The efficient spatial transport of molecular excitations remains a paramount goal for the development of next generation solar-to-chemical energy conversion technologies. However, established physical mechanisms of intermolecular excitation transfer necessitate donor and acceptor moieties to remain within sub-5 nm relative distances to enable measurable rates. The application of molecular light absorbers to solar-to-chemical energy conversion technologies will depend sensitively on overcoming these physical limits in novel device architectures. In this talk, I will provide an overview of the design, fabrication, and characterization of the coherent coupling between degenerate, model light harvesting molecules separated by over 100 nms mediated by the cavity multipolariton formation. Using steady-state spectra, I show the quasi-degeneracy of excitons localized on copper(II) tetraphenyl porphyrin (CuTPP) and free base tetraphenyl porphyrin (H2TPP) leads to the formation of formally dark middle polariton states comprised of almost no photonic content. Despite the low photonic composition of these states, they maintain a coherent mixture of excitations on H2TPP and CuTPP molecules located over 150 nm apart from one another. I present ultrafast spectroscopic measurements that suggest these coherent mixtures of excitonic states enable a novel form of intermolecular energy transfer not possible outside of the cavity environment. Using the spectroscopic signatures of localized porphyrin molecules, we show evidence of ground state recovery of CuTPP molecules coupled to cavity photons coincident with a rise of the excited state absorption of local H2TPP molecules. Comparisons with theoretical models allow us to examine the temporal evolution of polaritonic composition consistent with the measured sample kinetics. These results demonstrate the ability of cavity polariton formation to mediate fundamental physical interactions not possible outside of the cavity environment and the types of experimental device achitectures necessary to enable those polariton-mediated interactions.

Mesoscopic intermolecular energy transfer between degenerate molecular excitons mediated by cavity multipolariton formation

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