Squaraine dye

Squaraine Dyes are a class of organic dyes with very intense fluorescence spectra, typically in the red and near infrared region (Absorption maxima are found between 630 and 670nm and their emission maxima are between 650-700nm). They are characterized by their unique aromatic four membered ring system derived from squaric acid. Most squaraines are encumbered by nucleophilic attack of the central four membered ring, which is highly electron deficient. This encumbrance can be attenuated by the formation of a rotaxane around the dye to protect it from nucleophiles. They are currently used as sensors for ions and have recently, with the advent of protected squanaine derivatives, been exploited in biomedical imaging.

They are synthetically derived from squaric acid which undergoes an electrophilic aromatic substitution reaction with an aniline or another electron rich derivative to form a highly conjugated product with extensive charge distribution. For instance, squaraine dyes are also formed via reaction of squaric acid or its derivatives with so-called "methylene bases" like 2-methyl-indolenines, 2-metyl-benzthiazoles or 2-methyl-benzo-selenazoles. Indolenine-based squaraines combine good photostability including high quantum yields when bound to proteins and reactive versions of these dyes are commonly used as fluorescent probes and labels for biomedical applications.Fact|date=August 2008

References

# E. Terpetschnig and J.R. Lakowicz, "Synthesis and characterization of asymmetrical squarains - a novel class of cyanine dyes", Dyes and Pigments 21, 227 - 234 (1993).
# E. Terpetschnig, H. Szmacinski and J.R. Lakowicz, "Synthesis, spectral properties and photostabilities of symmetrical and unsymmetrical squarains, Anal. Chim. Acta 282, 633 -641 (1993).
#Squaraine Derived Rotaxanes: Sterically Protected Fluorescent Near-IR Dyes E. Arunkumar, C. C. Forbes, B. C. Noll, B. D. Smith. J. Am. Chem. Soc. 2005, 127, 3288-3289
# L. Patsenker, A. Tatarets, O. Kolosova, O. Obukhova, Ye. Povrozin, I. Fedyunyayeva, I. Yermolenko, and E. Terpetschnig, "Fluorescent Probes and Labels for Biomedical Applications", Ann. N.Y. Acad. Sci. 1130: 179–187 (2008).

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