The excited-state dynamics of a spiro-fused terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer (sTDI₂) in toluene and 2-methyltetrahydrofuran (mTHF) were investigated as a function of temperature. The spiro-conjugation and corresponding rigid geometry of this compound guarantees electronic coupling between the TDIs. Photoexcitation in toluene at 295 K results in ultrafast formation of a coherent mixed state with singlet ¹(S₁S₀), multiexciton ¹(T₁T₁), and charge transfer (CT) electronic character, which later decays to decorrelated triplet states, completing the process of intramolecular singlet fission (SF). Upon decreasing the temperature from 295 K to 200 K, the contribution of the ¹(T₁T₁) state to the mixed species decreases while that of the CT increases. We attribute this to the variation in vibrational energy level alignment between the states comprising the mixture due to changes in the temperature and hence the local dielectric environment. In contrast, photoexcitation of sTDI₂in more polar mTHF at 295 K results in the formation of a mixed singlet and CT state before undergoing symmetry-breaking charge separation, owing to the increased stabilization of the CT state in the medium. However, in glassy mTHF at 85 K, photoexcited sTDI₂exhibits behavior comparable to that observed in toluene at 200 K, which we rationalize by the similarity of the media’s dielectric constants under these two sets of conditions. These observations of varying diabatic mixed state contributions over the range of experimental conditions show that temperature and the static dielectric constant can directly control the composition of the electronically mixed excited state of sTDI₂ and thus the SF process.

Jonathan Schultz

Northwestern University Robert R McCormick School of Engineering and Applied Science

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