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

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′ 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′ position (Daly et al. Photochem. Photobiol. 2022). This work provides the first example of a TMR derivative with peak SWIR emission (λ_abs: 862 nm, λ_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.