Synthesis and optical properties of new isomeric core-fluorinated azo-containing bis(2-hydroxybenzaldehyde)s

The V-type azo chromophores 3,3'-{(2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl-4,4′-diyl)bis [oxy-4,1-phenylenediazene-2,1-diyl]}bis(6-hydroxybenzaldehyde) (ABA-I) and 3,3'-{(2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl-4,4′-diyl)bis[oxy-3,1-phenylenediazene-2,1-diyl]}bis(6-hydroxybenzaldehyde) (ABA-II) have been obtained by diazotization of octafluorobiphenylene-based diamines with the further azo-coupling of the prepared diazonium salts with salicylaldehyde. The particular feature of them is a simultaneous combination of an electron-withdrawing (A) 4,4′-octafluorobiphenylene central unit and two kinds of reactive groups (hydroxyl and aldehyde) in its electron-donating (D) component. The synthesized azo chromophores are well dissolved in a wide range of organic solvents. The structure of the obtained compounds has been characterized using 1H, 13C, 19F NMR, FTIR and UV–vis spectroscopy. By means of the UV spectroscopy the solvatochromic and photoisomeric properties in the solvents of different polarity and pH have been studied. It has been found out that the regulation of the absorption maxima of ABA-I and ABA-II within the range from 347 to 470 nm is realized due to the tautomeric transitions and ionized forms as a result of the change in a solvent concentration and pH of a medium. Having investigated the photoisomeric properties of ABA-I and ABA-II and those of their monofunctional analogue (M-HA) with one 2-hydroxy-5-(phenyldiazenyl)benzaldehyde fragment, it has been established that the solvent type and pH of a medium are a key factor that affects photoisomeric process. The photoisomerization rate constants of ABA-II and M-HA make up about 0.128 and 0.119 sec−1 respectively. The synthesized azo chromophores are promising as low-molecular dopants for constructing the polymer composites, which may be applied in stimuli-responsive systems and as the monomers for polymer synthesis through high-temperature polycondensation since the obtained azobenzenes are stable up to 270 °С.