Helical Ribbon Aggregate Composed of a Crown-Appended Cholesterol Derivative Which Acts as an Amphiphilic Gelator of Organic Solvents and as a Template for Chiral Silica Transcription

Abstract

New crown-appended cholesterol-based organogelator 1, which has 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, was synthesized, and the gelation ability was evaluated in organic solvents. It can gelate acetic acid, acetonitrile, acetone, ethanol, 1-butanol, 1-hexanol, DMSO, and DMF under 1.0 wt %, indicating that 1 acts as a versatile gelator of various organic solvents. To characterize the aggregation mode in the organogel system, we observed a CD spectrum of the acetic acid gel 1. In the CD spectrum, the lambda(theta)=0 value appears at 353 nm, which is the same as the absorption maximum lambda(max) = 353 nm. The positive sign for the first Cotton effect indicates that the dipole moments of azobenzene chromophores tend to orient in a clockwise direction. Very surprisingly, the TEM images of the 1 + acetic acid gel resulted in the helical ribbon and the tubular structures. Sol-gel polymerization of tetraethoxysilane was carried out using 1 in the gel phase. The silica obtained from the 1 + acetic acid gel showed the helical ribbon with 1700-1800-nm pitches and the tubular structure of the silica with approximately 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 R (right-handed) helicity, we consider that a microscopic helicity is reflected by a macroscopic helicity.