Three series of rigidified tri-, penta- and heptamethine merocyanine dyes were synthesised. A piperidyl moiety was chosen as the electron-donating substituent while the electron-accepting group was varied from ketones to malononitriles and cyanoacetates. The structures of the compounds in the solid state and in solution were elucidated by X-ray diffraction and NMR spectroscopy, respectively, while optical properties were investigated by absorption and emission spectroscopy. As a general trend, the acceptor properties decrease in the series malononitrile>cyanoacetate>ketone based on the analysis of their solvatochromic behaviour. The experimental results were further supported by calculations at the B3LYP 6-311+G(d) level of theory.
Keywords: UV/Vis spectroscopy; X-ray diffraction; density functional calculations; donor-acceptor systems; dyes/pigments; fluorescence spectroscopy.
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