Introduction of the Functional Amino Group at the meso Position of Cy3 and Cy5 Dyes: Synthesis, Stability, Spectra and Photolysis of 4-Amino-1-diazo-2-butanone Derivatives

Searching for photoconvertible or photoactivatable dyes, we considered trimethine (Cy3) and pentamethine (Cy5) fluorophores with a 4-amino-1-diazo-2-butanone fragment (H2N(CH2)2COCH=N2) attached to the meso (γ) position of the polymethine chain via the amino group. The Cy3 derivative was prepared. All Cy5 derivatives with the basic amino group at this position decomposed with breaking the polymethine chain. However, amino-squaraine based Cy5 dyes were found to be stable and accessible. Photolysis of Cy3 and squaraine-based Cy5 dyes with a 4-amino-1-diazo-2-butanone fragment was accompanied with Wolff rearrangement and led to pyrrolidones (in methanol and aprotic solvents) formed via intramolecular cyclization of the intermediate ketenes. In aqueous acetonitrile, ketenes reacted with water and gave carboxylic acids. Both products were non-fluorescent, though the model Cy5 dyes (without the squaraine fragment) with acylated amino and N-methylamino groups were found to be fluorescent.