Theory of Transient Excited State Absorptions in Molecular Crystals and Dimers : Effects of Electron Correlations and their Role in Singlet Fission

Abstract

Singlet fission (SF), in which a photoexcted spin singlet exciton dissociates into a pair of low lying triplet excitons has the potential to overcome the Shockley-Quessier limit of conventional solar cells. It is well understood that a key intermediate - the multiexcitonic, quantum entangled $^1$(TT)$_1$ state plays a vital role in making the process spin-allowed. Early experiments based on transient absorption spectroscopy (TA) conducted on molecular crystals and dimers of organic molecules reported highly efficient SF. However, with the emergence of optical probes in the infra-red (IR) regime, the ``bound'' triplet pair state has been shown to be stable against decoherence thereby raising questions on SF itself. In this thesis, within a correlated electron model, we report calculations of excited state absorptions (ESAs) from the singlet and triplet excitons and from the triplet-triplet biexciton for pentacene crystal and dimers. We formulate a broad theory of quantum entanglement of triplet-triplet state in these molecular systems and show that spectroscopic differences in the ESA signals between free and bound triplets exist beyond the visible. The triplet exciton primarily absorbs in the visible and any near IR (NIR) absorption depends on the strength of the intermolecular coupling. In contrast, the $^1$(TT)$_1$ state has additional weak ESA in the IR. In the dimers, we show that entanglement between triplets is a function of the proximity as well as the steric hindrance between the molecular chromophores. A weak signal from the singlet exciton is found in the IR and attributed to a transition to a different kind of triplet-triplet state. In summary, our theory of ESA can be extended to other weakly coupled bimolecular systems without any loss of generality.