The quantum control of ultrafast excited state dynamics remains an unachieved goal within the chemical physics community. In this study, we assess how strongly coupling to cavity photons affects the excited state dynamics of strongly coupled zinc (II) tetraphenyl porphyrin (ZnTPP) and copper (II) tetraphenyl porphyrin (CuTPP) molecules. By varying the concentration of each chromophore within different Fabry–Pérot (FP) structures, we control the collective vacuum Rabi splitting between the energies of cavity polariton states formed through the strong coupling of molecular electrons and cavity photons. Using ultrafast transient reflectivity and transmission measurements probing optical transitions of individual ZnTPP and CuTPP molecules, we find that the polaritonic states localize into uncoupled excited states of these chromophores through different mechanisms. For ZnTPP, we build a simple kinetic model including a direct channel of relaxation between the polaritonic states. We find that our models necessitate a small contribution from this interpolaritonic relaxation channel to explain both our steady-state and transient optical spectroscopic measurements adequately. In contrast, we propose that strong cavity coupling slows the internal conversion between electronic states of CuTPP not directly interacting with the photons of FP structures. These results suggest that researchers must consider the vibrational structure and excited state properties of the strongly coupled chromophores when attempting to use polariton formation as a tool to control the dynamics of molecules central to photo-sensitizing and light harvesting applications.

The rational control of nonradiative relaxation remains an unfulfilled goal of synthetic chemistry. In this study, we show how strongly coupling an ensemble of molecules to the virtual photons of an electromagnetic cavity provides a rational handle over ultrafast, nonradiative dynamics. Specifically, we control the concentration of zinc tetraphenyl porphyrin molecules within nanoscale Fabry−Perot cavity structures to show a variable collective vacuum Rabi splitting between cavity polaritons coincides with systematic changes in internal conversion rates. We find these changes deviate from the predictions of so-called gap laws. We also show that simple theories of structural changes caused by polariton formation cannot explain discrepancies between our results and established theoretical predictions. In light of these deficiencies, we explore other ways to explain the dependence of the internal conversion rate on polariton parameters. Our results demonstrate cavity polariton formation controls the photophysics of light harvesting and photocatalytic molecular moieties and the gap remaining in our fundamental understanding of mechanisms enabling this control.

The properties of mid-band-gap electronic states are central to the potential application of self-assembled, hybrid organic–inorganic perovskite-like quantum wells in optoelectronic technologies. We investigate broadband light emission from mid-band-gap states in fast-forming hybrid organic lead iodide quantum wells at room temperature. By comparing temperature- and intensity-dependent photoluminescence (PL) spectra emitted from butyl ammonium spaced inorganic layers, we propose that structural defects in a metastable material phase trap excitons and cause broadband light emission spanning wavelengths between 600 and 1000 nm. We use temperature-dependent terahertz time-domain spectroscopy to correlate changes in the subgap PL emission with changes in the chemical bonding of the inorganic octahedral layer. Our results provide new fundamental physical insights into the array of mechanisms capable of inducing broadband light emission from low-dimensional perovskite-like materials central to their application in future optoelectronic technologies and novel spectroscopic tools to characterize these states.

The formation of exciton polaritons by strongly coupling molecular electronic transitions to spatially confined photons may change photochemical processes. However, the fundamental physical drivers of these proposed changes remain unclear. To understand the role of changes to the molecular structure caused by polariton formation in these photochemical processes, we develop an experimentally motivated theoretical description of resonance Raman scattering from exciton polaritons. By modeling the structural parameters of light absorption in zinc tetraphenyl porphyrin and exciton polaritons formed from this molecule in nanostructured Fabry–Pérot cavities, we simulate resonance Raman scattering excitation spectra from exciton polaritons and assess how these spectra depend on the polariton characteristics. We find that the detuning of the cavity mode energy from that of the molecular transition can affect the structure of the resulting polaritonic states, as inferred from our simulated spectra. We also find previously unreported interference effects between the quantum paths contributing to the resonance Raman scattering pathways changes in the presence of exciton polaritons. Finally, we discuss possible experimental configurations capable of extracting the predicted polariton spectral signatures from the background of the signal from molecules not participating in strong cavity–molecule coupling. These results suggest resonance Raman spectroscopy could be a powerful tool to assess the role changes to the molecular structure plays in the amended photophysics and photochemistry of exciton polaritons.

In this study, we explore how one can use cavity polariton formation and a non-Condon vibronic coupling mechanism to form a type of hybrid light−matter state we denote as Herzberg−Teller (HT) vibronic polaritons. We use simple models to define the basic characteristics of these hybrid light−matter excitations including their dispersive energies. Experimentally, we find evidence of HT polaritons in the light emission spectra from copper(II) tetraphenylporphyrin (CuTPP) molecules strongly coupled to both single and multimode Fabry−Perot resonator structures. For specific resonator designs, we find evidence of significant enhancement of light emission from a short-lived sing-doublet state of CuTPP, which couples to a higher energy singlet state via a non-Condon vibronic mechanism. The results of a two-state model support the conclusion that this enhancement and the temperature-dependent dispersion of the light emission peak energy stem from radiative relaxation into cavity photon states dressed by collective vibrations of the molecules participating in polariton formation. These results show how researchers can leverage the complex interplay of electronic and nuclear degrees of freedom in light absorbing molecules to form a vaster array of coherent light−matter states and potentially transform platforms in optoelectronic and photocatalytic technologies.