Solid-Phase Synthesis of Asymmetric Cyanine Dyes

Cyanine dyes (CD) are a functional class of organic molecules used in several applications ranging from photography to bioimaging. CDs most well-known features resides on high molar extinction coefficients up to 105 L mol-1cm-1 and on the absorption spectra, ranging from 500 to 1000 nm, which can be fine-tuned both by extending the length of the central methylene bridge or by modulating the terminal heterocycles. In the last decades, new synthetic methodologies, namely microwave-assisted and the solid-phase procedure, have been developed to overcome the limitation of the classical synthetic protocols. While microwave approach usually reduces the exposure time of the reagents and products to thermal degradation, the solid-phase methodology allows easier synthetic protocols which is translated in higher yields and simpler products purification. In the present review a comprehensive analysis of the solid-phase methods for the synthesis of asymmetrical CDs is discussed, with a critical evaluation of the difference among the currently available solid-state approaches.