Cavity Catalysis of an Enantioselective Reaction under Vibrational Strong Coupling.

Strong light-matter interaction is emerging as an exciting tool for controlling chemical reactions. Here, we demonstrate an L-proline-catalyzed direct asymmetric Aldol reaction under vibrational strong coupling. Both the reactants (4-nitrobenzaldehyde and acetone) carbonyl bands are coupled to an infrared photon and react in the presence of L-proline. The reaction mixture is eluted from the cavity, and the conversion yields and enantiomeric excess are quantified using NMR and chiral HPLC. The conversion yields increase by up to 90 % in ON-resonance conditions. Interestingly, a large increase in the conversion yield does not affect the enantiomeric excess. Further control experiments were carried out by varying the temperature, and we propose that the rate-limiting step may not be the deciding factor in enantioselectivity. Whereas the formation of the enamine intermediate is modified by cavity coupling experiments. For this class of enantioselective reactions, strong coupling does not change the enantiomeric excess, possibly due to the large energy difference in chiral transition states. Strong coupling can boost the formation of enamine intermediate, thereby favouring the product yield. This gives more hope to test polaritonic chemistry based on enantioselective reactions in which the branching ratios can be controlled.

Understanding the Nature of Vibro-Polaritonic States in Water and Heavy Water.

Very recent experiments on vibrational strong coupling tend to modify chemical reactivity, energy transfer, and many other physical properties of the coupled system. This is achieved without external stimuli and is very sensitive to the vibrational envelope. Water is an excellent vibrational oscillator, which is being used for similar experiments. However, the inhomogeneously broad OH/OD stretching vibrational band make it complicated to characterize the vibro-polaritonic states spectroscopically. In this paper, we performed vibrational strong coupling and mapped the evolution of vibro-polaritonic branches from low to high concentration of H2 O and measured the on-set of strong coupling. The refractive index dispersion is correlated with the cavity tuning experiments. These results are further compared with transfer matrix simulations. Simulated data deviate as noted in the dispersion spectra as the system enters into ultra-strong coupling due to enhanced self-dipolar interactions. Hopfield coefficients calculation shows that even at ±400 cm-1 detuning, the vibro-polaritonic states still possess hybrid characteristics. We systematically varied the concentration of H2 O and mapped the weak, intermediate, and strong coupling regimes to understand the role of inhomogeneously broad OH/OD stretching vibrational band. Our finding may help to better understand the role of H2 O/D2 O strong coupling in the recent polaritonic chemistry experiments.

Long-Range Energy Transfer in Strongly Coupled Donor-Acceptor Phototransistors.

Strong light-matter coupling offers a way to tailor the optoelectronic properties of materials. Energy transfer between strongly coupled donor-acceptor pairs shows remarkable efficiency beyond the Förster distance via coupling through a confined photon. This long-range energy transfer is facilitated through the collective nature of polaritonic states. Here, the cooperative, strong coupling of a donor (MoS2 monolayer) and an acceptor (BRK) generates mixed polaritonic states. The photocurrent spectrum of the MoS2 monolayer is measured in a field effect transistor while coupling the two oscillators to the confined cavity mode. The strongly coupled system shows efficient energy transfer, which is observed through the photoresponsivity even the donor and acceptor are physically separated by 500 Å. These studies are further correlated with the Hopfield coefficients and the overlap integral of the lower polaritonic and uncoupled/dark states. Cavity detuning and distance-dependent studies support the above evidence. These observations open new avenues for using long-range interaction of polaritonic states in optoelectronic devices.

Solvent Dependence on Cooperative Vibrational Strong Coupling and Cavity Catalysis.

Strong light-matter coupling offers a unique way to control chemical reactions at the molecular level. Here, we compare the solvent effect on an ester solvolysis process under cooperative vibrational strong coupling (VSC). Three reactants, para-nitrophenylacetate, 3-methyl-para-nitrophenylbenzoate, and bis-(2, 4-dinitrophenyl) oxalate are chosen to study the effect of VSC on the solvolysis reaction rates. Two solvents, ethyl acetate and cyclopentanone, are also considered to compare the cavity catalysis by coupling the C=O stretching band of the reactant and the solvent molecules to a Fabry-Perot cavity mode. Interestingly, both solvents enhance the chemical reaction rate of para-nitrophenylacetate and 3-methyl-para-nitrophenylbenzoate under cooperative VSC conditions. However, the resonance effect is observed at different temperatures for different solvents, which is further confirmed by thermodynamic studies. Bis-(2, 4-dinitrophenyl) oxalate doesn't respond to VSC in either of the solvent systems due to poor overlap of reactant and solvent C=O vibrational bands. Cavity detuning and other control experiments suggest that cooperative VSC of the solvent plays a crucial role in modifying the activation free-energy of the reaction. These findings, along with other observations, cement the concept of polaritonic chemistry.

Cavity Catalysis by Cooperative Vibrational Strong Coupling of Reactant and Solvent Molecules.

Here, we report the catalytic effect of vibrational strong coupling (VSC) on the solvolysis of para-nitrophenyl acetate (PNPA), which increases the reaction rate by an order of magnitude. This is observed when the microfluidic Fabry-Perot cavity in which the VSC is generated is tuned to the C=O vibrational stretching mode of both the reactant and solvent molecules. Thermodynamic experiments confirm the catalytic nature of VSC in the system. The change in the reaction rate follows an exponential relation with respect to the coupling strength of the solvent, indicating a cooperative effect between the solvent molecules and the reactant. Furthermore, the study of the solvent kinetic isotope effect clearly shows that the vibrational overlap of the C=O vibrational bands of the reactant and the strongly coupled solvent molecules is critical for the catalysis in this reaction. The combination of cooperative effects and cavity catalysis confirms the potential of VSC as a new frontier in chemistry.

Modification of Enzyme Activity by Vibrational Strong Coupling of Water.

Vibrational strong coupling (VSC) has recently emerged as a completely new tool for influencing chemical reactivity. It harnesses electromagnetic vacuum fluctuations through the creation of hybrid states of light and matter, called polaritonic states, in an optical cavity resonant to a molecular absorption band. Here, we investigate the effect of vibrational strong coupling of water on the enzymatic activity of pepsin, where a water molecule is directly involved in the enzyme's chemical mechanism. We observe an approximately 4.5-fold decrease of the apparent second-order rate constant kcat /Km when coupling the water stretching vibration, whereas no effect was detected for the strong coupling of the bending vibration. The possibility of modifying enzymatic activity by coupling water demonstrates the potential of VSC as a new tool to study biochemical reactivity.

Voltage-Controlled Switching of Strong Light-Matter Interactions using Liquid Crystals.

We experimentally demonstrate a fine control over the coupling strength of vibrational light-matter hybrid states by controlling the orientation of a nematic liquid crystal. Through an external voltage, the liquid crystal is seamlessly switched between two orthogonal directions. Using these features, for the first time, we demonstrate electrical switching and increased Rabi splitting through transition dipole moment alignment. The C-Nstr vibration on the liquid crystal molecule is coupled to a cavity mode, and FT-IR is used to probe the formed vibropolaritonic states. A switching ratio of the Rabi splitting of 1.78 is demonstrated between the parallel and the perpendicular orientation. Furthermore, the orientational order increases the Rabi splitting by 41 % as compared to an isotropic liquid. Finally, by examining the influence of molecular alignment on the Rabi splitting, the scalar product used in theoretical modeling between light and matter in the strong coupling regime is verified.

Energy Transfer between Spatially Separated Entangled Molecules.

Light-matter strong coupling allows for the possibility of entangling the wave functions of different molecules through the light field. We hereby present direct evidence of non-radiative energy transfer well beyond the Förster limit for spatially separated donor and acceptor cyanine dyes strongly coupled to a cavity. The transient dynamics and the static spectra show an energy transfer efficiency approaching 37 % for donor-acceptor distances ≥100 nm. In such systems, the energy transfer process becomes independent of distance as long as the coupling strength is maintained. This is consistent with the entangled and delocalized nature of the polaritonic states.

Multiple Rabi Splittings under Ultrastrong Vibrational Coupling.

From the high vibrational dipolar strength offered by molecular liquids, we demonstrate that a molecular vibration can be ultrastrongly coupled to multiple IR cavity modes, with Rabi splittings reaching 24% of the vibration frequencies. As a proof of the ultrastrong coupling regime, our experimental data unambiguously reveal the contributions to the polaritonic dynamics coming from the antiresonant terms in the interaction energy and from the dipolar self-energy of the molecular vibrations themselves. In particular, we measure the opening of a genuine vibrational polaritonic band gap of ca. 60 meV. We also demonstrate that the multimode splitting effect defines a whole vibrational ladder of heavy polaritonic states perfectly resolved. These findings reveal the broad possibilities in the vibrational ultrastrong coupling regime which impact both the optical and the molecular properties of such coupled systems, in particular, in the context of mode-selective chemistry.

Non-Radiative Energy Transfer Mediated by Hybrid Light-Matter States.

We present direct evidence of enhanced non-radiative energy transfer between two J-aggregated cyanine dyes strongly coupled to the vacuum field of a cavity. Excitation spectroscopy and femtosecond pump-probe measurements show that the energy transfer is highly efficient when both the donor and acceptor form light-matter hybrid states with the vacuum field. The rate of energy transfer is increased by a factor of seven under those conditions as compared to the normal situation outside the cavity, with a corresponding effect on the energy transfer efficiency. The delocalized hybrid states connect the donor and acceptor molecules and clearly play the role of a bridge to enhance the rate of energy transfer. This finding has fundamental implications for coherent energy transport and light-energy harvesting.

Ground-State Chemical Reactivity under Vibrational Coupling to the Vacuum Electromagnetic Field.

The ground-state deprotection of a simple alkynylsilane is studied under vibrational strong coupling to the zero-point fluctuations, or vacuum electromagnetic field, of a resonant IR microfluidic cavity. The reaction rate decreased by a factor of up to 5.5 when the Si-C vibrational stretching modes of the reactant were strongly coupled. The relative change in the reaction rate under strong coupling depends on the Rabi splitting energy. Product analysis by GC-MS confirmed the kinetic results. Temperature dependence shows that the activation enthalpy and entropy change significantly, suggesting that the transition state is modified from an associative to a dissociative type. These findings show that vibrational strong coupling provides a powerful approach for modifying and controlling chemical landscapes and for understanding reaction mechanisms.

Enhanced Raman Scattering from Vibro-Polariton Hybrid States.

Ground-state molecular vibrations can be hybridized through strong coupling with the vacuum field of a cavity optical mode in the infrared region, leading to the formation of two new coherent vibro-polariton states. The spontaneous Raman scattering from such hybridized light-matter states was studied, showing that the collective Rabi splitting occurs at the level of a single selected bond. Moreover, the coherent nature of the vibro-polariton states boosts the Raman scattering cross-section by two to three orders of magnitude, revealing a new enhancement mechanism as a result of vibrational strong coupling. This observation has fundamental consequences for the understanding of light-molecule strong coupling and for molecular science.

Thermodynamics of molecules strongly coupled to the vacuum field.

The thermodynamics of strong coupling between molecules and the vacuum field is analyzed and the Gibbs free energy, the enthalpy, and entropy of the coupling process are determined for the first time. The thermodynamic parameters are a function of the Rabi splitting and the microscopic solvation. The results provide a new framework for understanding light-molecule strong coupling.

Improving Enzyme Catalytic Efficiency by Co-operative Vibrational Strong Coupling of Water.

Here, we report enhancement of catalytic efficiency of an enzymatic reaction by co-operative vibrational strong coupling (VSC) of water and the enzyme α-chymotrypsin. Selective strong coupling of the O-H stretching mode of water along with O-H and N-H stretching modes of the enzyme modify the rate of the enzymatic ester hydrolysis, increasing the catalytic efficiency by more than 7 times. This is specifically achieved by controlling the rate-determining proton-transfer process through a co-operative mechanism. Here, VSC is also used as a spectroscopic tool to understand the mechanism of the enzymatic reaction, suggesting its potential applications in chemistry.

Enhanced Charge Transport in Two-Dimensional Materials through Light-Matter Strong Coupling.

Strong light-matter interaction of functional materials is emerging as a promising area of research. Recent experiments suggest that material properties like charge transport can be controlled by coupling to a vacuum electromagnetic field. Here, we explored the design of a Fabry-Perot cavity in a field-effect transistor configuration and studied the charge transport in two-dimensional materials. The optical and electrical measurements of strongly coupled WS2 suggest an enhancement of electron transport at room temperature. Electron mobility is enhanced more than 50 times at ON resonance conditions. Similarly, Ion/Ioff ratio of the device increased by 2 orders of magnitude without chemical modification of the active layer. Cavity tuning and coupling strength-dependent studies support the evidence of modifying the electronic properties of the coupled system. A clear correlation in the effective mass of the polaritonic state and Schottky barrier height indicates a collective nature of light-matter interaction.

Boosting Self-interaction of Molecular Vibrations under Ultrastrong Coupling Condition.

In this letter, we investigated the modification of the oscillator strength of an asymmetric stretching band of CS2 by strong coupling to an infrared cavity photon. This is achieved by placing liquid CS2 in a Fabry-Perot resonator and tuning the cavity mode position to match the molecular vibrational transition. Ultrastrong coupling leads to an increase in the effective oscillator strength of the asymmetric stretching band of CS2. We proved this experimentally by taking the area ratio of the asymmetric stretching and combination bands by selectively coupling the former. A nonlinear increase in the oscillator strength of the asymmetric stretching band is observed upon varying the coupling strength. This is explained by a quantum mechanical model that predicts quadratic behavior under ultrastrong coupling conditions. These findings will set up a new paradigm for understanding chemical reaction modifications by vacuum field coupling.

Cavity catalysis: modifying linear free-energy relationship under cooperative vibrational strong coupling.

Here, we used an unconventional idea of cooperative vibrational strong coupling of solute and solvent molecules to enhance the rate of an esterification reaction. Different derivatives of p-nitrophenyl benzoate (solute) and isopropyl acetate (solvent) are cooperatively coupled to an infrared Fabry-Perot cavity mode. The apparent rates are increased by more than six times at the ON resonance condition, and the rate enhancement follows the lineshape of the vibrational envelope. Very interestingly, a strongly coupled system doesn't obey the Hammett relations. Thermodynamics suggests that the reaction mechanism remains intact for cavity and non-cavity conditions. Temperature-dependent experiments show an entropy-driven process for the coupled molecules. Vacuum field coupling decreases the free energy of activation by 2-5 kJ mol-1, supporting a catalysis process. The non-linear rate enhancement can be due to the reshuffling of the energy distribution between the substituents and the reaction center across the aromatic ring. These findings underline the non-equilibrium behavior of cavity catalysis.

Surface plasmon coupled circular dichroism of Au nanoparticles on peptide nanotubes.

Au nanoparticles grown on D- and L-isomers of diphenylalanine peptide nanotubes showed a bisignated CD signal at their surface plasmon frequency with positive and negative couplets, respectively. The surface plasmon coupled CD signal in these hybrid systems originates from the asymmetric organization of Au nanoparticles on peptide nanotubes. Mirror image relationship in the CD spectra clearly indicates that the chiral molecules on the nanotubes drive the organization of nanoparticles in two different ways.

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.

Chiral Plasmons: Au Nanoparticle Assemblies on Thermoresponsive Organic Templates.

Template-assisted strategies are widely used to fabricate nanostructured materials. By taking these strategies a step forward, herein we report the design of two chiral plasmonic nanostructures based on Au nanoparticle (NP) assemblies organized in clockwise and anticlockwise directions, having opposite response to circularly polarized light. The chiral plasmonic nanostructures are obtained by growing Au NPs on chiral templates based on d- and l-forms of alanine functionalized phenyleneethynylenes. Interestingly, Au NP assemblies show mirror symmetrical electronic circular dichroism (ECD) bands at their surface plasmon frequency originating through their asymmetric organization. Upon increasing the temperature, the chiral templates dissociate as evident from the disappearance of their ECD signal. The profound advantage of the thermoresponsive nature of the templates is employed to obtain free-standing chiral plasmonic nanostructures. The tilt angle high-resolution transmission electron microscopic measurements indicate that the NP assemblies, grown on a template based on the d-isomer, organize in clockwise direction ( P-form) and on l-isomer in anticlockwise direction ( M-form). The inherent chirality prevailing on the surface of the template drives the helical growth of the Au NPs in opposite directions. Experimental results are rationalized by a model which accounts for the large polarizability of Au NPs. The large polarizability leads to large oscillating dipole moments whose effects become prominent when interparticle distances are comparable to the particle size.