Counterion‐Paired Bright Heptamethine Fluorophores with NIR‐II Excitation and Emission Enable Multiplexed Biomedical Imaging

Abstract

Photon excitation and emission at the NIR‐II spectral window enable high‐contrast deep‐tissue bioimaging. However, multiplexed imaging with NIR‐II excitation and emission has been hampered by the limited chemical strategies to develop bright fluorophores with tunable absorption in this spectral regime. Herein, we developed a series of heptamethine cyanines (HCs) with varied absorption/emission maxima spanning from 1100 to 1600 nm through a physical organic approach. A bulky counterion paired to HCs was found to elicit substantial improvements in absorptivity (7‐fold), brightness (14‐fold), and spectral profiles in water, addressing a notorious quenching problem of NIR‐II cyanines due to aggregation and polarization. We demonstrated the utilities of HC1222 and HC1342 for high‐contrast dual‐color imaging of circulatory system, lymphatic structures, tumor, and organ function in living mice under 1120 nm and 1319 nm excitation, showing HCs as a promising platform for non‐invasive bioimaging.

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

“…Cyanine dyes consist of a conjugated polymethine bridge between two nitrogen terminals and have been extensively applied in the photographic industry and bioimaging. [53][54][55][56][57][58][59][60] In yet another approach, donor-acceptor (D-A) templates are inspired by reported systems which have shown efficient intramolecular charge transfer and a low band gap with decent emission quantum yield. [61][62][63][64][65] Prototypes are 6-TPA-NO and 6-TPA-NT, consisting of electron-withdrawing moieties, naphtho [1,2-c:5,6-c 0 ]bis [1,2,5]oxadiazole (NO) or naphtho[1,2-c:5,6-c 0 ]bis [1,2,5]thiadiazole (NT), [66][67][68][69][70][71][72][73] respectively, and electron-donating groups, triphenylamines (TPAs).…”

Quantitative Energy Transfer in Organic Nanoparticles Based on Small-Molecule Ionic Isolation Lattices for UV Light Harvesting

“…First, the close packing of traditional organic dyes in crystals or aggregated states leads to strong electronic coupling between the chromophores, inducing spectral shifts and new electronic transitions, often with low radiative yields as a consequence. − Second, even when a specific structure has a high radiative yield, the photon-generated excitons easily migrate between the densely packed dyes to nonemissive defects, e.g., on the surface of the NPs, resulting in significant quenching of fluorescence. − Different strategies have been proposed to reduce aggregation caused quenching and improve the brightness of organic dye-loaded fluorescent NPs. The most common of these includes: covalent introduction of bulky side groups onto organic dyes and conjugated polymers, pairing cationic dyes with large hydrophobic counterions, supramolecular encapsulation, and dilution in matrices, e.g., polymers, silica, and ionic liquids. ,− Another approach is the use of special dye types that are less susceptible to quenching, this includes the so-called, aggregation-induced emission dyes. − …”

The pursuit of polymethine fluorophores with NIR-II emission and high brightness forin vivoapplications

“…3B ). 39 As a proof of concept, among these dyes, HC1222 and HC1342 displayed approximately 14-fold and 13-fold enhancements of fluorescent brightness in aqueous solution after counterion exchange with bulky tetrakis(pentafluorophenyl) borate (F5-TPB) reference to their parent forms. This work provides a successful paradigm to overcome the problem of fluorophore self-aggregation and fluorescence self-quenching and reserve the NIR-II multiplexed imaging capacity for high-contrast dual-color bioimaging.…”

“… 38 And the dye 5H5 is also a partially rigidified polymethine fluorophore with outstanding NIR-II optical performance. 35 In addition to partially restrained terminal methine moieties, five-membered or six-membered rings were introduced in the center of polymethine chain structure to prevent the conformational transition to improve the brightness, such as FD-1080, 26 HCs, 39 and Flav7, 24 etc.…”