Interpretational Pitfalls of Singlet Fission Studies by Impulsively Generated Vibrational Wavepackets

A conceptual overview of singlet exciton fission and its investigation by broadband impulsive vibrational spectroscopy (IVS) is presented, with special emphasis on putative interpretational difficulties. The generation and properties of coherent vibrational wavepackets are first discussed because of their importance in the pertinent experiment technique, focusing in particular on their role in singlet fission and the insight they may give into the mechanisms of the processes contributing to that phenomenon. The contrast between totally and nontotally symmetric normal modes is emphasized. Several interpretationally misleading crucial subtleties are identified and illustrated with examples from the literature and the authors’ experience. This is set in the context of related results of other experimental techniques and theoretical approaches. Some recent articles are discussed with emphasis on the pressing demand for complementary experimental/theoretical collaboration in investigating how wavepacket properties depend on the method of its generation and how this affects singlet fission studies.