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

Precise understanding of the electronic structures of optically dark triplet-triplet multiexcitons that are the intermediate states in singlet fission (SF) continues to be a challenge. This is particularly true for intramolecular singlet fission (iSF) chromophores, that are oligomers of large monomer molecules. We have performed quantum 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 use an exciton basis that gives physical pictorial descriptions of all eigenstates, and are performed over an active space of twenty-eight monomer molecular orbitals, including configuration interaction with all relevant quadruple excitations within the active space, thereby ensuring very high precision. We discuss the many-electron structures of the optical predominantly intramonomer spin-singlets, intermonomer charge-transfer excitations, and most importantly, the complete set of low energy covalent triplet-triplet multiexcitons. We are able to explain the weak binding energy of the pentacene-tetracene triplet-triplet eigenstate that is generated following photoexcitation. We explain the increase in lifetime with increasing numbers of tetracene monomers of the transient absorption associated with contiguous pentacene-tetracene triplet-triplet in this family of oligomers. We are consequently 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.