Introduction of the Functional Amino Group at the <i>meso</i> 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 polymethine chain via amino group. The Cy3 derivative was prepared. All Cy5 derivatives basic group at this decomposed breaking chain. However, amino-squaraine based dyes were found be stable accessible. Photolysis squaraine-based accompanied Wolff rearrangement led pyrrolidones (in methanol aprotic solvents) formed intramolecular cyclization intermediate ketenes. In aqueous acetonitrile, ketenes reacted water gave carboxylic acids. Both products non-fluorescent, though model (without squaraine fragment) acylated N-methylamino groups fluorescent.