Asymmetric Ketocyanine Dyes with an Extended Polymethine Chain

A series of long-chain ketocyanines (KCYs), in which chromophore asymmetry is achieved through the noncentral positioning of the acceptor carbonyl group and variation in the electron-donating ability of the end groups, is synthesized via sequential condensation reactions and isolated with good preparative yields. The obtained dyes exhibit positive solvatochromism, with its range increasing as the donor strength of the terminal groups grows and upon transition to higher vinylogs. The new KCYs are efficient fluorophores, with fluorescence quantum yields up to 40% and large Stokes shifts, while their fluorescence bands extend into the near-infrared (NIR) spectral region. Furthermore, the asymmetric KCYs are successfully transformed into the cationic polymethine-styryl derivatives, which exhibit absorption and fluorescence spectra that are significantly shifted toward longer wavelengths.