Synthesis and characterization of novel liquid-crystalline azo-dyes bearing two amino-nitro substituted azobenzene units and a well-defined, oligo(ethylene glycol) spacer

Four novel liquid-crystalline azo-dyes bearing two amino-nitro substituted azobenzene units linked by a well-defined oligo(ethylene glycol) spacer (DIRED-PEG series): (E)-N,N′-(2,2′-oxybis(ethane-2,1-diyl))bis(N-methyl-4-((E)-(4-nitrophenyl)diazenyl) benzenamine) (DIRED-PEG-2), (E)-N,N′-(2,2′-(ethane-1,2-diylbis(oxy)) bis(ethane-2,1-diyl)) bis(N-methyl-4-((E)-(4-nitrophenyl) diazenyl) benzenamine) (DIRED-PEG-3), (E)-N,N′-(2,2′-(2,2′-oxybis (ethane-2,1-diyl) bis(oxy)) bis(ethane-2,1-diyl)) bis(N-methyl-4-((E)-(4-nitrophenyl) diazenyl) benzenamine) (DIRED-PEG-4) and N1,N17-dimethyl-N1,N17-bis (4-((E)-(4-nitrophenyl) diazenyl) phenyl)-3,6,9,12,15-pentaoxaheptadecane-1,17-diamine (DIRED-PEG-6) have been synthesized. These dyes were fully characterized by FTIR, 1H and 13C NMR spectroscopies, and their thermal and optical properties were studied. Besides, the liquid-crystalline behaviour of these compounds was monitored in function of the temperature by light polarized microscopy. Finally, Langmuir films were prepared with these dyes.

► Novel dyes containing azobenzene and oligo(ethylene glycol) were synthesized.
► Absorption spectra of RED-PEG dyes in CHCl3 showed λmax = 440–480 nm.
► Absorption spectra of RED-PEG dyes in film revealed the formation of H-aggregates.
► Their liquid-crystalline behaviour was confirmed by light polarized microscopy.
► These liquid-crystalline dyes can form Langmuir films on the air–water interface.