Femtosecond stimulated Raman spectroscopy (FSRS) is a vibrationally selective ultrafast technique, producing structurally sensitive signatures that can have both high spectral and temporal resolution (~10 cm-1 and &lt;100 fs). This allows for facile probing of evolving structural dynamics of a reaction on the timescale of molecular motion. A photoexcitation pulse promotes the sample to an excited state which is probed at different time delays by the Raman pump and probe. Following the evolution of these structural signals allows us to track changing nuclear motions and determine the mechanism of the reaction. Here we use FSRS to determine the mechanism by which organic crystals undergo photoinduced charge transfer. While many studies have investigated ways to improve the efficiency and cost effectiveness of photovoltaic devices, a lack of understanding relating the driving force in the mechanism of charge transfer to specific vibrational modes and using that knowledge to rationally design better organic solar cells remains. Here, we use FSRS to study structural changes during charge transfer in a canonical charge transfer crystal comprised of perylene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). In Figure 1, we see the appearance of photoexcited perylene (1260 cm-1), followed by a shift to the perylene cation (1260 cm-1) concurrently with the appearance of the TCNQ anion (1360 cm-1). These results suggest that the charge transfer process occurs through an intermediate in under 100 fs. By studying the spectral changes as a function of time after photoexcitation, we can determine which modes show signatures of charge transfer occurring on these fast timescales. Ultimately, this work will lead to a better understanding as to which molecular structures enhance charge transfer and which vibrational modes are involved in electron transfer, leading to improved organic solar cell design.

Probing the mechanism of charge transfer in organic crystals using femtosecond stimulated Raman spectroscopy

255th National Meeting (2018)

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