Chain length effects of $p$-oligophenyls with comparison of benzene by Raman scattering

TL;DR

This study uses Raman scattering to analyze how chain length influences vibrational modes in p-oligophenyls and benzene, identifying a specific librational mode as a reliable indicator of chain length effects.

Contribution

It provides detailed vibrational analysis of p-oligophenyls and benzene, highlighting a simple, chain-length-dependent librational mode as a new indicator for molecular property studies.

Findings

Vibrational modes vary with chain length, but are complex and fluctuating.

The intensity ratio of specific modes is laser-dependent.

A low-energy librational mode correlates monotonically with chain length.

Abstract

Raman scattering measurements are performed on benzene and a number of $p$-oligophenyls including biphenyl, $p$-terphenyl, $p$-quaterphenyl, $p$-quinquephenyl, and $p$-sexiphenyl at ambient conditions. The vibrational modes of the intra- and intermolecular terms in these materials are analyzed and compared. Chain length effects on the vibrational properties are examined for the C-H in-plane bending mode and the inter-ring C-C stretching mode at around 1200 cm$^{-1}$ and 1280 cm$^{-1}$, respectively, and the C-C stretching modes at around 1600 cm$^{-1}$. The complex and fluctuating properties of these modes result in an imprecise estimation of the chain length of these molecules. Meanwhile, the obtained ratio of the intensities of the 1200 cm$^{-1}$ mode and 1280 cm$^{-1}$ mode is sensitive to the applied lasers. A librational motion mode with the lowest energy is found to have a monotonous change with the increase in the chain length. This mode is simply relevant to the $c$ axis of the unit cell. Such an obvious trend makes it a better indicator for determining the chain length effects on the physical and chemical properties in these molecules.