Defects Cause Subgap Luminescence from a Crystalline Tetracene Derivative

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission

Materials that undergo singlet fission are of interest for their use in light-harvesting, photocatalysis, and quantum information science, but their ability to undergo fission can be sensitive to local variations in molecular packing. Herein we employ transient absorption microscopy, molecular dynamics simulations, and electronic structure calculations to interrogate how structures found at the edges of orthorhombic rubrene crystals impact singlet fission. Within a micrometer-scale spatial region at the edges of rubrene crystals, we find that the rate of singlet fission increases nearly 4-fold. This observation is consistent with formation of a region at crystal edges with reduced order that accelerates singlet fission by disrupting the symmetry found in rubrene's orthorhombic crystal structure. Our work demonstrates that structural distortions of singlet fission materials can be used to control fission in time and in space, potentially offering a means of controlling this process in light harvesting and quantum information applications.

Local molecular probes of ultrafast relaxation channels in strongly coupled metalloporphyrin-cavity systems

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.

Room-Temperature Broadband Light Emission from Hybrid Lead Iodide Perovskite-Like Quantum Wells: Terahertz Spectroscopic Investigation of Metastable Defects

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.