Infrared Spectroscopic Signatures of the Fluorous Effect Arise from a Change of Conformational Dynamics

Per- and polyfluoroalkyl substances (PFAS) are synthetic compounds widely employed in society due to their chemical inertness. These substances accumulate in the environment, from where they enter human bodies, leading to harmful effects like cancer. PFAS exhibit omniphobic properties, which often cause them to separate from both aqueous and organic phases, forming a fluorous phase. Yet, the physical nature of this fluorous effect is unknown. Here, we shed light on the fluorous effect by analyzing the infrared absorption spectra of perfluorinated and semifluorinated alkanes in various solvents. We find that specific bands in the C-F stretching vibrational region exhibit selective behaviors in fluorous and nonfluorous environments. In a fluorous environment, these bands undergo significant broadening, and the asymmetric CF₃ stretching bands decrease in intensity. Using static density functional theory calculations and force-field molecular dynamics simulations, we decipher the underlying molecular mechanisms: The decrease in absorption intensities is related to the intermolecular vibrational coupling of the perfluoroalkyl chains, while an acceleration of conformational changes in the carbon backbone of the molecules causes the observed band broadening. Given the high specificity of the reported spectral changes to a fluorous environment, bands in the C-F stretching range can serve as spectroscopic markers for the fluorous phase, facilitating the study of PFAS aggregation. Such knowledge can lead to the rational design of absorber materials for PFAS, which are aimed at mitigating their environmental impact.