Structural Engineering of Cyanine Dyes to Access Shortwave Infrared-Emissive J-Aggregates

Low-cost, safe materials that absorb and emit light at near-infrared (NIR) and shortwave infrared (SWIR) wavelengths are of interest for optical communications, sensors, and diagnostics. One promising avenue to access the NIR/SWIR regions is to self-assemble chromophores into J-aggregates. However, the molecular design of two-dimensional (2D) or tubular J-aggregates with NIR and SWIR emissions is challenging, as small monomeric changes can drastically affect the resulting aggregates' morphologies and photophysical properties. Here, by synthesizing four new heptamethine benzothiazole dyes with increasing steric bulk on the polymethine chain, we rationally design SWIR-emissive 2D J-aggregates. Adding steric bulk drastically changes the self-assembled mesophases and photophysical properties, leading to an unprecedented 3-4 distinct aggregates for each dye. We isolate a total of thirteen new NIR/SWIR aggregates, out of which nine exhibit room-temperature emission. Excitingly, one of the aggregates also shows a tubular morphology─the first SWIR tubular aggregate characterized. A series of interconversion studies, structural characterizations, and modeling provide insights into the molecular packing arrangements of each mesophase. This thorough understanding of the J-aggregate mesophases allowed us to perform four-color imaging using a single chromophore scaffold by selective excitation of the monomeric fluorophore and distinct aggregate structures. Overall, this work demonstrates a rational design strategy for accessing NIR/SWIR emission via supramolecular aggregation and highlights opportunities in energy transport and multiplexed imaging derived from this fundamental understanding of long-wavelength J-aggregates.