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.
