Correlated-Electron Theory of Triplet-Triplet Multiexciton States in Polypentacene
We present correlated-electron calculations of optical spin-singlet and triplet-triplet multiexciton states in three- and four-unit pentacene oligomers as models for polypentacene. The calculations use the Pariser-Parr-Pople Hamiltonian, multi-reference singles and doubles configuration interaction, ad a molecular exciton basis that resolves Frenkel, charge-transfer and triplet-pair $^1$(T$_1$T$_1$) configurations in real space. We find that the complete set of $^1$(T$_1$T$_1$) eigenstates lies in a narrow, nearly degenerate energy window near the lowest optical exciton and that no eigenstate can be identified with a single localized triplet-pair configuration. Instead each triplet-pair eigenstate is a quantum superposition of configurations containing all accessible intertriplet separations. This electronic structure explains the perceived absence of intermolecular triplet diffusion in pentacene oligomers, polypentacene, and polytetracene solutions, while leaving open the possibility of intermolecular singlet fission in films with appreciable interchain interactions.