Tuning the charge transfer character of the multiexciton state in singlet fission

Intramolecular singlet fission (SF) produces the multiexciton correlated triplet pair state, (T₁T₁), prior to the formation of free triplet excitons. The nature of the multiexciton state is complex, as generation of the (T₁T₁) state may involve a charge transfer (CT) intermediate and has been shown to have both mixed electronic and spin characters. According to transient absorption spectroscopy, a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI₂) exhibits solvent-dependent excited-state dynamics. As solvent polarity increases from 1,2,4-trichlorobenzene (ε = 2.2) to chlorobenzene (ε = 5.6) to 1,2-dichlorobenzene (ε = 9.9), the SF rate in TDI₂ increases and the multiexciton state, which can be thought of as a linear combination of the ¹(S₁S₀), CT, and (T₁T₁) states, gains more CT character. Eventually, the CT state becomes a trap state as indicated by symmetry-breaking charge separation in TDI₂ in pyridine (ε = 12.3). The dielectric environment influences not only the SF rate and the relative contributions of the ¹(S₁S₀), CT, and (T₁T₁) states to the overall multiexciton state but also the rate at which the state mixing evolves, with faster dynamics in higher polarity solvents. More importantly, the tunability and presence of strong CT character in the multiexciton state have implications for SF applications since they often rely on electron transfer from the free triplet excitons. This enhanced CT character in the (T₁T₁) state may assist with two-electron transfer directly from the (T₁T₁) state, allowing for facile extraction of charges in intramolecular SF systems whose (T₁T₁) states do not always efficiently dissociate to two triplet excitons.