S301 (Seminar Room), South Laboratory Building 3F

   Coupling of the transition dipole moment of an organic chromophore can create new photophysical relaxation pathways and/or change the energetics of the excited states. I will give a selection of examples using three different coupling scenarios. The first scenario deals with weak exciton coupling, which enables energy transfer between chromophores. I will discuss how the energy transfer, commonly referred to as FRET, can be used as a multiplicity converter.^{1, 2} The second scenario deals with strong exciton coupling, which results in new delocalized states with distinctly different energies than the uncoupled chromophore (so-called J-aggregates). I will show that such systems can not only be used to red shift the absorption, but the delocalized nature results in a breakdown of the so-called energy gap law, giving a surprisingly high emission quantum yield in the NIR region of the electromagnetic spectrum.³ The third and last scenario deals with strong light-matter coupling. Here the molecular transition dipole moment is strongly coupled to the electromagnetic field inside an optical cavity, producing so-called polaritonic states. This phenomenon opens a plethora of options for manipulating the properties of molecules, with a broad range of applications in physics, chemistry, and materials science. I will start with introducing the fundamentals of strong light-mater coupling.^{4, 5} How an optical cavity can be used to increase the electromagnetic field experienced by a molecule to the point where polaritons, emerge. I will go through the basic properties of these states, how to model them,⁶ and then give examples of possible uses. The first example discusses the effect of transitions from a polaritonic state to a molecular centred state. The reorganization energy of the polaritonic state is negligible, thus allowing for a redshifted absorbance (when comparing the polaritonic absorbance compared to the uncoupled molecular absorbance) in a photochemical transformation such as a photoswitching event or in a photovoltaic device.⁷ Furthermore, the polaritonic states are delocalized, a transition to a molecular centred state can therefore be used to funnel excitation energy to a reactive site. I will discuss how such a funnelling could be used in an organic solar cell setting, to use the delocalized nature of polaritons to shuttle excitation energy to charge-transfer states at material interfaces, thus the enabling of planar heterojunction solar cells in low exciton diffusion materials.⁸ I will end this presentation with discussing transitions from a molecular centred state to a polaritonic state. In such transitions, driving forces can be manipulated, and I will focus on showing how the energy difference between triplet states and polaritonic states can affect photophysics.^9-11

References

1. Cravcenco, A.; Hertzog, M.; Ye, C.; Iqbal, M. N.; Mueller, U.; Eriksson, L.; Börjesson, K. Multiplicity conversion based on intramolecular triplet-to-singlet energy transfer. Sci. Adv. 2019, 5 (9), 5.
2. Cravcenco, A.; Ye, C.; Gräfenstein, J.; Börjesson, K. Interplay between Förster and Dexter Energy Transfer Rates in Isomeric Donor–Bridge–Acceptor Systems. J. Phys. Chem. A . 2020, 124 (36), 7219-7227.
3. Cravcenco, A.; Yu, Y.; Edhborg, F.; Goebel, J. F.; Takacs, Z.; Yang, Y.; Albinsson, B.; Börjesson, K. Exciton Delocalization Counteracts the Energy Gap: A New Pathway toward NIR-Emissive Dyes. J. Am. Chem. Soc. 2021, 143 (45), 19232-19239.
4. Hertzog, M.; Wang, M.; Mony, J.; Börjesson, K. Strong light–matter interactions: a new direction within chemistry. Chem. Soc. Rev. 2019, 48 (3), 937-961.
5. Bhuyan, R.; Mony, J.; Kotov, O.; Castellanos, G. W.; Gómez Rivas, J.; Shegai, T. O.; Börjesson, K. The Rise and Current Status of Polaritonic Photochemistry and Photophysics. Chemical Reviews 2023, 123 (18), 10877-10919.
6. Mukherjee, A.; Feist, J.; Börjesson, K. Quantitative Investigation of the Rate of Intersystem Crossing in the Strong Exciton–Photon Coupling Regime. J. Am. Chem. Soc. 2023, 145 (9), 5155-5162.
7. Mony, J.; Climent, C.; Petersen, A. U.; Moth-Poulsen, K.; Feist, J.; Börjesson, K. Photoisomerization Efficiency of a Solar Thermal Fuel in the Strong Coupling Regime. Adv. Funct. Mater. 2021, 31 (21), 2010737.
8. Wang, M.; Hertzog, M.; Börjesson, K. Polariton-assisted excitation energy channeling in organic heterojunctions. Nat. Commun. 2021, 12 (1), 1874.
9. Ye, C.; Mallick, S.; Hertzog, M.; Kowalewski, M.; Börjesson, K. Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation. J. Am. Chem. Soc. 2021, 143 (19), 7501-7508.
10. Yu, Y.; Mallick, S.; Wang, M.; Börjesson, K. Barrier-free reverse-intersystem crossing in organic molecules by strong light-matter coupling. Nat. Commun. 2021, 12 (1), 3255.
11. Stranius, K.; Hertzog, M.; Börjesson, K. Selective manipulation of electronically excited states through strong light-matter interactions. Nat. Commun. 2018, 9, 2273.