Dynamics of Singlet Fission in the TIPS-Pn Cluster: Endothermic or Exothermic?

Singlet fission (SF) capable of multi-exciton generation is a well-accepted concept for solar cells and also promises an intermediate quintet state with long coherence time for quantum information science. As a multi-step process involving both energy conservation and spin conservation, the thermodynamics of SF in the typical pentacene derivative 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) is still ambiguous. The difficulty lies in the experimental visualization of the high-spin states. Here, we combine the temperature-dependent time-resolved electron paramagnetic resonance (TrEPR) and transient photoluminescence (TrPL) techniques to reexamine the multi-step SF process in the TIPS-Pn cluster. An amorphous solid prepared by co-melting of TIPS-Pn with host 1,3,5-tri(1-naphthyl)benzene promises an intense TrEPR signal even at room temperature. The TrEPR intensities of the intermediate quintet triplet pair (⁵TT) increase with the temperature, revealing that the conversion from the singlet triplet pair (¹TT) to ⁵TT is an endothermic process for their positive exchanged coupling between two involved triplet excitons. In contrast, the first step of SF, from the first excited singlet to ¹TT, is exothermic, as expected from the positive temperature dependence of lifetimes extracted from the TrPL decay. As a result, the total SF processes within the TIPS-Pn cluster exhibit an endothermic characteristic, as the TrEPR signals of the resultant triplet increase with the temperature. This study clarifies the thermodynamics of multi-step SF processes and highlights the powerful TrEPR technique in visualizing spin-involving processes.