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.
Highlights
Conjugated polymers have attracted more interests as the novel properties resulted from the delocalized π electrons
We have reported Raman scattering spectra of benzene, biphenyl, p-terphenyl, p-quaterphenyl, p-quinquephenyl, and p-sexiphenyl
The difference of the frequencies between these two modes has an anomaly at P2P. This is because the interaction of the phenyl rings is different between P2P and POPs containing more than two phenyl rings
Summary
Conjugated polymers have attracted more interests as the novel properties resulted from the delocalized π electrons. The structural transition of biphenyl (P2P) from monoclinic to triclinic has been suggested to result from the nonplanar molecules at low temperature.[21] This phase transformation was confirmed by Raman spectroscopy measurements.[32] For POPs with a long C-C chain length, the ratio of the integral intensity of the 1280 cm 1 and 1220 cm 1 modes in the Raman spectra has often been used to distinguish the chain lengths between different compounds.[33,34,35] It serves as an indicator of the planarization of molecules at ambient conditions[36,37] or high pressures.[38,39] the features of the two peaks at around 1600 cm 1 is particular These two peaks have been suggested to estimate the conjugation of the molecules.[34,35,38,39] The identification of the chain length effects on POPs will be more clear if the basic starting compound benzene is included. The results are compared with previously published data.[33,34,35,36,37,38,39] A new method is introduced to estimate the chain length of POPs and PPP materials
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