Evidence for Charge-Transfer Mediation in the Primary Events of Singlet Fission in a Weakly Coupled Pentacene Dimer

Singlet fission (SF) is a process that converts one singlet excited state into two triplet states. The mechanism of SF is still not well understood. Here, we report on the use of a combination of transient absorption and electron paramagnetic resonance spectroscopies in conjunction with theoretical calculations to probe SF in a pentacene dimer linked by a non-conjugated, 1,4-diethynylbicyclo[2.2.2]octane spacer. Next to observing the key intermediates in solution-based SF, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons, we demonstrate that the coupling is sufficiently weak that SF is essentially inoperative in non-polar media. Transitioning to a polar medium, however, amplifies the coupling strength such that SF becomes operative and supports long triplet lifetimes. Our results offer strong evidence for a charge-transfer mediation and, in turn, provide a solid framework for decoding the complete mechanism of SF in systems beyond pentacene. The ever-increasing demand for energy with the simultaneous need of protecting the environment is one of the grand challenges of humankind. On the one hand, this concerns the conversion of energy from renewable sources, and on the other, innovative solutions for the energy storage and transport are of utmost importance. Singlet fission (SF) is the molecular analog of multiple-exciton generation (MEG). It transforms a molecular singlet excited state into two triplet states, each with half the energy of the initial singlet. SF has the potential to break the conventional barrier of one electron per photon and, in turn, push light-conversion efficiencies beyond the Shockley-Queisser limit. Silicon-based solar cells are approaching the thermodynamic limit of efficiency (Shockley-Queisser limit). Simultaneously, fossil fuels are strongly linked to climate changes. Consequently, new approaches are necessary to satisfy the world's steadily increasing energy demand. Singlet fission (SF) is a process overcoming the core assumptions that Shockley and Queisser postulated for their calculations: it is predicted to generate two charges per photon rather than only one! Basel et al. provide evidence for a charge-transfer-mediated mechanism of SF in a non-conjugated, rigid pentacene dimer.