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Nineteen cholesterol derivatives containing a variety of azobenzene moieties coupled to C-3 of a steroidal moiety through an ester linkage were synthesized. We employed two different esterification methods by which cholesterol derivatives with the natural (.^-configuration at C-3 and those with the inverted (/^-configuration at C-3 were obtained (the latter derivatives are indicated by a prime). Among them, cholesterol derivatives bearing a p-alkoxyazobenzene moiety (2R and 2R') acted as excellent thermally-reversible gelators of various organic fluids, but the gelation ability is fairly different between 2R and 2R': 2R could gelatinize hydrocarbons such as n-hexane, n-octane, and toluene, halogen solvents such as 1,2-dichloroethane and dichloromethane, ether solvents such as diethyl ether and THF, and alcohols such as ethanol and 1-butanol whereas 2R' could gelatinize ketones, methanol, and polysiloxanes. In general, the solubility of 2R' in apolar solvents is superior to that of 2R, so 2R is useful for gelation of apolar solvents whereas 2R' is useful for gelation of polar solvents. We found that the sol-gel phase transition is sensitively "read-out" by a change in the circular dichroism (CD) spectrum: the gel phase is CD-active whereas the sol phase is totally CD-silent. For example, the 2Me-l-butanol gel gave a positive exciton coupling band with (R)-chirality whereas the 2EFmethanol gel gave a negative exciton coupling band with (5)-chirality. These results mean that dipoles in the azobenzene moiety are oriented in a clockwise (in (R)-chirality) or anticlockwise (in (5")-chirality) direction when they interact in the excited state. Strangely, we accidentally found that the CD sign of the gels prepared from 2Pr, 2Bu, and 3Me? (azobenzene-linked cholesterol derivative with p-NMe2) is frequently inverted. After careful examination of the gel preparation conditions, we found that inversion takes place only when the cooling speed is fast. The scanning electron microscopic studies established that gelators form three-dimensional networks with helical fibrils. Interestingly, we found that in the 3Me' gel prepared from cyclohexane the gel with (R)-chirality in CD possesses a right-handed helix whereas the gel with (S)-chirality in the CD possesses a left-handed helix. The sol-gel phase transition was also induced by photoresponsive cis-trans isomerism of the azobenzene moiety: the gel formed from the franr-isomer was efficiently converted to the sol when trans-to-cis isomerization was photochemically induced, and this process can be repeated reversibly. The photoinduced sol-gel phase transition was also "read-out" as a change in CD spectroscopy.

The lock‐and‐key principle of natural systems is based on complex interactions like hydrogen bonding. Many synthetic systems that attempt to mimic natural systems have also used hydrogen bonding as the main binding force and have met with great success in non‐hydrogen‐bonding solvents that do not compete with the guest for the binding pocket. In contrast, natural systems function in water, a very competitive solvent. Synthetic hydrogen‐bonding systems may yet evolve to be successful in water. If this transition can not be made, synthetic answers can nevertheless take inspiration from nature without slavishly following the blue print. This is not an attempt to reinvent the “lock”: a new locking mechanism merely replaces the existing one. The inspiration might be the view of the hydrogen bond as an easily reversible “covalent” bond. Screening the literature we rediscovered boronic acids, which have been known for over 100 years. Conveniently, boronic acids rapidly and reversibly form cyclic esters with diols in basic aqueous media. Saccharides and other related “keys” contain a contiguous array of cyclic alcohols. In this work we hope to demonstrate that saccharide “keys” and boronic acid “locks” can open the door to a new and exciting field of research.

Mankind's fascination with shapes and patterns, many examples of which come from nature, has greatly influenced areas such as art and architecture. Science too has long since been interested in the origin of shapes and structures found in nature. Whereas organic chemistry in general, and supramolecular chemistry especially, has been very successful in creating large superstructures of often stunning morphology, inorganic chemistry has lagged behind. Over the last decade, however, researchers in various fields of chemistry have been studying novel methods through which the shape of inorganic materials can be controlled at the micro- or even nanoscopic level. A method that has proven very successful is the formation of inorganic structures under the influence of (bio)organic templates, which has resulted in the generation of a large variety of structured inorganic structures that are currently unattainable through any other method.

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