Persistence of Triplet-Exciton Spin Entanglement in -acene Organic Materials

We analyze the quantum beats observed after impulsive excitation of triplet exciton pairs entangled into an overall singlet state to determine the intrinsic mechanism that can lead to the randomization of the spin state, as distinguished from possible spin-dephasing effects that are also a possible cause for the decay of fluorescence quantum beats. We perform experiment in single-crystal tetracene and rubrene under an applied magnetic field, which gives single frequency quantum beats, and we determine their lifetime for different beat frequencies and magnetic field orientations in order to determine the main mechanism that limits the stability of the triplet exciton spin and entangled states in these materials.