Distinct properties of the triplet pair state from singlet fission

Singlet fission, the conversion of a singlet exciton (S₁) to two triplets (2 × T₁), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state ¹(TT). Despite extensive research, the nature of the ¹(TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound ¹(TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S₁ state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to ¹(TT)→S_n and S₁→S_n' transitions; S_n and S_n' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S₂ state localized on each pentacene unit in the dimer molecule. The ¹(TT)→S_n transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound ¹(TT) state also shows chemical reactivity that is distinctively different from that of an individual T₁ state. Using an electron-accepting iron oxide molecular cluster [Fe₈O₄] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T₁ in pentacene but not from the tightly bound ¹(TT) state. Thus, reducing intertriplet electronic coupling in ¹(TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.