Periodic Reporting for period 4 - STRONG (Strong Coupling Between Molecules and Vacuum Fields: New Molecular Properties)
Reporting period: 2022-08-01 to 2023-01-31
A central physical property of the molecule is its ability to interact with light. Plant leaves are green because they absorb light. However, less known is that this light-matter interaction can be enhanced to the point where it is so strong so that the photon and molecule cannot be regarded as separate entities, but as a system with unique properties. So called strong coupling occurs when exchange of energy between light and matter is stronger than any dissipation process and it leads to the formation of hybrid states with new physical and chemical properties. Little is actually known on how these interactions can change the chemistry, photochemistry and photophysics of organic molecules. Innovative studies exploring the scope of strong light-matter interactions are thus needed.
From a societal perspective, methods of lowering the energy of excited singlet states below that of the corresponding triplet state are of importance. This because it would create an energetically driven triplet to singlet conversion of organic molecular states. This is of high importance in applications and technologies within the fields of molecular electronics, where energy positions and alignment as well as spin state is crucial. STRONG will also influence the perception of the possibility of manipulating energy levels in molecules on a fundamental level.
STRONG used a chemical viewpoint to develop unique molecules optimized for strong light-matter interactions, and with these examined excited state processes of strongly coupled systems. The aim was to demonstrate that strong light-matter coupling enables selective manipulation of energy levels. By so doing allowing for a singlet ground and first excited state, thus challenge Hund’s rule and change how the basic rules of electronic state energetics are envisioned. The idea was that this would enable channelling of all excitation energy, irrespectively of origin, through a singlet pathway, which is of great technological importance in organic electronics. Furthermore, the project used reversible oriented molecules to enhance the coupling to examine the relationship between orientation of molecules and strong light-matter coupling.
The conclusions drawn from STRONG is the following. It is clear that it is possible to change the energetics of a system that is based on an organic molecule using strong exciton-photon coupling, for instance invert the energetics of singlet and triplet states. It is also possible to change the excited state relaxation pathways by strong exciton-photon coupling, for instance by allowing barrier-less channeling of triplet energy to a singlet state. What is not clear at the moment is the scope of these processes. So far this type of channeling has been seen in systems where more traditional relaxation pathways were slow. To draw conclusions on the possibility of future improvements on rates more knowledge about delocalization in strongly coupled systems are needed. Furthermore, by using molecules that can change orientation, the molecular orientation dependence in strongly coupled systems has been experimentally determined.
From a societal perspective, methods of lowering the energy of excited singlet states below that of the corresponding triplet state are of importance. This because it would create an energetically driven triplet to singlet conversion of organic molecular states. This is of high importance in applications and technologies within the fields of molecular electronics, where energy positions and alignment as well as spin state is crucial. STRONG will also influence the perception of the possibility of manipulating energy levels in molecules on a fundamental level.
STRONG used a chemical viewpoint to develop unique molecules optimized for strong light-matter interactions, and with these examined excited state processes of strongly coupled systems. The aim was to demonstrate that strong light-matter coupling enables selective manipulation of energy levels. By so doing allowing for a singlet ground and first excited state, thus challenge Hund’s rule and change how the basic rules of electronic state energetics are envisioned. The idea was that this would enable channelling of all excitation energy, irrespectively of origin, through a singlet pathway, which is of great technological importance in organic electronics. Furthermore, the project used reversible oriented molecules to enhance the coupling to examine the relationship between orientation of molecules and strong light-matter coupling.
The conclusions drawn from STRONG is the following. It is clear that it is possible to change the energetics of a system that is based on an organic molecule using strong exciton-photon coupling, for instance invert the energetics of singlet and triplet states. It is also possible to change the excited state relaxation pathways by strong exciton-photon coupling, for instance by allowing barrier-less channeling of triplet energy to a singlet state. What is not clear at the moment is the scope of these processes. So far this type of channeling has been seen in systems where more traditional relaxation pathways were slow. To draw conclusions on the possibility of future improvements on rates more knowledge about delocalization in strongly coupled systems are needed. Furthermore, by using molecules that can change orientation, the molecular orientation dependence in strongly coupled systems has been experimentally determined.
This overview of achievements within STRONG follows two main lines. Technologies for improving processability of organic molecules in order for them to work better in strong exciton photon coupling experiments (related to WP1) and developing the concept of strong exciton photon coupling (related to WP2-3).
Regarding the processability of molecules, we have showed that it is possible to create an organic dye, which aggregation state is a liquid at room temperature (see Kushwaha et al.), and further expanded the scope to molecules that act as dye glasses at room temperature (see Schäfer et al.). This by the use of entropic mixing, which is a concept invented in this project. We forsee that especially the concept of making dye glasses will be of significant practical importance within the fields of strong exciton photon coupling and organic electronics in the future.
Regarding the fundamentals of strong exciton photon coupling, we have confirmed several theoretical predictions experimentally. For instance, we have shown that the lifetime of polariton emission is angular independent (see Mony et al) and have demonstrated the molecular orientation dependence on the light-matter coupling strength (see Hertzog et al). We have furthermore introduced strong-light matter coupling to be a concept that can change the alignment between singlet and triplet states (see Stranius et al, Ye et al, and Yu et al), and by so doing changed the photophysical behavior of the system. We see this last part as the most important achievement and foresee that this is what STRONG will be remembered from.
Regarding the processability of molecules, we have showed that it is possible to create an organic dye, which aggregation state is a liquid at room temperature (see Kushwaha et al.), and further expanded the scope to molecules that act as dye glasses at room temperature (see Schäfer et al.). This by the use of entropic mixing, which is a concept invented in this project. We forsee that especially the concept of making dye glasses will be of significant practical importance within the fields of strong exciton photon coupling and organic electronics in the future.
Regarding the fundamentals of strong exciton photon coupling, we have confirmed several theoretical predictions experimentally. For instance, we have shown that the lifetime of polariton emission is angular independent (see Mony et al) and have demonstrated the molecular orientation dependence on the light-matter coupling strength (see Hertzog et al). We have furthermore introduced strong-light matter coupling to be a concept that can change the alignment between singlet and triplet states (see Stranius et al, Ye et al, and Yu et al), and by so doing changed the photophysical behavior of the system. We see this last part as the most important achievement and foresee that this is what STRONG will be remembered from.
The largest advance beyond the scope of STRONG that has been made is the concept of entropic mixing. The concept was vaguely outlined in the proposal, but it has now been named, and the theoretical foundation of the concept has been formalized. More material science-oriented scientists have now taken this concept to their hearts in order to enhance the stability of solar cells. This concept is therefore in a state of becoming generalized for the materials chemistry community.
The main goal of STRONG was to explore the connection between hybrid-and triplet states. This concept in itself goes beyond state of the art. We have shown that it is possible to change the energetics of a system that is based on an organic molecule using strong exciton-photon coupling, and by so doing invert the energetics of singlet and triplet states. It is also possible to change the excited state relaxation pathways by strong exciton-photon coupling, for instance by allowing barrier-less channeling of triplet energy to a singlet state. What is not clear at the moment is the scope of these processes. So far this type of channeling has been seen in systems where more traditional relaxation pathways were slow. But we have shown that there is a communication link between hybrid and molecular centered states, which is large enough to be detectable, and this is very important.
The main goal of STRONG was to explore the connection between hybrid-and triplet states. This concept in itself goes beyond state of the art. We have shown that it is possible to change the energetics of a system that is based on an organic molecule using strong exciton-photon coupling, and by so doing invert the energetics of singlet and triplet states. It is also possible to change the excited state relaxation pathways by strong exciton-photon coupling, for instance by allowing barrier-less channeling of triplet energy to a singlet state. What is not clear at the moment is the scope of these processes. So far this type of channeling has been seen in systems where more traditional relaxation pathways were slow. But we have shown that there is a communication link between hybrid and molecular centered states, which is large enough to be detectable, and this is very important.