It is known that the performance of organic photovoltaic (OPV) material is affected by its molecular morphology. The frequency of ester carbonyl (C=O) stretch in a widely used electron accepting material, phenyl-C61-butyric acid methyl ester (PCBM), was reported to be sensitive to its local environment. Vibrational solvatochromic shift has been observed for dilute PCBM in a range of organic solvents (including dichloromethane, trichloromethane, and benzene), but the spectral shift cannot be explained by vibrational Stark shift in terms of solvent polarity. Here we applied a mixed quantum/classical approach to calculate the ester C=O stretch infrared (IR) spectra for PCBM in these solvents. In this approach the ester carbonyl group is treated quantum mechanically and the rest of the system is described by molecular dynamics (MD) simulations, and an excellent agreement is reached between experimental and calculated IR spectra. We find that the electrostatic interaction between the C=O group and the solvent molecules in the first solvation shell is primarily responsible for the spectral shift. Furthermore, we attribute the difference in line-width to different solvation dynamics of the C=O group in different solvents. Our results relate the vibrational features of the ester group in PCBM to its surrounding environment, suggesting a wider use of its vibrational spectroscopy in understanding structures and dynamics in photovoltaic materials.

Understanding the vibrational solvatochromism of the ester carbonyl vibration in dilute PCBM solutions

2019 Fall National Meeting

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