Tracking ultrafast reactions in organic materials through vibrational coherence: vibronic coupling mechanisms in singlet fission

A multitude of ultrafast photoinduced reactions in organic semiconductors are governed by the close interplay between nuclear and electronic degrees of freedom. From biological light-harvesting and photoprotection to organic solar cells, the critical electronic dynamics are often precisely synchronized with and driven by nuclear motions, in a breakdown of the Born-Oppenheimer approximation. Ultrafast time-domain Raman methods exploit impulsive excitation to generate nuclear wavepackets and track their coherent evolution through these reaction pathways in real time. This tool of vibrational coherence has recently been applied to study singlet fission, a carrier multiplication process with the potential to boost solar cell efficiencies which has been under intense mechanistic investigation for the past decade. In this review, we present the essential features of the spectroscopic techniques and discuss how they have been used to elaborate a new perspective on the singlet fission mechanism. It is now established that ultrafast triplet-pair formation is driven by vibronic coupling, whether fission is exothermic or endothermic, and thus that full understanding of singlet fission requires explicit consideration of nuclear dynamics. Despite broad qualitative agreement between different vibrational coherence methods, differences in the detailed observations and interpretation raise important questions and pose new challenges for future research