Energy transfer (ET) between donor and acceptor molecules in free space relies on specific physical conditions. When considering molecular singlet excited states, ET necessitates the fluorescence spectrum of the donor overlaps with the absorption spectrum of the acceptor. This restriction indicates molecules with very low fluorescence quantum yields cannot act as efficient ET donors. Moreover, one would expect almost no ET when the donor and acceptor molecules possess very similar optical spectra. We propose cavity polariton formation in specially designed resonator structures represents one method to overcome these limitations. To test this proposal, we form cavity polaritons using two porphyrin molecules whose intense Soret absorption peaks overlap almost completely, free base tetraphenyl porphyrin (H2TPP) and copper (II) tetraphenyl porphyrin (CuTPP). The ultrafast lifetime of the B state in CuTPP (&lt;50 fs) indicates one should expect very inefficient ET from this state. By considering the strong coupling of the nearly degenerate Soret peaks of each molecule to cavity photons in multilayer Fabry-Perot cavities, we use a 3 oscillator Hamiltonian to show the polaritonic states of these structures controllably mix excitations on each chromophore. In addition, the angular dependence of the Hopfield coefficients characterizing each polariton state indicate one can use this controllable mixing to drive ET from an upper polariton (UP) comprised mostly of a CuTPP excitons to a lower polariton (LP) state largely localized on H2TPP molecules across the 450 nm length of our structures. To test this prediction, we probe the ultrafast evolution of excited state absorption of H2TPP at 1.9 eV following direct excitation of the UP state with a 3.1 eV pump pulse. We tune the angle of the probe to ensure that we investigate the LP state at a position in which the H2TPP fraction is dominant. We assess how the temporal characteristics of the H2TPP ESA signal indicate polariton-mediate ET from a CuTPP-rich donor layer. These results demonstrate that polaritons formed using quasi degenerate porphyrin excitons enable novel forms of ET inaccessible for these molecules outside of the cavity’s structured photonic environment.

Polariton-mediated energy transfer between spatially separated porphyrin excitons

ACS Spring 2022

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