Heteroatom‐Substituted Xanthene Fluorophores Enter the Shortwave‐Infrared Region^†

Abstract

This article is a highlight of the paper by Ivanic and Schnermann et al. in this issue of Photochemistry and Photobiology (Daly et al. Photochem. Photobiol. 2022). The collaborative team utilized computational approaches to investigate the influence of electron-withdrawing groups at the 10 0 position of tetramethylrhodamine (TMR). Leveraging this information, the team was able to extend the emission of the TMR scaffold into the shortwave-infrared region (SWIR, 1000-2500 nm) by incorporation of a ketone functional group at the 10 0 position (Daly et al. Photochem. Photobiol. 2022). This work provides the first example of a TMR derivative with peak SWIR emission (k abs : 862 nm, k em : 1058 nm). The authors utilize the ketone rhodamine scaffold to generate fluorogenic, pH-responsive reporters. This work demonstrates the potential of the classic xanthene scaffold for use as a SWIR reporter, an important step in the ultimate expansion of the repertoire of small-molecule organic fluorophore scaffolds available for deep-tissue imaging applications.

Synthesis of silicon-substituted hemicyanines for multimodal SWIR imaging

“…Heteroatom substitution is a versatile strategy employed to shift the absorbance and emission profile of various dye platforms to longer wavelengths, with fluorescein and related molecules being the most notable examples. Substitutions include oxygen to boron, 24 carbon, 25,26 silicon, [27][28][29] phosphorus, 30,31 sulfur, 32,33 as well as several other group 14 elements. 34 From this list, we identified silicon as the most optimal heteroatom for our dye remodeling campaign.…”

Evaluation of the Cytosolic Uptake of HaloTag Using a pH-Sensitive Dye

“…Simple substitutions within xanthene dyes enable the tuning of their spectroscopic properties. − For example, substitutions can bias the dye toward a closed lactone form, which has negligible fluorescence, or an open zwitterionic form, which is highly fluorescent. This equilibrium can also be affected by the microenvironment of the fluorophore …”

A New Boron–Rhodamine-Containing Carboxylic Acid as a Sugar Chemosensor

“…The substituted benzene ring enables various chemical modifications, and many kinds of chemosensors based on the rhodamine skeleton have been developed [ 16 , 17 , 18 , 19 ]. Furthermore, recent reports have demonstrated interesting features of fluorescent dyes in which the oxygen atom of rhodamine is replaced with heteroatoms [ 20 , 21 , 22 , 23 , 24 ], such as silicon [ 25 , 26 , 27 , 28 , 29 , 30 ], phosphorus [ 31 , 32 , 33 , 34 ], sulfur [ 35 , 36 ], and boron [ 14 , 15 ]. Only two cases of boron-substituted rhodamine have been reported and there is still much room for further investigation.…”