Low-melting dyes of tetra-alkyl azobenzene-3,3′,5,5′-tetracarboxylates

Abstract

A series of tetraalkyl azobenzene-m-tetracarboxylates was synthesized and characterized by polarized-light optical microscopy (POM), differential scanning calorimetry (DSC) and by UV-visible spectroscopy investigations. The dyes were readily obtained from esterification reactions of azobenzene-m-tetracarboxylic acid and alcohols of different carbon chains (methyl, ethyl, propyl, i-propyl, butyl, s-butyl, i-butyl, pentyl, i-pentyl, hexyl, 2-ethylhexyl and octyl). All compounds showed good thermal stability with temperatures of decomposition above 300 °C. The increase and linearity of alkyl chains were important to give low-melting products. Branched alkyl substituents gave products with slightly higher melting points compared to their linear alkyl isomers. The octyl and racemic 2-ethylhexyl substituents gave liquid azo dye products. Photoisomerization of the azo group in the final compounds was investigated in solution. Compounds showed a good resistance to photoisomerization and the branched alkyl substituents gave products with considerable faster back relaxation, from cis to trans configuration, when compared to the compounds with linear alkyl substituents.