Trifluoroacetylated Trimethine, Pentamethine, and Heptamethine Cyanine Dyes: Syntheses, Structures, and Photochemical Properties

Herein, the rapid, concise, and straightforward synthesis of trifluoroacetylated trimethine, pentamethine, and heptamethine cyanine dyes is reported via the direct trifluoroacetylation of trimethine, pentamethine, and heptamethine cyanine dyes using commercially available trifluoroacetic anhydride. Their structures and optical properties by single crystal X-ray analysis are described. The unique solvent responsiveness of trifluoroacetylated trimethine cyanine dyes is discussed.

Polymethine cyanine dyes have a heterocyclic structure containing nitrogen at both ends of the polymethine chain. They have been extensively investigated in material sciences for numerous applications. The synthesis of cyanine dyes with various functional groups on the methine chain has attracted significant attention because it significantly alters the properties of the dye. However, few synthetic methods have been reported for directly introducing groups onto the methine chains of cyanine dyes. Ketone functional groups are essential in synthetic organic chemistry, biochemistry, and materials chemistry. However, there are no reports on the direct introduction of ketone groups into cyanine dyes. Trifluoroacetylation is performed on the methine chains of polymethine cyanine dyes. This is achieved by directly using anhydrous trifluoroacetic acid to synthesize trifluoroacetylated trimethine, pentamethine, and heptamethine cyanine dyes in good yields. Single-crystal X-ray structural analysis is performed to determine the substitution positions in the synthesized dyes. The results reveal that trifluoroacetylation of the methine chain shortens the maximum absorption wavelength and decreases the molar absorption coefficient in dichloromethane solution and poly(methyl methacrylate) film compared to an unsubstituted cyanine dye, and decreases the fluorescence quantum yield. This study highlights the unique properties of trifluoroacetylated polymethine cyanine dyes.

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