Small Molecule NIR-II Dyes for Switchable Photoluminescence via Host -Guest Complexation and Supramolecular Assembly with Carbon Dots

Abstract

Small molecular NIR-II dyes are highly desirable for various biomedical applications. However, NIR-II probes are still limited due to the complex synthetic processes and inadequate availability of fluorescent core. Herein, the design and synthesis of three small molecular NIR-II dyes are reported. These dyes can be excited at 850-915 nm and emitted at 1280-1290 nm with a large stokes shift (≈375 nm). Experimental and computational results indicate a 2:1 preferable host-guest assembly between the cucurbit[8]uril (CB) and dye molecules. Interestingly, the dyes when self-assembled in presence of CB leads to the formation of nanocubes (≈200 nm) and exhibits marked enhancement in fluorescence emission intensity (Switch-On). However, the addition of red carbon dots (rCDots, ≈10 nm) quenches the fluorescence of these host-guest complexes (Switch-Off) providing flexibility in the user-defined tuning of photoluminescence. The turn-ON complex found to have comparable quantum yield to the commercially available near-infrared fluorophore, IR-26. The aqueous dispersibility, cellular and blood compatibility, and NIR-II bioimaging capability of the inclusion complexes is also explored. Thus, a switchable fluorescence behavior, driven by host-guest complexation and supramolecular self-assembly, is demonstrated here for three new NIR-II dyes.

Keywords: NIR-II emission; carbon dots; host-guest complexation; supramolecular assembly; switchable fluorescence.

Scheme 1

Reaction of different 4‐bromo‐N, N‐diarylaniline compounds with naphtho[1,2‐c:5,6‐c′bis[1,2,5]thiadiazole‐5,10‐diboronic acid bis(pinacol) ester. Reaction Conditions: Naphtho[1,2‐c:5,6‐c′bis[1,2,5]thiadiazole‐5,10‐diboronic acid bis(pinacol) ester, 1 (3.3 mmol), 4‐bromo‐N,N‐diarylaniline (8.2 mmol), Pd(PPh₃)₄ (0.17 mmol), K₂CO₃ (16.5 mmol), toluene (80 mL), H₂O (10 mL), N₂, 115 °C, 48 h.

Figure 1

Emission spectra of a) D₁AD₁ upon excitation at 915 nm wavelength, D₂AD₂ and D₃AD₃ upon excitation at 850 nm wavelength. The color of the DMSO solution of the NIR‐II dyes has been showed in inset upon irradiation of UV light (365 nm). Emission spectra and (inset)emission intensity changes of b) D₁AD₁ at 915 nm, c) D₂AD₂ and d) D₃AD₃ at 850 nm upon gradual addition of CB[8] in DMSO solution.

Figure 2

¹H NMR spectra of a) CB[8] (green spectra) and D₂AD₂ (red spectra). The NMR experiments were carried out in d6‐DMSO solvent for CB[8] and CDCl₃ solvent for D₂AD₂. b) NMR titration studies between D₂AD₂ and CB[8] demonstrate the formation of host–guest complex, D₂AD₂⊂CB[8]. The chemical shifts for 6 separate sets of protons (i–vi) are also assigned.

Figure 3

a) UV–vis and fluorescence spectra of red carbon dots. Emission spectra of b) D₁AD₁⊂CB, c) D₂AD₂⊂CB, and d) D₃AD₃⊂CB host–guest complexes upon gradual addition of aqueous solution of rCDots in DMSO solvent. D₁AD₁⊂CB was irradiated at 915 nm, whereas D₂AD₂⊂CB and D₃AD₃⊂CB complexes were irradiated at 850 nm.

Figure 4

Transmission electron microscopic images of a) D₁AD₁⊂CB, c) D₂AD₂⊂CB, and e) D₃AD₃⊂CB host–guest complexes along with their most favorable docked geometries. Corresponding images of the supramolecular assemblies when red carbon dots are embedded in b) D₁AD₁⊂CB, d) D₂AD₂⊂CB, and f) D₃AD₃⊂CB complexes are also shown with their most favored docked geometries.

Figure 5

Schematic illustration of the switchable photoluminescence behavior via host–guest complexation between NIR‐II dyes and CB[8] followed by self‐aggregation with carbon dots. The scale bar of the inset TEM images indicates 100 nm for A and C and 5 nm for B.

Figure 6

a) Average hydrodynamic diameter of D₃AD₃ NIR‐II dye before and after the addition of CB[8] and rCD demonstrating the dispersibility of the particles in water. b) Cellular toxicity of D₃AD₃ conjugates before and after the addition of CB[8] and rCD in HEK293 cells. Blood biocompatibility of d) 400 × 10⁻⁶ m of D₃AD₃, e) 400 × 10⁻⁶ m of D₃AD₃ and 800 × 10⁻⁶ m of CB[8], and f) 400 × 10⁻⁶ m of D₃AD₃, 800 × 10⁻⁶ m of CB[8] and 2 mg mL⁻¹ solution of rCD in 1X PBS compared to c) nontreated blood.

Figure 7

Ex vivo NIR‐II imaging for tissue and surgical navigation: a) Site of the agent administration; b) heart; c) colon and d) under the skin tissue recorded within 60 min after the contrast injection. Scale bar: 25 µm.