The energy barrier in singlet fission can be overcome through coherent coupling and entropic gain.

One strategy to improve solar-cell efficiency is to generate two excited electrons from just one photon through singlet fission, which is the conversion of a singlet (S(1)) into two triplet (T(1)) excitons. For efficient singlet fission it is believed that the cumulative energy of the triplet states should be no more than that of S(1). However, molecular analogues that satisfy this energetic requirement do not show appreciable singlet fission, whereas crystalline tetracene displays endothermic singlet fission with near-unity quantum yield. Here we probe singlet fission in tetracene by directly following the intermediate multiexciton (ME) state. The ME state is isoenergetic with 2 × T(1), but fission is not activated thermally. Rather, an S(1) ⇔ ME superposition formed through a quantum-coherent process allows access to the higher-energy ME. We attribute entropic gain in crystalline tetracene as the driving force for the subsequent decay of S(1) ⇔ ME into 2 × T(1), which leads to a high singlet-fission yield.