Robust synthesis of NIR-emissive P-rhodamine fluorophores

Design, Synthesis, and Evaluation of Near-Infrared Fluorescent Molecules Based on 4H-1-Benzopyran Core

A series of novel fluorescent 4H-1-benzopyrans was designed and developed as near-infrared fluorescent molecules with a compact donor–acceptor-donor architecture. Spectral intensity of the fluorescent molecules M-1, M-2, M-3 varied significantly with the increasing polarities of solvents, where M-3 showed high viscosity sensitivity in glycerol-ethanol system with a 3-fold increase in emission intensity. Increasing concentrations of compound M-3 to 5% BSA in PBS elicited a 4-fold increase in fluorescence intensity, exhibiting a superior environmental sensitivity. Furthermore, the in vitro cellular uptake behavior and CLSM assay of cancer cell lines demonstrated that M-3 could easily enter the cell nucleus and bind to proteins with low toxicity. Therefore, the synthesized near-infrared fluorescent molecules could provide a new direction for the development of optical imaging probes and potential further drugs.

Late-stage functionalisation of alkyne-modified phospha-xanthene dyes: lysosomal imaging using an off-on-off type of pH probe

Near-infrared (NIR) fluorescent molecules are of great importance for the visualisation of biological processes. Among the most promising dye scaffolds for this purpose are P[double bond, length as m-dash]O-substituted phospha-xanthene (POX) dyes, which show NIR emission with high photostability. Their practical utility for in vitro and in vivo imaging has recently been demonstrated. Although classical modification methods have been used to produce POX-based fluorescent probes, it is still a challenge to introduce additional functional groups to control the localisation of the probe in cells. Herein, we report on the development of POXs that bear a 4-ethynylphenyl group on the phosphorus atom. These dyes can subsequently be functionalised with azide-tagged biomolecules via a late-stage Cu-catalysed azide/alkyne cycloaddition (CuAAC) reaction, thus achieving target-selective labelling. To demonstrate the practical utility of the functionalised POXs, we designed a sophisticated NIR probe that exhibits a bell-shaped off-on-off pH-response and is able to assess the degree of endosomal maturation.

Modular Synthetic Approach to Silicon-Rhodamine Homologues and Analogues via Bis-aryllanthanum Reagents

A modular synthetic approach toward diverse analogues of the far-red fluorophore silicon-rhodamine (SiR), based on a regioselective double nucleophilic addition of aryllanthanum reagents to esters, anhydrides, and lactones, is proposed. The reaction has improved functional group tolerance and represents a unified strategy toward cell-permeant, spontaneously blinking, and photoactivatable SiR fluorescent labels. In tandem with Pd-catalyzed hydroxy- or aminocarbonylation, it serves a streamlined synthetic pathway to a series of validated live-cell-compatible fluorescent dyes.

Far-red imaging of β-galactosidase through a phospha-fluorescein

The introduction of phosphine oxide into a fluorescein scaffold has yielded phospha-fluorescein with bathochromically shifted spectra, reliable photostability and solubility. Moreover, ratiometric and turn-on fluorescence in the decaging process has ensured that the phospha-fluorescein is a unique scaffold for fluorescence bioimaging. Probe DiMe-PF-Gal without further structural decoration was designed for accurately monitoring β-galactosidase in vivo.

Systematic study of synthesizing various heteroatom-substituted rhodamines from diaryl ether analogues

The dye rhodamine, as the most popular scaffold to construct fluorescent labels and probes, has been explored extensively on its structure-fluorescence relationships. Particularly, the replacement of the oxygen atom in the 10th position with heteroatoms obtained various new rhodamines with improved photophysical properties, such as brightness, photostability, red-shifted emission and fluorogenicity. However, the applications of heteroatom-substituted rhodamines have been hindered by difficult synthetic routes. Herein, we explored the condensation strategy of diaryl ether analogues and o-tolualdehyde to synthesize various heteroatom-substituted rhodamines. We found that the electron property and steric effect in the rhodamine 10th position determined the synthetic yield. It's concluded that this condensation method was more suitable for the synthesis of heteroatom-substituted rhodamines with small or electron-donating groups like rhodamine, S-rhodamine and Si-rhodamine. We hope these results will benefit the design and synthesis of heteroatom-substituted rhodamines.

A general method to optimize and functionalize red-shifted rhodamine dyes

Expanding the palette of fluorescent dyes is vital to push the frontier of biological imaging. Although rhodamine dyes remain the premier type of small-molecule fluorophore due to their bioavailability and brightness, variants excited with far-red or near-infrared light suffer from poor performance due to their propensity to adopt a lipophilic, nonfluorescent form. We report a framework for rationalizing rhodamine behavior in biological environments and a general chemical modification for rhodamines that optimizes long-wavelength variants and enables facile functionalization with different chemical groups.