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.