Elsevier

Dyes and Pigments

Volume 99, Issue 3, December 2013, Pages 1107-1116
Dyes and Pigments

Review
Synthesis and applications of unsymmetrical carbocyanine dyes

https://doi.org/10.1016/j.dyepig.2013.08.001Get rights and content

Highlights

  • Carbocyanine dyes are an immense class of compounds with extensive applications.

  • Synthetic methods of unsymmetrical carbocyanine dyes.

  • Microwave and solid-phase conditions can be used for unsymmetrical dye synthesis.

  • Absorption and fluorescence wavelengths can be modified using unsymmetrical dyes.

  • Unsymmetrical dyes can alter polarity and solubility causing less aggregation.

Abstract

Carbocyanine dyes are a unique class of organic molecules with high molar extinction coefficients and characteristically long absorption bands with absorption maxima ranging from 500 to 1000 nm making them extremely useful in numerous applications ranging from photography to medicine. Using various synthetic procedures, the conjugated system of these compounds can be modified to yield specific absorption and fluorescence spectra. One way to alter these conjugated systems is to include two different heterocycles thereby producing unsymmetrical dyes. The attractiveness of unsymmetrical carbocyanines has increased significantly over the last decade. For monomethine carbocyanines, there are many methods to synthesize unsymmetrical dyes, but for tri-, penta- and heptamethines the synthesis is more complicated. The synthesis of unsymmetrical carbocyanine dyes is excellent for modifying a dyes absorption wavelength, preparing it for conjugation, or altering the dyes polarity or solubility. The few known unsymmetrical carbocyanine dyes have found use in DNA labeling, dye-sensitized solar cells, and many other applications. There is a great lack of unsymmetrical carbocyanines in the literature and no collective review of them therefore we have put together this review detailing their synthesis and applications.

Section snippets

Introduction to carbocyanine dyes

In 1856, C.H.G. Williams synthesized the first carbocyanine dye by heating quinoline with N-amyl lepidinium iodide in ammonia. The compound produced displayed a “magnificent blue color,” thus the Latin word cyanos, meaning blue, provided the general carbocyanine dye name [1]. This vast class of dyes shows absorption that covers a wider range of the electronic spectrum, from the ultraviolet to the infrared, than any other class of dyes; however, due to their extreme sensitivity to light, the

Synthesis of monomethine carbocyanine dyes

Monomethine carbocyanine dyes typically absorb in the visible region. Most literature on unsymmetrical dyes consists of monomethine dyes as they are the easiest to prepare. The majority of monomethine dyes are unsymmetrical as the synthesis is based on the condensation of one methyl-substituted quaternary ammonium salt with a second salt containing a different substitution and no methine linker is used. This allows for only one product to form. Unsymmetrical monomethine carbocyanines are the

Synthesis of tri-, penta-, and heptamethine carbocyanine dyes

The synthesis of tri-, penta-, and heptamethine unsymmetrical carbocyanine dyes is performed through two main synthetic routes that are very similar. The aldehyde method involves the addition of a short chain with an aldehyde to a quaternary ammonium salt for reaction with a second salt to form unsymmetrical dye. The hemicyanine method involves the synthesis of hemicyanine or “half dye” and its purification from any dye that forms. Hemicyanine dyes are sometimes hydrolyzed into aldehydes for

Microwave assisted solid-phase synthesis

As microwave synthesis becomes more popular due to the ability to avoid time-consuming reaction steps and increase purity [41], [42], [43], [44]. Lopalco et al. designed a solid-phase synthesis approach to the hemicyanine method for tri-, penta-, and heptamethine carbocyanines using microwave-assisted synthesis. As shown in Scheme 12, this solid-phase method first reduces reaction time by synthesizing quaternary ammonium salts at 150 °C in acetonitrile. Next, the alkylated indolenines are

Conclusion

Carbocyanine dyes have found a wide range of applications due to their diverse functionality. They are generally non-toxic [46], stable and exhibit exceptional biocompatibility making their cellular use tremendously appealing. One of the main reasons for carbocyanine dyes extensive applications is that they are highly modifiable. The synthesis of unsymmetrical carbocyanines allows for further modification of these compounds whether it will be for solubility, conjugation, or in vivo

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