Nanometer-level sol-gel transcription of cholesterol assemblies into monodisperse inner helical hollows of the silica

Dimeric azobenzene-appended cholesterol organogel 1 was synthesized and its gelation ability was evaluated in organic solvents. It can gelate 1-hexanol, 1-octanol, toluene, m-xylene, and p-xylene, under 5.0 wt %, indicating that 1 acts as a versatile gelator of various organic solvents. To obtain visual insights into the aggregation mode, we observed the xerogel structure of 1-octanol and m-xylene gels 1 by TEM and SEM. They revealed a typical helical fiber structure with 50-100 nm of outer diameters, suggesting that the organogel 1 grows into the bundled fiber structure in the absence of any additive. Also, we observed the CD spectrum of 1-octanol gel 1 to characterize the aggregation mode in the gel phase. In the CD spectrum, the λ_θ=0 value appeared at 365 nm, which is consistent with the absorption maximum at λ_max = 365 nm. It is known that azobenzene-appended cholesterol gelators with natural (S) C-3 configuration tend to give a negative sign for the first Cotton effect, indicating that the dipole moments of azobenzene chromophores tend to orient in the anticlockwise direction. Sol-gel polymerization of tetraethoxysilane (TEOS) was carried out using the organogel 1 as a template. We observed the SEM pictures of the silica obtained from 1-octanol gel 1. The silica nanotube showed the fibrous structure with ca. 30-nm outer diameter and a few micrometers length. Very surprisingly, the TEM picture revealed the inner helical structure of a silica nanotube with ca. 7.5-nm inner diameter and long helical pitch, which is comparable with the width of one-dimensional molecular stacking 1 on the basis of the interdigitated van der Waals interaction between cholesterol moieties. These results indicate that the organogel 1 was successfully transcribed into the silica nanotube by the intermolecular hydrogen-bonding interaction between the amino group of the gelator and anionic silica particles.