Vibrational strong coupling (VSC)
occurs when molecular
vibrations
hybridize with the modes of an optical cavity, an interaction mediated
by vacuum fluctuations. VSC has been shown to influence the rates
and selectivity of chemical reactions. However, a clear understanding
of the mechanism at play remains elusive. Here, we show that VSC affects
the polarity of solvents, which is a parameter well-known to influence
reactivity. The strong solvatochromic response of Reichardt’s
dye (RD) was used to quantify the polarity of a series of alcohol
solvents at visible wavelengths. We observed that, by simultaneously
coupling the OH and CH vibrational bands of the alcohols, the absorption
maximum of Reichardt’s dye redshifted by up to ∼15.1
nm, corresponding to an energy change of 5.1 kJ·mol–1. With aliphatic alcohols, the magnitude of the absorption change
of RD was observed to be related to the length of the alkyl chain,
the molecular surface area, and the polarizability, indicating that
dispersion forces are impacted by strong coupling. Therefore, we propose
that dispersion interactions, which themselves originate from vacuum
fluctuations, are impacted under strong coupling and are therefore
critical to understanding how VSC influences chemistry.