The entangled triplet pair in rubrene: transport

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. The individual excitons in a pair can undergo independent diffusion, randomly hopping in the crystal lattice. The probability that a photon is emitted by triplet-pair is then proportional to the probability that the two excitons in the pair meet again. We show that the photons created in rubrene on the occasion of geminate annihilation of a triplet pair can serve as a probe for exciton transport in the crystal lattice. By using a sub-picosecond pulse for photoexcitation, we obtain the time-dependence of the probability of photon emission over 6 time-decades using time-correlated single photon counting. Transport in one, two, or three dimensions can be distinguished and identified on different time scales after photoexcitation, from picoseconds to microseconds. Hopping times and hopping anisotropies can be derived, and temperature and magnetic-field dependence can then provide additional information.