Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State Using Femtosecond Mid-Infrared Spectroscopy

Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, ¹(S₁S₀), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, ¹(T₁T₁), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (XanTDI₂). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton ¹(T₁T₁) state has absorptions characteristic of the T₁ state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the C=C stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CH₂Cl₂ and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character.