Synthesis and self-assembly of new light-sensitive Gemini surfactants containing an azobenzene group

A homologue of dissymmetric quaternary ammonium Gemini surfactants, hexanediyl-α-(butylazobenzene dimethylammonium)-ω-(alkyldimethylammonium) dibromides (referred to as a4-6-m, m = 12, 14, 16), and a symmetric quaternary ammonium Gemini surfactant, hexanediyl-α,ω-bis(butylazobenzene dimethylammonium bromide) (referred to as a4-6-4a), have been synthesized. The Krafft temperature at 1 wt% concentration was determined to be 17–32 °C for dissymmetric a4-6-m homologues depending on the length of aliphatic tail and 42.5 °C for symmetric a4-6-4a. After UV light irradiation, the content of their cis-azobenzene isomers reaching photostationary state was estimated to be ca. 67–71% for the dilution. The critical micelle concentration (cmc) of a4-6-m is close to that of 12-6-m, indicating that the both are equivalent from the self-assembly point of view. But the cmc of a4-6-4a is higher than that of a4-6-12 or 12-6-12, showing the effect of the steric inhibition on aggregation owing to the two rigid terminal azobenzene groups per molecule. After UV light irradiation, the twisted cis-azobenzene groups in both a4-6-m and a4-6-4a molecules shows more obvious steric inhibition, resulting in a larger cmc than that of the trans-surfactants.