhancing photovoltaicenergy conversion.