A Correlated Many-Electron Approach to Singlet Fission

π−conjugated organic systems present numerous opportunities for exploration of fundamental physics due to the prevalent role of electron correlations in the optoelectronic properties of these systems, as well as potentially being useful for many technological applications. In particular, they may serve as strong candidates for harnessing singlet fission (SF), a spin-allowed photophysical process in which a single incident photon excites the material to the dipole-allowed spin singlet state which then undergoes internal conversion to a pair of triplet excitons bound into a state of total spin 0. These triplets may in principle then dissociate and each contribute an electron-hole (e-h) pair to the resulting photocurrent that can be used in a solar cell, thereby doubling the output of charge carriers generated upon initial photoexcitation in comparison with traditional technologies. Thus, SF represents one possible avenue for surpassing current fundamental solar cell efficiency limits and poses significant promise for addressing outstanding challenges in renewable energy. Complete description and understanding of SF relies on characterization of the triplet pair intermediate state, an inherently multiparticle excited state, as well as potentially charge transfer (CT) states that have been theorized to mediate the SF process. From a theoretical perspective, this requires the use of correlated electronic structure models to properly capture the quantum many-body nature of these excited states. We present results of such calculations based on multireference configuration interaction (CI) techniques applied to the many-body Pariser-Parr-Pople (PPP) model for a series of SF candidate systems based on dimers of acene monomers linked by bridge molecules. Through comprehensive analysis of the excited states relevant for SF and linear and transient absorption spectra, we develop a theory of triplet pair generation in these compounds and present conclusive evidence that this process is mediated by CT states whose specific form depends sensitively on seemingly minute changes to the molecular structures. Comparison of the calculated ground and excited state absorption spectra with experimental results based on transient absorption (TA) spectroscopy enables direct mapping of the calculated optoelectronic properties of these compounds to measured SF dynamics such as the time to generate the triplet pair. Our results thus establish the central role of CT in the SF process and may inform future searches for potential SF candidate systems.