Deep Red to NIR Emitting Xanthene Hybrids: Xanthene-Hemicyanine Hybrids and Xanthene-Coumarin Hybrids

Abstract

Xanthene dyes exhibit remarkable photophysical properties and photostability. Their ability to switch fluorescence on/off depending on the external environment makes them a promising candidate for imaging/sensing/tracking/labeling probes. However, their emission ≤600 nm and Stokes shift ∼34 nm restrict their use in biological applications. A well-known approach to shifting the emission of xanthene dyes in the deep red to Near infrared (NIR) is by forming xanthene hybrids. This review gives a comprehensive list of the deep red to NIR xanthene-hemicyanine and xanthene-coumarin hybrids. We have outlined general synthetic strategies and a detailed discussion of substituent effect, π-conjugation, and ring effect on the photophysical properties of these dyes. A section on applications of deep red to NIR xanthene hybrids based on fluorophore-linker strategy and spirocyclization is included. This review highlights the structure-spectroscopic relationships, which will be useful in designing new deep red to NIR xanthene analogs.