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 wellknown 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.

Rational Design and Biological Application of Hybrid Fluorophores

“…In 1996, Czerney et al pioneered the design and construction of the first hybrid fluorophore (merocyanine A), with a maximum absorption wavelength of approximately 700 nm. [23] The hybrid fluorophore was created by integrating xanthene and hemicyanine into an olefinic bond (Figure 2). [33] [24][25][26][27][28][29] and enable fluorescence off-on switching mode.…”

“…The hybridization of xanthene and cyanine greatly improved the absorption and emission wavelengths of the fluorophores (Figure 4), but their performance remained insufficient, such as the Stokes shift (Table 1). To solve this problem, in 2013, Lin et al reported a series of xanthene-coumarin hybrid fluoro- [23,34] Figure 3. Molecular structures of xanthene-cyanine conjugated hybrid fluorophores.…”

“…Hybridization is a practical strategy for molecular engineering of fluorophores. [8,[22][23][24] In the context of this review, hybridization refers to the combination of two or more fluorophores at different positions with distinct structural characteristics to create a new type of fluorophore, known as hybrid fluorophores (Figure 1a). The primary objective of hybridization is to fuse fluorophores at the molecular level, utilizing their individual strengths and mitigating their respective limitations while conferring new desirable properties unique to the hybrid.…”

“…In 2018, Huang et al summarized the hybridization, modification and biological application of rhodamine fluorophores, [8] while in 2023, Sekar et al provided a specific summary of the hybridization fluorophores of xanthene with cyanine and coumarin. [23] However, a comprehensive review regarding the biological applications of hybridized fluorophores remains lacking. In this review, we provide a comprehensive overview of the strategies employed to hybridize four classical fluorophores (xanthene, cyanine, coumarin, and BODIPY) and their biological applications.…”

“…To overcome these challenges, significant contributions have been made to the molecular engineering of fluorophores to improve their properties. Hybridization is a practical strategy for molecular engineering of fluorophores [8,22–24] …”