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 applications. A well-known approach to shifting the emission of xanthene dyes in the far-red to NIR is the element substitution strategy and extending ᴨ-conjugation in O-xanthene with carbon at the periphery. In this review, we have outlined the element substitution strategy, ᴨ-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-rhodamine, (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.