Exploring the design of superradiant J-aggregates from amphiphilic monomer units.

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. Using the well-studied amphiphilic cyanine dye C8S3 as a template-known to assemble into tubular excitonic aggregates-we synthesize several redshifted variants and study their self-assembly and photophysics. The new pentamethine dyes retain their tubular self-assembly and demonstrate nearly identical bathochromic shifts and lineshapes well into near-infrared wavelengths. However, detailed photophysical analysis finds that the new aggregates show a significant decline in superradiance. Additionally, cryo-TEM reveals that these aggregates readily form short bundles of nanotubes that have nearly half the radii of their trimethine comparators. We employ computational screening to gain intuition on how the structural components of these new aggregates affect their excitonic states, finding that the narrower tubes are able to assemble into a larger number of arrangements, resulting in more disordered aggregates (i.e. less superradiant) with highly similar degrees of redshift.

Counterion Pairing Effects on a Flavylium Heptamethine Dye.

Polymethine fluorophores have facilitated the advance of biological and material sciences, due to their advantageous photophysical properties. However, the need to maintain a monomeric state can severely limit the utility and processability of dyes. High concentrations of fluorophore can lead to aggregation and negate the beneficial photophysical properties of monomers. Another concern is "crossing the cyanine limit" in which delocalization within the polymethine scaffold is broken, producing the presence of an asymmetric state diminishing its photophysical behavior. Herein, we attempt to overcome these limitations by exploring anion exchange on a cationic flavylium heptamethine scaffold. By increasing the size and hydrophobicity of the anion, we can effectively tune the degree of ion pairing within the polymethine dye. Interestingly, we found that the effect of ion pairing on photophysical properties was subtle for the flavylium heptamethine scaffold in comparison to the more commonly used indolenine cyanine dye. Utilizing larger weakly coordinating anions enabled solubility of the flavylium heptamethine fluorophore in nonpolar solvents, which could otherwise not be achieved. Even with more subtle effects than classic cyanine dyes, anion exchange on flavylium polymethine dyes holds potential for further manipulation of the properties of these low energy dyes.

Photophysical Tuning of Shortwave Infrared Flavylium Heptamethine Dyes via Substituent Placement.

Optical imaging in the shortwave infrared region (SWIR, 1000-2000 nm) provides high-resolution images in complex systems. Here we explore substituent placement on dimethylamino flavylium polymethine dyes, a class of SWIR fluorophores. We find that the position of the substituent significantly affects the λmax and fluorescence quantum yield. Quantum-mechanical calculations suggest that steric clashes control the extent of π-conjugation. These insights provide design principles for the development of fluorophores for enhanced SWIR imaging.

Silicon incorporation in polymethine dyes.

Methods to red-shift fluorophores have garnered considerable interest due to the broad utility of low energy light. The incorporation of silicon into xanthene and coumarin scaffolds has resulted in an array of visible and near-infrared fluorophores. Here, we extend this approach to polymethine dyes, another popular fluorophore class, performing experimental and computational analyses. We found that when oxygen was replaced with SiMe2, bathochromic shifts of up to 121 nm and fluorophores with emission above 900 nm were achieved.

Shortwave infrared polymethine fluorophores matched to excitation lasers enable non-invasive, multicolour in vivo imaging in real time.

High-resolution, multiplexed experiments are a staple in cellular imaging. Analogous experiments in animals are challenging, however, due to substantial scattering and autofluorescence in tissue at visible (350-700 nm) and near-infrared (700-1,000 nm) wavelengths. Here, we enable real-time, non-invasive multicolour imaging experiments in animals through the design of optical contrast agents for the shortwave infrared (SWIR, 1,000-2,000 nm) region and complementary advances in imaging technologies. We developed tunable, SWIR-emissive flavylium polymethine dyes and established relationships between structure and photophysical properties for this class of bright SWIR contrast agents. In parallel, we designed an imaging system with variable near-infrared/SWIR excitation and single-channel detection, facilitating video-rate multicolour SWIR imaging for optically guided surgery and imaging of awake and moving mice with multiplexed detection. Optimized dyes matched to 980 nm and 1,064 nm lasers, combined with the clinically approved indocyanine green, enabled real-time, three-colour imaging with high temporal and spatial resolutions.