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.