Formation Mechanism of Achiral Amphiphile-Templated Helical Mesoporous Silicas

Abstract

A series of achiral cationic and anionic amphiphiles employed as templates for mesoporous silica, all showed an excellent ability to form highly ordered helical mesoporous silica (HMS) crystals. It was supposed that the helical mesostructure may originate from the helical propeller-like micelle formed by the achiral amphiphilic molecules due to their instantaneous asymmetric shape survived in the micelle. The existence of the mirror-imaged conformations of equal proportion may result in the racemic nature of the achiral amphiphile-templated HMSs. The helicity of the HMS greatly depends on the templating molecules which may be modulated by the twisting power of the helical propeller-like packing of the amphiphiles in the micelle. The dependence of the pitch length on different factors was explained by mechanical analysis indicating that the pitch length should be in direct proportion to the rod diameter but inversely proportional to the moment of micelle (M0). Additionally, chiral amphiphiles with opposite charge were used as chiral dopants to control the enantiopurity of the achiral amphiphile-templated HMSs. The absolute enantiomeric excess (ee) of the HMSs increased with increasing amounts of dopant.