The strong coupling of an IR-active molecular transition with an optical mode of the cavity results in vibrational polaritons, which opens a new way to control chemical reactivity via confined electromagnetic fields of the cavity. In this study, we design a voltagetunable open microcavity and we show the formation of multiple vibrational polaritons in methyl salicylate. A Rabi splitting and polariton anticrossing behavior is observed when the cavity mode hybridizes with the CO stretching vibration of methyl salicylate. Furthermore, the proposed theoretical model based on coupled harmonic oscillators reveals that the absorption of uncoupled molecules must also be considered to model the experimental spectra properly and that simultaneous coupling of multiple molecular vibrations to the same cavity mode has a significant influence on the transmission spectral profile.

Changes in the Raman spectra under vibrational strong coupling do not necessarily result from the coupling effect but rather they can be caused by the surface enhancement effect.

Over the years, probing and controlling tautomerization has attracted significant attention because of its fundamental importance in various chemical and biological phenomena. So far, light, force, electrons, and electric field have been employed to alter the tautomerization characteristics in porphyrin and phthalocyanine derivatives. Here, we show that engineering the photophysics of molecules through interaction with the vacuum electromagnetic field in an optical microcavity can be used to control the tautomerization of single phthalocyanine molecules. Compared to the molecules embedded in a polymer matrix in open space, the average fluorescence lifetime inside the resonant microcavity decreases significantly due to the Purcell effect. The decreased lifetime reduces the possibility of a molecule entering into the triplet state, i.e., the photoactive tautomerization channel. As a result, the photoinduced tautomerization in phthalocyanine can be significantly altered. Our results demonstrate that the weak coupling between the excited state of a molecule and the vacuum electromagnetic field via a cavity mode can lead to significant changes in photoreactivity.

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