Design and Synthesis of RhodIndolizine Dyes with Improved Stability and Shortwave Infrared Emission up to 1250 nm

Abstract

The design of shortwave infrared (SWIR) emissive small molecules with good stability in water remains an important challenge for fluorescence biological imaging applications. A series of four SWIR emissive rhodindolizine (RI) dyes were rationally designed and synthesized to probe the effects of nonconjugated substituents, conjugated donor groups, and nanoencapsulation in a water-soluble polymer on the stability and optical properties of the dyes. Steric protecting groups were added at the site of a significant LUMO presence to probe the effects on stability. Indolizine donor groups with added dimethylaniline groups were added to reduce the electrophilicity of the dyes toward nucleophiles such as water. All of the dyes were found to absorb (920–1096 nm peak values) and emit (1082–1256 nm peak values) within the SWIR region. Among xanthene-based emissive dyes, emission values >1200 nm are exceptional with 1256 nm peak emission being a longer emission than the recent record setting VIX-4 xanthene-based dye. Half-lives were improved from ∼5 to >24 h through the incorporation of either steric-based core protection groups or donors with increased donation strength. Importantly, the nanoencapsulation of the dyes in a water-soluble surfactant (Triton-X) allows for the use of these dyes in biological imaging applications.

Molecular Engineering of Stabilized Silicon-Rosindolizine Shortwave Infrared Fluorophores

“…Even though the 1Ph donors have comparable Φ F to the 2Ph derivatives, the MB values exhibit the importance of the 1Ph derivatives having ∼30% higher ε than the highest 2Ph derivative, which results in a greater MB. While the Φ F values reported herein are on par with what would be expected for fluorophores in this region, 6,16,28 the actual Φ F values are likely slightly higher than the reported values due to solvent reabsorption of emitted light (Figure S10, seen as the dips in the emission spectra) and the InGaAs detector efficiency drop off at 1550 nm since a previous work shows the emission of these fluorophore tails out to ∼1800 nm. 32 Accounting for these losses, the Φ F is likely ∼10−20% greater than the values reported herein based on best estimates.…”

“…Single-crystal X-ray diffraction (XRD) studies were pursued to better understand the orientation and geometry of the molecular structure. The 2Ph-Me derivative was selected for comparison to Ph RosIndz as well as other indolizine-donor-based fluorophore derivatives previously studied through XRD. ,, Several techniques with a range of solvent combinations were tried including vapor diffusion, slow evaporation, and hot/cold recrystallization. Of these techniques, only one successful batch of crystals was grown via the vapor diffusion of hexanes into a saturated solution of 2Ph-Me in fluorobenzene.…”

“…For this reason, SWIR emitting materials are being researched for their potential as fluorescence-based biological imaging agents. Within the past decade, there has been significant research toward small-molecule organic fluorophores emitting past or near 1100 nm with representatives from a variety of fluorophore classes including cyanines, ,− BODIPYs, , squaraines, benzothiadiazoles, − and xanthenes. ,− However, despite intense research into SWIR emitting fluorophores, there remain few small-molecule organic fluorophores reported that emit past 1300 nm. , Another issue encountered with SWIR emitting fluorophores is chemical stability. As the absorption and emission of a fluorophore shift to longer wavelengths, the optical energy gap must decrease either by raising the highest occupied molecular orbital (HOMO) or by lowering the lowest unoccupied molecular orbital (LUMO).…”

“…While the electrophilicity of these molecules is unavoidable, appropriate synthetic modifications can be made to block electrophilic sites from nucleophilic attack. Thus, dye designs allowing for kinetic persistence of xanthene fluorophores in nucleophile-rich aqueous environments are desirable. ,, Herein, the photophysical and chemical stability properties of SWIR emitting fluorophores containing a silicon-xanthene core and indolizine donors, termed silicon-rosindolizine (SiRos), are tuned via alteration of the silicon alkyl groups, substitution of the indolizine donors, and modulation of the steric environment imparted by an aryl group appendage at the core of the fluorophore (Figure ).…”

Rigid and Photostable Shortwave Infrared Dye Absorbing/Emitting beyond 1200 nm for High-Contrast Multiplexed Imaging

“…However, this has not been achieved for cyanine dyes with a longer backbone than heptamethine (Cy7). Use of alternative head groups, e.g., benz­[ c , d ]­indole, , flavylium, , and rhodindolizine, afforded SWIR cyanine dyes absorbing beyond 1100 nm with a Cy7 backbone. The thiopyrilynium , head group pushed the absorption beyond 1200 nm.…”