Raman Optical Activity on a Solid Sample: Identification of Atropisomers of Perfluoroalkyl Chains Having a Helical...

Perfluoroalkyl (R_f) chains have a specific helical conformation due to the steric repulsion between the adjacent CF₂ units. Although R_f chains have no chiral center, two chiral structures, i.e., the right-handed (R) and left-handed (L) helices, are available as the most stable conformations, which are atropisomers to each other. According to the stratified dipole array (SDA) theory, the helical structure about the chain axis plays a key role in the spontaneous molecular aggregation of R_f chains in a two-dimensional manner, and the R_f chains having the same chirality tend to be aggregated spontaneously to generate molecular domains. This implies that an R_f compound in a solid state should be a mixture of the R and L domains, and each domain should exhibit distinguishable optical activity. To identify molecular domains with different atropisomers, in this study, Raman optical activity (ROA) measurements were performed on a Raman imaging spectrometer. Through the ROA measurements of recrystallized solid samples of an R_f compound, each particle exhibits an apparent optical activity, and the two atropisomers were readily distinguished. As a result, an R_f compound with the same helicity is found to be spontaneously aggregated as expected by the SDA theory.