Ultrafast Dynamics of Long-Lived Bound Triplet Pair Generated by Singlet Fission in 6,13-Bis(triisopropylsilylethynyl) Pentacene

Singlet fission (SF) converts a singlet exciton into two triplet excitons and holds great potential in the photovoltaic field. The triplet pair state is proposed to play an important role in SF. However, the existence and character of the triplet pair state are still ambiguous to date. In this study, we directly observed the ultrafast formation of the long-lived bound triplet pair state in 6,13-bis(triisopropylsilylethynyl) pentacene polycrystalline film using sub-10-fs broadband transient absorption spectroscopy. The triplet pair state has strong absorption in the near-infrared region and shows distinct dynamic characteristics. It shows faster formation and relaxation compared to free triplet states. Diffusion-limited triplet–triplet annihilation was proved to be the major relaxation channel for both free triplets and bound triplet pairs. Moreover, the larger constraint coefficient obtained from triplet–triplet annihilation of the bound triplet pair state reveals the inherent coupling nature between its two constituent triplets.