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Polaritonic chemistry is emerging as a powerful approach to modifying the properties and reactivity of molecules and materials. However, probing how the electronics and dynamics of molecular systems change under strong coupling has been challenging due to the narrow range of spectroscopic techniques that can be applied in situ. Here we develop microfluidic optical cavities for vibrational strong coupling (VSC) that are compatible with nuclear magnetic resonance (NMR) spectroscopy using standard liquid NMR tubes. VSC is shown to influence the equilibrium between two conformations of a molecular balance sensitive to London dispersion forces, revealing an apparent change in the equilibrium constant under VSC. In all compounds studied, VSC does not induce detectable changes in chemical shifts, J‐couplings, or spin‐lattice relaxation times. This unexpected finding indicates that VSC does not substantially affect molecular electron density distributions, and in turn has profound implications for the possible mechanisms at play in polaritonic chemistry under VSC and suggests that the emergence of collective behavior is critical. [ABSTRACT FROM AUTHOR]

Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

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.

The scattering properties of metallic optical antennas are typically examined through the lens of their plasmonic resonances. However, non-plasmonic transition metals also sustain surface waves in the visible. We experimentally investigate in this work the far-field diffraction properties of apertured optical antennas milled on non-plasmonic W films and compare the results with plasmonic references in Ag and Au. The polarization-dependent diffraction patterns and the leakage signal emerging from apertured antennas in both kinds of metals are recorded and analyzed. This thorough comparison with surface plasmon waves reveals that surface waves are launched on W and that they have the common abilities to confine the visible light at metal-dielectric interfaces offering the possibility to tailor the far-field emission. The results have been analyzed through theoretical models accounting for the propagation of a long range surface mode launched by subwavelength apertures, that is scattered in free space by the antenna. This surface mode on W can be qualitatively described as an analogy in the visible of the Zenneck wave in the radio regime. The nature of the new surface waves have been elucidated from a careful analysis of the asymptotic expansion of the electromagnetic propagators, which provides a convenient representation for explaining the Zenneck-like character of the excited waves and opens new ways to fundamental studies of surface waves at the nanoscale beyond plasmonics.