Thermodynamic Control over Molecular Aggregate Assembly Enables Tunable Excitonic Properties across the Visible and Near-Infrared

Abstract

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.

Tubular J-aggregates of cyanine dyes in the near-infrared

“…To probe the aggregate morphologies of the new pentamethine dyes, we both screened 2a and 2b at a constant concentration and varied the methanol/water ratio, as well as varied the concentration at a constant methanol/water ratio. 34 Absorbance spectra for monomers and aggregates of different compounds are shown in Figure 2A. In this exploration, we found that the pentamethine dye aggregates display similar photophysical properties to the trimethine dyes, forming two primary aggregates (confirmed by non-negative matrix factorization, Figure S3) depending on the concentration and methanol ratio.…”

Tubular J-aggregates of cyanine dyes in the near-infrared

“…To probe the aggregate morphologies of the new pentamethine dyes, we both screened 2a and 2b at varied concentrations while maintaining a constant methanol/water ratio (Figure 2A). 35 Aggregates of each chromophore were prepared by first creating a stock solution of the dye in methanol. To a small volume of this stock solution was added ultrapure H2O.…”

Bridging the Gap between H- and J-Aggregates: Classification and Supramolecular Tunability for Excitonic Band Structures in 2-Dimensional Molecular Aggregates

“…Using this approach, we were able to stabilize the 2D sheet-like morphology of all the dyes studied here (SI Figure S2). 36 We also include the well-known dye 3,3'-bis(4-sulfobutyl)-5,5',6,6'-tetrachloro-1,1'-diethylbenzimidacarbocyanine, sodium salt (TDBC), commonly used in photonic applications. 37,38 All 2D aggregates are prepared by injecting a methanol solution of the dye in water or aqueous NaCl (see experimental section for exact conditions), followed by equilibration in the dark for 24 h. By changing the dye lengths as well substitutions on the central position of the cyanine bridge (Figure 3a), we afford extended 2D aggregates with different brick lengths and slips.…”

“…48 Although, this is unlikely in our case, since all aggregates had up to micron scale sizes as seen from cryoEM images (SI Figure S2). 36 Screening aggregate geometries using stochastic modeling of the Hamiltonian: In Figure 5a-b, we…”