Consequences of Vibrational Strong Coupling on Supramolecular Polymerization of Porphyrins.

Supramolecular polymers display interesting optoelectronic properties and, thus, deploy multiple applications based on their molecular arrangement. However, controlling supramolecular interactions to achieve a desirable molecular organization is not straightforward. Over the past decade, light-matter strong coupling has emerged as a new tool for modifying chemical and material properties. This novel approach has also been shown to alter the morphology of supramolecular organization by coupling the vibrational bands of solute and solvent to the optical modes of a Fabry-Perot cavity (vibrational strong coupling, VSC). Here, we study the effect of VSC on the supramolecular polymerization of chiral zinc-porphyrins (S-Zn) via a cooperative effect. Electronic circular dichroism (ECD) measurements indicate that the elongation temperature (Te) of the supramolecular polymerization is lowered by ∼10 °C under VSC. We have also generalized this effect by exploring other supramolecular systems under strong coupling conditions. The results indicate that the solute-solvent interactions are modified under VSC, which destabilizes the nuclei of the supramolecular polymer at higher temperatures. These findings demonstrate that the VSC can indeed be used as a tool to control the energy landscape of supramolecular polymerization. Furthermore, we use this unique approach to switch between the states formed under ON- and OFF-resonance conditions, achieved by simply tuning the optical cavity in and out of resonance.

Polaritonic Chemistry Enabled by Non-Local Metasurfaces.

Vibrational strong coupling can modify chemical reaction pathways in unconventional ways. Thus far, Fabry-Perot cavities formed by pairs of facing mirrors have been mostly utilized to achieve vibrational strong coupling. In this study, we demonstrate the application of non-local metasurfaces that can sustain surface lattice resonances, enabling chemical reactions under vibrational strong coupling. We show that the solvolysis kinetics of para-nitrophenyl acetate can be accelerated by a factor of 2.7 by strong coupling to the carbonyl bond of the solvent and the solute with a surface lattice resonance. Our work introduces a new platform to investigate polaritonic chemical reactions. In contrast to Fabry-Perot cavities, metasurfaces define open optical cavities with single surfaces, which removes alignment hurdles, facilitating polaritonic chemistry across large areas.

Solvent Polarity under Vibrational Strong Coupling.

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.

Supramolecular Assembly of Conjugated Polymers under Vibrational Strong Coupling.

Strong coupling plays a significant role in influencing chemical reactions and tuning material properties by modifying the energy landscapes of the systems. Here we study the effect of vibrational strong coupling (VSC) on supramolecular organization. For this purpose, a rigid-rod conjugated polymer known to form gels was strongly coupled together with its solvent in a microfluidic IR Fabry-Perot cavity. Absorption and fluorescence studies indicate a large modification of the self-assembly under such cooperative VSC. Electron microscopy confirms that in this case, the supramolecular morphology is totally different from that observed in the absence of strong coupling. In addition, the self-assembly kinetics are altered and depend on the solvent vibration under VSC. The results are compared to kinetic isotope effects on the self-assembly to help clarify the role of different parameters under strong coupling. These findings indicate that VSC is a valuable new tool for controlling supramolecular assemblies with broad implications for the molecular and material sciences.

On the Role of Symmetry in Vibrational Strong Coupling: The Case of Charge-Transfer Complexation.

It is well known that symmetry plays a key role in chemical reactivity. Here we explore its role in vibrational strong coupling (VSC) for a charge-transfer (CT) complexation reaction. By studying the trimethylated-benzene-I2 CT complex, we find that VSC induces large changes in the equilibrium constant KDA of the CT complex, reflecting modifications in the ΔG° value of the reaction. Furthermore, by tuning the microfluidic cavity modes to the different IR vibrations of the trimethylated benzene, ΔG° either increases or decreases depending only on the symmetry of the normal mode that is coupled. This result reveals the critical role of symmetry in VSC and, in turn, provides an explanation for why the magnitude of chemical changes induced by VSC are much greater than the Rabi splitting, that is, the energy perturbation caused by VSC. These findings further confirm that VSC is powerful and versatile tool for the molecular sciences.

Tunable emission from H-type supramolecular polymers in optical nanocavities.

H-type supramolecular polymers with preferred helicity and highly efficient emission have been prepared from the self-assembly of chiral tetraphenylene-based monomers. Implementation of the one-dimensional fibers into dielectric nanoparticle arrays allows for a significant reshaping of fluorescence due to weak light-matter coupling.

Manipulating the Self-Assembly of Phenyleneethynylenes under Vibrational Strong Coupling.

The chemical and physical properties of molecules and materials are known to be modified significantly under vibrational strong coupling (VSC). To gain insight into the effects of VSC on π-π interactions involved in molecular self-assembly, themselves sensitive to vacuum electromagnetic field fluctuations, the aggregation of two structural isomers (linear and V-shaped) of phenyleneethynylene under cooperative coupling was investigated. By coupling the aromatic C═C stretching band, the assembly of one of the molecules results in the formation of spheres as opposed to flakes under normal conditions. As a consequence, the electronic absorption and emission spectra of the self-assembled structures are also modified significantly. The VSC-induced changes depend not only on the type of vibration that is coupled but also on the symmetry of the phenyleneethynylene isomer. These results confirm that VSC can be used to drive molecular assemblies to new structural minima and thereby provide a new tool for supramolecular chemistry.

Conductivity and Photoconductivity of a p-Type Organic Semiconductor under Ultrastrong Coupling.

During the past decade, it has been shown that light-matter strong coupling of materials can lead to modified and often improved properties which has stimulated considerable interest. While charge transport can be enhanced in n-type organic semiconductors by coupling the electronic transition and thereby splitting the conduction band into polaritonic states, it is not clear whether the same process can also influence carrier transport in the valence band of p-type semiconductors. Here we demonstrate that it is indeed possible to enhance both the conductivity and photoconductivity of a p-type semiconductor rr-P3HT that is ultrastrongly coupled to plasmonic modes. It is due to the hybrid light-matter character of the virtual polaritonic excitations affecting the linear response of the material. Furthermore, in addition to being enhanced, the photoconductivity of rr-P3HT shows a modified spectral response due to the formation of the hybrid polaritonic states. This illustrates the potential of engineering the vacuum electromagnetic environment to improve the optoelectronic properties of organic materials.