Vibrational strong coupling and the formation of vibrational polaritons are a result of strong light–matter interaction between a cavity photon and a molecular vibrational mode. The Rabi splitting parameter, which reflects the microscopic light–matter interaction strength, reveals information about the molecular alignment and concerted vibrational motion inside the cavity. We have investigated vibrational strong coupling of 4-cyano-4′-octylbiphenyl liquid crystal molecules in isotropic and smectic A phases. We observed a ∼30% change in the Rabi splitting with the phase transition from isotropic to smectic A by controlling the temperature, together with the onset of polarization-dependent anisotropy of the Rabi splitting in the smectic A phase. Based on the estimated orientational distribution function, we show that the observed Rabi splitting difference in the isotropic and smectic A phases can only be explained by taking into account the influence of collective vibrational motion in the cavity, which affects the molecular properties under the vibrational strong coupling regime.

We developed a spacerless flow-cell cavity for the observation of vibrational strong coupling and demonstrate its availability in two samples with a CN bond: a metal complex (aq) and an ionic liquid. It is shown that the cavity length can be tuned over a wide range to investigate coupling with different order Fabry-Pérot cavity modes without reassembling the cavity. In the ionic liquid, analyses based on the coupled harmonic oscillator model with multiple vibrational modes show that the Rabi splitting parameters and the square root of the integrated absorption intensity are proportional among the three neighboring vibrational modes. Our spacerless cell structure simplifies the comparison of the different vibrational strong coupling measurements, such as the mode order dependence and the coupling to different molecular vibrations.