Raman study of mesogenic transitions in 4,4'-di-n-pentyloxyazoxybenzene (C5)

A Raman scattering study of the solid-smectic transition in 4,4′-di-n-pentyloxyazoxybenzene (C5) has been carried out. C5 is known to exhibit a previously unrecognized, highly ordered smectic phase. We have investigated the Raman spectra of C5 in its solid, smectic, nematic, and isotropic phases, with special attention to the external mode region below 100 cm−1. At 300 K the solid phase exhibits several distinct low frequency features. These are progressively attenuated on approaching the solid–smectic transition which occurs at 342 K. The accordion mode of the pentyl chains is observable at 318 cm−1 for the solid phase, but it practically disappears in the smectic phase. This is in accord with the inference of Prasad et al. that the solid–smectic transition is accompanied by a configurational melting of the pentyl end chains. In the smectic phase, an inelastic wing due to a highly damped mode is seen below 60 cm−1; this unique signature of the smectic phase disappears in the nematic and isotropic phases. The plausible origin of this mode and its damping are considered in terms of the molecular correlation within the smectic layers.