Exploring the Isotropic to Nematic Phase Transition in a Liquid Crystal under Vibrational Strong Coupling

We investigate how vibrational strong coupling (VSC) between a mid-infrared cavity mode and selected molecular vibrational transitions influences the isotropic-to-nematic phase transition of the prototypical liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Using a tunable Fabry–Pérot microcavity, we achieve VSC with prominent stretching and bending vibrational modes of 5CB mesogens in the isotropic phase. Through temperature-dependent optical birefringence, Fourier-transform infrared, and UV–visible spectroscopic measurements, we directly compare cavity-coupled and control samples. Surprisingly, we find that the isotropic-to-nematic phase transition is robust against VSC, indicating that the formation of vibro-polaritonic states does not appreciably perturb the isotropic phase of 5CB. More broadly, our results show that VSC of the studied vibrational modes of 5CB (-C≡N, -C═C-, −C-H) does not produce a measurable change in the I → N transition under the experimental conditions investigated.