Quantum-Entangled Triplet-Triplet Excitons in Acene Dimers and their role in Singlet Fission

Singlet fission (SF) in which a photoexcited spin singlet dissociates into a pair of low energy triplet excitons has been claimed in covalently linked acene dimers. Internal covnersion to a quantum entangled triplet-triplet state prior to dissociation makes the process spin-allowed. However, recent experiments based on electron spin resonance as well as transient absorption spectroscopy have both shown this intermediate biexciton to be stable against further decoherence thereby raising questions on SF itself. A disconnect between theory and experimental results have led us to calculate the excited state energies and their optical properties using a correlated electron model (Pariser-Parr-Pople Hamiltonian). We formulate a broad theory of quantum entanglement of triplet-triplet state in symmetric and asymmetric dimers. Based on our calculations, we conclude that i) spectroscopic differences in the ESA signals between free and bound triplets exist beyond the visible, ii) entanglement between the triplets is reduced as proximity between acene units is reduced or in the presence of significant steric hindrance, and iii) ESA from singlet exciton shows absorption in the short-wave infrared due to a transition to an optically forbidden even parity state.