Scheme 1. Framework of polyenes (above) with even number of p-centres and polymethines (below) with odd number of p-centres. A = acceptor, D = donor.Scheme 3. Synthesised merocyanine derivatives. For better comparison of experimental and theoretical data a common atom numbering system was used, which is different from IUPAC numbering. Hex = hexyl.
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.
All chemicals were obtained from the supplier and used without further purification. Dry solvents were prepared by common laboratory methods. The eluents for chromatography petroleum ether (low-boiling) and ethyl acetate were distilled prior to use.Thinlayer chromatography silica gel 60 F254 glass plates (layer thickness of 0.20 mm on aluminium, pore size 60 Å) from the company Merck were used. Column chromatography
Summary4-Cyano-1,1'-biphenyl derivatives bearing ω-hydroxyalkyl substituents were reacted with methyl 3-chloro-3-oxopropionate or cyanoacetic acid, giving liquid-crystalline linear malonates and cyanoacetates. These compounds formed monotropic nematic phases at 62 °C down to ambient temperature upon cooling from the isotropic liquid. The mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction (WAXS).
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