Creation of Double Silica Nanotubes by Using Crown‐Appended Cholesterol Nanotubes

Abstract

New crown-appended cholesterol-based organogelators 1–3, which have one or two cholesterol skeletons as a chiral aggregate-forming site, two amino groups as an acidic proton binding site, and one crown moiety as a cation binding site, were synthesized, and the gelation ability was evaluated in organic solvents. These gelators could gelatinize several organic solvents under 1.0 wt %, indicating that 1–3 act as a versatile gelator of various organic solvents. We observed CD spectra of the acetic acid or propionic acid gels of 1–3 to characterize the aggregation mode in the organogel system. In the CD spectrum of the acetic acid gel 1, the positive sign for the first Cotton effect indicates that the dipole moments of azobenzene chromophores tend to orient into the clockwise direction. On the other hand, propionic acid gels 2 and 3, bearing only one cholesterol, moiety exhibit a negative sign for the first Cotton effect, strongly suggesting that the dipole moments of the azobenzene chromophores orient into the anticlockwise direction. The TEM images of the 1+acetic acid gel resulted in the helical ribbon and tubular structures. Sol-gel polycondensation of tetraethoxysilane (TEOS) was carried out with 1–3 as templates in the gel phase. The silica obtained from the 1+acetic acid gel showed the helical ribbon with 200–1700 nm width and the tubular structure of the silica with constant about 560 nm outer diameter. As far as can be recognized, all the helicity possesses a right-handed helical motif. Since the exciton coupling band of the organogel also shows P (right-handed) helicity, we consider that a microscopic helicity is reflected by a macroscopic helicity. On the other hand, the tubular structure of the silica obtained from the organogels 2 and 3 is somewhat different from that prepared from the organogel 1. The careful examination of SEM and TEM pictures revealed that the tube wall of the silica features a roll-paper-like multilayer structure. Thus, this paper demonstrates successful and rare examples for precise transcription of gel superstructures into inorganic silica materials.