Far-red to NIR emitting xanthene-based fluorophores

Xanthene dyes are well known for their remarkable photophysical property, photostability, and ability to switch fluorescence on-off depending upon the external environment. Such intriguing traits find a wide number of applications, albeit their emission (<= 600 nm) and stokes shifts (-34 nm) limit their use in biological applica-tions. A well-known approach to shifting the emission of xanthene dyes in the far-red to NIR is the element substitution strategy and extending -conjugation in O-xanthene with carbon at the periphery. In this review, we have outlined the element substitution strategy, -extended O-xanthene dyes with carbon at the periphery, and triangulenium ion. A section on general synthetic strategies, a detailed discussion of the substituent effect on the photophysical properties, and applications of far-red to NIR emitting xanthene dyes are included. A comparative study on the photophysical properties of (a) Si-rhodamine - amino Si-rhodamine, (b) Si-rhodamine - P-rhoda-mine, (c) P-rhodamine, P-fluorescein, and P-rhodol, (d) Te-rhodamine -Te=O rhodamine, and (e) crystal violet, rhodamine 3b, 2,6,10-tris(dialkylamino)trioxatriangulenium ions is discussed. This review highlights the structure-spectroscopic relationships, which will be useful in designing new far-red to NIR xanthene dyes.

Automated summary

Xanthene dyes are widely used due to their unique photophysical properties, photostability, and ability to switch fluorescence based on the external environment. However, their emission range and stokes shifts impose limitations on their use in biological applications. This review focuses on the element substitution strategy and extending -conjugation in O-xanthene with carbon at the periphery to shift the emission of xanthene dyes towards the far-red to NIR range. The review also discusses the impact of substituents on photophysical properties and presents applications of far-red to NIR emitting xanthene dyes.