Rapid and repeated photon cycling has enabled precision
metrology
and the development of quantum information systems using atoms and
simple molecules. Extending optical cycling to structurally complex
molecules would provide new capabilities in these areas, as well as
in ultracold chemistry. Increased molecular complexity, however, makes
realizing closed optical transitions more difficult. Building on already
established strong optical cycling of diatomic, linear triatomic,
and symmetric top molecules, recent work has pointed the way to cycling
of larger molecules, including phenoxides. The paradigm for these
systems is an optical cycling center bonded to a molecular ligand.
Theory has suggested that cycling may be extended to even larger ligands,
like naphthalene, pyrene, and coronene. Herein, we study optical excitation
and fluorescent vibrational branching of CaO-, SrO-, and CaO- and find only weak decay to excited vibrational
states, indicating a promising path to full quantum control and laser
cooling of large arene-based molecules.
Specific
molecular arrangements within H-/J-aggregates of cyanine
dyes enable extraordinary photophysical properties, including long-range
exciton delocalization, extreme blue/red shifts, and excitonic superradiance.
Despite extensive literature on cyanine aggregates, design principles
that drive the self-assembly to a preferred H- or J-aggregated state
are unknown. We tune the thermodynamics of self-assembly via independent
control of the solvent/nonsolvent ratio, ionic strength, or dye concentration,
obtaining a broad range of conditions that predictably stabilize the
monomer (H-/J-aggregate). Diffusion-ordered spectroscopy, cryo-electron
microscopy, and atomic force microscopy together reveal a dynamic
equilibrium between monomers, H-aggregated dimers, and extended J-aggregated
2D monolayers. We construct a model that predicts the equilibrium
composition for a range of standard Gibbs free energies, providing
a vast aggregation space which we access using the aforementioned
solvation factors. We demonstrate the universality of this approach
among several sheet-forming cyanine dyes with tunable absorptions
spanning visible, near, and shortwave infrared wavelengths.
Function-oriented design of supramolecular self-assemblies represents a foundational goal in chemistry. Especially, chromophore self-assemblies display extreme photophysical changes from their monomers that are sensitive to the nanoscale molecular arrangements. Slip-stacked arrangements in J-aggregates lead to red shifts (>200 nm) and enhanced quantum yields. Actively introducing specific molecular arrangements using supramolecular chemistry provides a platform to tune the excitonic couplings and avail exotic photophysical properties. However, the nanoscale molecular arrangements have not yet been directly observed in these solution-state assemblies. Here, we present a high-resolution structure of the prototypical biomimetic light harvesting nanotubes (LHNs) of an amphiphilic cyanine dye (C8S3-Cl). We achieve a 3.3 Å resolution with helical reconstruction of cryo-EM images, directly visualizing the atomic scale parameters and packing arrangements that control the excitonic properties. Our structure clearly shows a brick layer arrangement of the molecules as opposed to the previously thought herringbone arrangement. Furthermore, we identify a new non-biological supramolecular motif – interlocking sulfonates, that may be responsible for the slip-stacked packing and ultimately the J-aggregate nature of LHNs. This motif may be relevant towards other amphiphilic self-assemblies, in general and provides a new chemical handle on predictable self-assemblies.
Excitonic chromophore aggregates have wide-ranging applicability in fields such as imaging and energy harvesting, however their rational design requires adapting principles of self-assembly to the requirements of excited state coupling....
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