In photonics, matter is used to control light. The inverse has been much more difficult to achieve. However, in the past several years, it has been shown experimentally that (resonant) photonic cavities can be used to alter chemical reactivity through vibro-polariton formation via a technique called vibrational strong coupling or VSC. This includes reaction kinetics, product distributions and binding thermodynamics. I have personally co-pioneered this while working in the group of Prof. Thomas Ebbesen at the University of Strasbourg. Besides chemical properties, VSC can also be used to alter physical properties. The most impressive example is making polystyrene, an insulator, semi-conducting in a Fabry-Perot structure.
Vibrational strong coupling entails the formation of hybrid light-matter states, called polaritonic states, generated by coupling an IR-active vibration of a molecule and a resonant photonic structure.When the energy of a cavity mode matches the transition energy of an IR vibration, a resonant energy exchange interaction occurs, coupling molecules and the EM field. When this light-matter interaction dominates all other loss processes, the so-called strong coupling regime is reached. The wavefunctions of molecular vibration and EM field hybridize to form two new polaritonic states, entangling both. This is akin the formation of molecular orbitals from atomic orbitals during chemical bonding, or excitonic coupling between chromophores. Because the molecular wavefunction determines the physical and chemical properties of a compound, one can expect these to become modified. Because of their part-matter, part-photonic character, polaritonic states display many unusual properties. Most importantly, they are collective, coherent states encompassing all molecules coupled to the cavity, which is a drastic difference from ‘normal’ molecular states which are entirely localized. Furthermore, as theory shows, strong coupling can occur even in the dark because it involves the EM fluctuations of the cavity field, the so-called “vacuum field”. This renders VSC apart from other approaches for reaction control, even those that employ light, like mode-selective chemistry or coherent control: VSC does not require any light to be used. It only requires a reaction to occur in a photonic cavity on resonance to a specific molecular vibration.
From a practical point-of-view, using polaritons in chemistry comes down to the idea using photonic structures to steer, control and/or enhance chemical processes. Because this requires only a suitable photonic cavity, this approach would be much more environmentally friendly then the traditional methods for enhancing and controlling chemical reactions, like e.g. high temperatures, high pressures and catalysts based on rare elements. The possibility of modifying chemical reactivity and other material properties with polaritons has generated considerable excitement around the world in both academia and industry. For instance, dedicated centers on the topic are being established in the US (see recent DARPA call) and in Europe. The ERC has also funded several projects on strong coupling.
Yet despite such increasing activity, nobody is yet able to predict the outcome of chemical reactions under VSC. This lack of understanding is not only a fundamental problem, but also hampers all efforts in applying VSC to catalysis in both academia and industry. This project aims to reach a new level of understanding of VSC chemistry using a systematic experimental approach focussed on examining nucleophilic acyl substitution and carbonyl chemistry systematically and in depth.
