Singlet fission is a spin-allowed photophysical process in which one photon can be converted into two electrons, and is promising for the development of efficient organic photovoltaics. The mechanism of the fission process, in which one excited singlet state evolves into two triplet states, has been the source of considerable debate for the last several decades. Both a direct mechanism, as well as a mechanism involving a charge-transfer intermediate, have found support through computational and experimental studies, although this work is challenging due to the rapid timescale of fission as well as the complexity of the involved potential energy surfaces. Here, we use a structurally-sensitive ultrafast Raman technique which is capable of following the dynamics as they occur in order to probe the mechanism of fission in canonical systems such as crystalline pentacene and rubrene. In pentacene, we observe the formation of an intermediate before the appearance of triplet states. This intermediate is characterized by pairs of excited state peaks that are red- and blue-shifted relative to the ground state features. We hypothesize that these features arise from the formation of both a partially cationic species and a partially anionic species due to the partial transfer of electron density from one pentacene molecule to a neighboring molecule. These observations provide experimental evidence for the role of states with significant charge-transfer character which facilitate the singlet fission process in pentacene. Our work both provides insight into the mechanism of singlet fission and demonstrates the utility of structurally-sensitive time-resolved ultrafast spectroscopic techniques in examining sub-picosecond photophysical processes.

Probing the mechanism of singlet fission with femtosecond stimulated Raman spectroscopy

253rd National Meeting (2017)

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