1 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom.
2 Fritz Haber Institute of the Max Planck Society, Department of Physical Chemistry, Faradayweg, 4-614195 Berlin, Germany.
3 Eberhard-Karls Universität Tübingen, Institut für Angewandte Physik, Auf der Morgenstelle 10, 72076 Tübingen, Germany.
4 Department of Chemical System Engineering, The University of Tokyo, Tokyo 113-8656, Japan.
5 Dipartimento di Chimica Industriale "Toso Montanari", Università di Bologna and INSTM-UdR Bologna, Viale Risorgimento 4, 40136 Bologna, Italy.
6 Institut Néel, CNRS and Grenoble Alpes University, F-38042 Grenoble, France.
7 Department of Physics, Department of Chemistry and Biochemistry, Centre for Research in Molecular Modeling (CERMM), Centre for NanoScience Research (CeNSR), Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec H4B 1R6, Canada.
8 Laboratory for Chemistry of Novel Materials, Department of Chemistry, Université de Mons, Place du Parc 20, 7000 Mons, Belgium.

Singlet exciton fission is a spin-allowed process in organic semiconductors by which one absorbed photon generates two triplet excitons. Theory predicts that singlet fission is mediated by intermolecular charge-transfer states in solid-state materials with appropriate singlet-triplet energy spacing, but direct evidence for the involvement of such states in the process has not been provided yet. Here, we report on the observation of subpicosecond singlet fission in mixed films of pentacene and perfluoropentacene. By combining transient spectroscopy measurements to nonadiabatic quantum-dynamics simulations, we show that direct excitation in the charge-transfer absorption band of the mixed films leads to the formation of triplet excitons, unambiguously proving that they act as intermediate states in the fission process.