Abstract

In the last two decades, various kinds of the low-molecular-weight organogelators (LMOGs) have been investigated in terms of technological applications in various fields as well as their fundamental scientific properties. The process of gelation is generally considered to arise from immobilization of the solvents in the three-dimensional networks formed by the assembly of gelator molecules through weak intermolecular noncovalent interactions. From these points of view a huge number of organogelators have been developed so far. In the course of our research on LMOGs we have noticed a mixture of two gelators could show a different trend in gelation compared to the single gelator. It is well known that the catecholic moiety easily forms cyclic boronate esters with the boronic acid. Thus, we have investigated the two-component system based on cyclic boronate esters formed by the catechols and a boronic acid in terms of the control of gelation capability. Basic gelation properties of the constituent catecholic gelators have also been clarified. The catecholic gelators with the amide unit form no gel by addition of the boronic acid. In contrast, the catecholic gelators with the glutamic acid moiety improve their gelation abilities by mixing with the boronic acid. Furthermore, the gelation ability of the catecholic gelators having the urea unit is maintained after addition of the boronic acid. It has been found that gelation abilities of the catecholic gelators are highly affected by addition of the boronic acid. In terms of practical applications some gels can be obtained by on-site mixture of two kinds of solutions.

Highlights

  • From this point of view we have developed organogelators based on the cyclophane skeletons [23,24], the europium complex [25], coumarin structure [26], and ferrocene unit [27]

  • The compounds 5a–d were synthesized synthesized by condensation of the corresponding glutamic acid derivatives and 3,4by condensation of the corresponding glutamic acid derivatives and 3,4-dimethoxybenzoic acid using dimethoxybenzoic acid using diethyl cyanophosphonate (DECP)

  • By treating 5a–d with BBr3 the catecholic derivatives having the catecholic derivatives having the glutamic acid moiety 6a–d were obtained in the yields of 50–76%

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Summary

Introduction

It seems difficult to predict the whole structural requirements for molecules to show gelation properties in organic solvents, the information obtained from a wide variety of organogelators helps to rationalize what kinds of chemical structures are necessary for building organogels. These required structures are closely related to non-covalent bonding such as hydrogen bonds, electrostatic attraction, hydrophobic interaction, and π–π interactions. In the last few decades a large number of organogelators have been discussed in terms of structural features, effects of noncovalent interactions, properties of gels, and aspects of their functions [16,17,18,19,20,21,22]. From this point of view we have developed organogelators based on the cyclophane skeletons [23,24], the europium complex [25], coumarin structure [26], and ferrocene unit [27]

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