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.

单重态裂变（SF）是将一个单重态激发态转换为两个三重态的过程。SF的机制仍不太清楚。在这里，我们报告结合瞬态吸收和电子顺磁共振光谱与理论计算的结合使用，以探测由非共轭的1,4-二乙炔基双环[2.2.2]辛烷间隔基连接的并五苯二聚体中的SF。在观察基于溶液的SF中的关键中间体（包括并五苯三重态激子形成之前的五重态的形成和衰变）之后，我们证明了偶联足够弱，以至于SF在非极性介质中基本上不起作用。但是，过渡到极性介质会放大耦合强度，从而使SF起作用并支持较长的三重态寿命。