Structural Engineering of Cyanine Dyes to Access Highly Redshifted and Emissive J-aggregates

Molecular design of two-dimensional (2D) and tubular excitonic aggregates would enable access to extraordinary and unusual photophysical properties via control over supramolecular structure. Here, we synthesize four heptamethine cyanines that enable a rational investigation of the role of steric bulk on aggregate self-assembly, morphology and photophysics. Despite near identical monomer photophysics, minute changes to structure and solvation lead to extreme differences in resultant J-aggregate morphology and photophysics. Additional steric bulk not only redshifts aggregate absorption, but also alters the kinetics/thermodynamics of self-assembly, yielding a variety of connected morphological supramolecular phases. We employ cryo-electron and atomic force microscopy, dynamic light scattering and computational screening to characterize 3-4 2D/tubular J-aggregates for each dye, enabling us to predict the monomer packing arrangements. Differentiating and tuning each unique morphology demonstrates approaches to structurally engineer highly redshifted molecular 2D molecular aggregates via synthetic design, achieving extramolecular control over their photophysics.