Paths to Singlet Fission in Rubrene

In crystalline organic semiconductors that consist of tightly packed small molecules, photon absorption can result in the creation of a pair of triplet excitons in an entangled spin state with zero total spin. Some well-known materials where this happens are single-crystal tetracene and rubrene.We studied the time-dynamics of singlet fission in rubrene by using two simultaneous non-collinear picosecond pulses to create a grating-like modulation in triplet density. Diffraction of a probe pulse from this grating is then used to detect the evolution of the triplet density during fission. By tuning the wavelength of light used for photoexcitation we can study fission from different excited states, and by changing the probe wavelength we isolate the response due to the two primary singlet fission processes in rubrene. While we found that the the primary fission process is essentially complete in 10 picosecond, we also isolate other contributions.