Exciton Basis Description of Ultrafast Triplet Separation in Pentacene-(Tetracene)2-Pentacene Intramolecular Singlet...

Precise understanding of the electronic structures of optically dark triplet-triplet multiexcitons of π-conjugated carbon-based systems requires computations incorporating configuration interaction (CI) with up to quadruple excitations from the single-particle ground state of many-electron Hamiltonians. This continues to be a challenge in the context of intramolecular singlet fission (iSF), where the systems of interest are oligomers of large monomer molecules, and CI matrices have dimensions of several million. We have performed many-body calculations of the complete set of excited states relevant to iSF in Pentacene-(Tetracene)2-Pentacene oligomers, consisting of two terminal pentacene monomers linked by two tetracene monomers. Our computations within the Pariser-Parr-Pople Hamiltonian use an exciton basis that gives physical pictorial descriptions of all of the eigenstates. They are performed over an active space of twenty-eight monomer molecular orbitals and include configuration interaction with all relevant quadruple excitations within the active space, thereby ensuring high accuracy. We discuss the many-electron structures of the lowest optical predominantly intramonomer spin-singlets, intermonomer charge-transfer excitations, and most importantly, the complete set of low-energy covalent triplet-triplet multiexcitons. We can explain the weak binding energy of the pentacene-tetracene triplet-triplet eigenstate that is generated immediately following photoexcitation as opposed to the large binding energy of the triplet-triplet in polypentacene. Our approach allows calculations of excited state absorptions (ESAs) from the optical singlet as well as the triplet-triplet, thereby making comparisons with experimental transient absorption measurements in Pentacene-(Tetracene)2-Pentacene feasible. We explain the increase in lifetime with increasing numbers of tetracene monomers of the transient absorption associated with contiguous pentacene-tetracene triplet-triplet oligomers in this family of oligomers. We are able to give a pictorial description of the triplet separation following generation of the initial triplet-triplet, leading to a state with individual triplets occupying only the two pentacene monomers. We expect many applications of our theoretical approach to triplet separation in the iSF.