Photodimerization of cinnamic acids controlled by molecular assemblies of surfactant amine N-oxides

Abstract

UV irradiation of cinnamic acids 1 involving a complex with surfactant amine N-oxides (C_nDAO, n = 12, 14 and 16) as vesicles in water leads to the formation of cyclodimers (i.e. β-, δ-truxinic and/or α-truxillic acids†). Decreasing the molar ratios of 1 to C_nDAO causes the vesicles to transform into rod-like micelles and the yield of the cyclodimers decreases. The addition of HCl or NaOH to aqueous solutions of 1 and C_nDAO brings about sharp changes in the self-assembly structures from vesicles to micro emulsions or rod-like micelles, accompanied by a change in product distribution in the photolysis of the complex 1. Upon addition of photoinactive carboxylic acids, phenylpropionic and palmitic acids to the 1 and C₁₆DAO system, the rod-like micelles change into vesicles by formation of a complex with C₁₆DAO leading to the observation of a dilution effect in the photodimerization upon addition of phenylpropionic acid. However, no dilution effect is observed for the palmitic acid. This is found to be attributable to the difference in degrees of mixing of 1 with the acids in the vesicles. These results show that photodimerization of 1 incorporated in C_nDAO is controlled by a variety of molecular assemblies, i.e. rod-like micelles and homogeneous or heterogeneous vesicles.