Periodic Reporting for period 2 - NOTsoQUANTUM (NOTsoQUANTUM: Realistic simulations of polaritonic chemistry)
Berichtszeitraum: 2024-09-01 bis 2025-08-31
In conventional chemistry, molecules interact with light in a regime termed “weak coupling”, meaning that during their interaction, light does not significantly perturb the molecules, but simply acts as an irreversible energy source that brings the molecules to a higher energy state. In more visual terms, light interacting with a molecule can be understood as a ball “kicked“ up-hill, where light provides the energy “kick” to the molecule. Depending on the energy of light, different states of the molecule are accessed. For instance, in solar cells, sunlight absorbed by molecules slightly redistributes their electrons, eventually converting it to electrical current. In the opposite regime, known as “strong coupling”, the interaction between light and matter is no longer “irreversible”: after light “kicks” a molecule, the molecule is able to “kick” the light back. Such back and forth “kicking“ essentially represents a continuous energy flow between the light and the molecule, and is responsible for the formation of new states called polaritons that do not exists in the standard “weak coupling” regime. In recent years, the field of polaritonic chemistry has become increasingly popular as a new means to manipulate chemical processes with light by the formation of these new polariton states. Because these states are mixed consisting of both light and matter, they have hybrid properties and can potentially harness the best of both worlds.
To fully exploit the potential of both weak and strong light-matter interactions for targeted applications with molecular platforms, it is essential to develop a detailed microscopic understanding of the underlying physical and chemical phenomena. Towards this end, the overall goal of the present proposal is to explore the effect of the environment, such as the role played by disorder and vibrations, as well as the interplay between different molecular spin states, and how these impact the nature of weak and strong light-matter interactions. A particular focus on which aspects may be understood by classical means without invoking the quantum nature of neither light nor matter will be given, paving the way towards more realistic simulations of molecular polaritons where a full quantum treatment is prohibitive.
To fully exploit the potential of both weak and strong light-matter interactions for targeted applications with molecular platforms, it is essential to develop a detailed microscopic understanding of the underlying physical and chemical phenomena. Towards this end, the overall goal of the present proposal is to explore the effect of the environment, such as the role played by disorder and vibrations, as well as the interplay between different molecular spin states, and how these impact the nature of weak and strong light-matter interactions. A particular focus on which aspects may be understood by classical means without invoking the quantum nature of neither light nor matter will be given, paving the way towards more realistic simulations of molecular polaritons where a full quantum treatment is prohibitive.
We have developed a fully classical theory that can account for the effect of both fast and slow (molecular) disorder on the polariton lineshape. One significant implication of our theory is that it allows to understand the interplay between the different timescales of the problem on the collectivity of the polariton. Given that a polariton is a collective state and by destroying its collectivity the polariton is lost, understanding how disorder affects its collectivity is critical for future experimental implementations. Importantly, our theory can be solved analytically, providing a physical and intuitive picture on the role of disorder. This work has been published in a manuscript entitled “Kubo-Anderson theory of polariton line shape” (DOI: 10.1103/PhysRevA.109.052809)
On a more applied note we have investigated how metallic plasmonic structures can enhance the absorption and emission of typically forbidden far-field transitions such as those governed by quadrupole moments instead of dipole ones. In collaboration with an experimental group, in the publication “Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities” (DOI: 10.1021/acsnano.4c01368) we report the first experimental example where quadrupole interaction terms (i) contribute to the absorption and photoluminescence of molecules embedded in plasmonic nanocavities and (ii) compete with standard dipole-driven transitions. In this work, a suitable molecule for probing quadrupole contributions was identified by theoretical arguments and computational results, and was subsequently employed in the experiments. Finally, by exploring the role of vibrations in the symmetry-forbidden dipole transition we revealed its interplay with the quadrupole contribution in the absorption and emission characteristics.
In the manuscript entitled “On the circularly polarized luminescence of individual triplet sublevels” (DOI: 10.1063/5.0159932) we investigated the possibility of using circularly polarized luminescence as a tool to probe the population of individual triplet spin subleves in a chiral molecule. Typically, spectroscopists unrelated to the spin physics/chemistry research areas are never concerned with the identity of each spin sublevel because these are equilibrated at room temperature. However, the development of the chiral induced spin selectivity (CISS) effect has evidenced that spin polarization can be substantial in chiral species even at room temperature. In this work we propose that by measuring both the total emission and the circular polarized luminescence of a phosphorescent chiral compound, the populations of each triplet sublevel could be retrieved, providing insight into the mechanism of coupled electronic-nuclear-spin dynamics.
I have disseminated the results obtained with the present proposal by delivering invited talks at two international conferences held in Mexico City and Stockholm. I also presented my results in four posters at an international conference held at the University of California San Diego, a winter school organized by the University of California Los Angeles (UCLA) and at two Gordon Research Conferences (GRC) held in the US.
On a more applied note we have investigated how metallic plasmonic structures can enhance the absorption and emission of typically forbidden far-field transitions such as those governed by quadrupole moments instead of dipole ones. In collaboration with an experimental group, in the publication “Influence of Quadrupolar Molecular Transitions within Plasmonic Cavities” (DOI: 10.1021/acsnano.4c01368) we report the first experimental example where quadrupole interaction terms (i) contribute to the absorption and photoluminescence of molecules embedded in plasmonic nanocavities and (ii) compete with standard dipole-driven transitions. In this work, a suitable molecule for probing quadrupole contributions was identified by theoretical arguments and computational results, and was subsequently employed in the experiments. Finally, by exploring the role of vibrations in the symmetry-forbidden dipole transition we revealed its interplay with the quadrupole contribution in the absorption and emission characteristics.
In the manuscript entitled “On the circularly polarized luminescence of individual triplet sublevels” (DOI: 10.1063/5.0159932) we investigated the possibility of using circularly polarized luminescence as a tool to probe the population of individual triplet spin subleves in a chiral molecule. Typically, spectroscopists unrelated to the spin physics/chemistry research areas are never concerned with the identity of each spin sublevel because these are equilibrated at room temperature. However, the development of the chiral induced spin selectivity (CISS) effect has evidenced that spin polarization can be substantial in chiral species even at room temperature. In this work we propose that by measuring both the total emission and the circular polarized luminescence of a phosphorescent chiral compound, the populations of each triplet sublevel could be retrieved, providing insight into the mechanism of coupled electronic-nuclear-spin dynamics.
I have disseminated the results obtained with the present proposal by delivering invited talks at two international conferences held in Mexico City and Stockholm. I also presented my results in four posters at an international conference held at the University of California San Diego, a winter school organized by the University of California Los Angeles (UCLA) and at two Gordon Research Conferences (GRC) held in the US.
The novel insights into weak and strong light-matter interactions developed in this proposal are expected to impact several research areas. (I) The influence of disorder on the spectroscopic signatures of polaritons is of utmost importance for future applications of molecular polaritonics in practical devices. The fully classical theory we have developed in this proposal provides a robust framework for future simulations of realistic experimental setups of polaritons. (II) The demonstration that traditionally forbidden transitions such as those governed by quadrupole interactions, can compete with dipole transitions in plasmonic setups, opens an avenue for future explorations to access optically inactive transitions with potential impact in spectroscopy and sensing. (III) The possibility to experimentally probe the population of individual triplet states is a new idea that represents a step forward towards understanding the mechanism of spin-polarized dynamics and within a broader context, its relationship with the chiral induced spin selectivity effect, with implications in spintronics and chiral catalysis.