Intermolecular singlet fission in a radical dianion system in solution phase.

Singlet fission (SF) is a spin-allowed exciton multiplication process, in which a photogenerated singlet separates efficiently into two free triplets. Herein, we report an experimental study on the solution-phase intermolecular SF (xSF) in a prototype radical dianion system of PTCDA2-, which is produced from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride) via a two-step consecutive photoinduced electron transfer mechanism. Our ultrafast spectroscopic results enable a comprehensive mapping of the elementary steps involved in the solution-phase xSF process of photoexcited PTCDA2-. Along the cascading xSF pathways, the three intermediates including excimer 1(S1S0), spin-correlated triplet pair 1(T1T1), and spatially separated triplet pair 1(T1·S0·T1) have been identified, with their formation/relaxation time constants being determined. This work demonstrates that the solution-phase xSF materials can be extended to charged radical systems and that the three-step model usually adopted to describe the crystalline-phase xSF can also be valid in describing solution-phase xSF.