Construction of Two- or Three-Component Low Molecular Weight Gel Systems Masamichi Yamanaka* and Ryohei Aoyama Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529 Received May 7, 2010; E-mail: smyaman@ipc.shizuoka.ac.jp Pyridyl group-bound tris-urea 2, whose structure issimilar to the tris-urea low molecular weight gelator (LMWG) 1, was synthesized. Addition of a trace amount of 2 and Pd(OAc) 2 to 1 resulted in a reduction in the critical gelation concentration (CGC) in acetone. Metal-ligand interaction between Pd(OAc) 2 and the pyridyl group of 2 assist the gelation. Assistance by intermolecular hydrogen bonding was also effective for gelation of acetone. An acetone gel of 1 was formed at half the value of the CGC of 1 alone by adding a trace amount of 2 and isophthalic acid. Scanning electron microscopy (SEM) measurements of these aggregates showed structural alterations caused by the additive. A mixture of 1 and a trace amount of 2 in EtOAc gave a gel. Addition of 2 was essential to form the gel, because 1 alone did not gel EtOAc. SEM measurements indicated that 2 produced extension of the fibrous aggregates. Directional regulated self-assemblyof preorganized small molecules enables construction of supramolecular polymers. 1 One of the attractive manifestations of these supramolecular polymers is the supramolecular gel. 2 Principally, a fibrous aggregate composed of a one-dimensional assemblyof small molecules, called a low molecular weight gelator (LMWG), is the unit constituent. Several types of single-component LMWGs have been reported, their gelations being achieved through a variety ofinteractions, such as hydrogen bonding, 3 ³-³, 4 dipole-dipole, 5 etc. Some of them showed a gel-sol phase transition responsive to photo-, 6 chemo-, 7 or other 8 stimuli in addition to the thermo-responsiveness that is potentially provided in supramolecular gels. Supramolecular gelformation as a resultof multicomponent assemblyof small molecules has also been achieved. 9 Formation of a fibrous aggregate via complementary hydrogen bonding pairs, 10 metal- ligand coordination bond, 11 and other interactions 12 has been reported. As a pioneering work, Hanabusa et al., reported a two-component gelling system using an equimolar mixture of 5-alkyl-2,4,6-triaminopyrimidine and 5,5-dialkylbarbituric acid. 10a Beck and Rowan developed a metallo-supramolecular gelling system that was based on a metal-ligand interaction between a bis-ligand monomer and a combination of metalions (transition-metalion and lanthanoid metalion). 11b Multicom- ponent supramolecular gels usuallymix each component in the ratioof appropriate whole numbers. In contrast, a supra- molecular gel composed of a main ingredient and a small quantity of accessory ingredient is hardly known. A supra- molecular gelformed from a small quantity of additive has the potential to be a smart material, because its rheology enables it to be converted by a trace amount of chemical. As a unique conception, Lee and co-worker reported that addition of a small quantity of rod-coil-rod molecule to a cylindrical object that was composed of a coil-rod-coil molecule induced gelation. 13 Partialintroduction of ligand or hydrogen bonding acceptor to a fibrous aggregate of LMWG wouldalter the gelation ability by adding a multidentate metalion or hydrogen bonding donor, depending on the type ofinteraction between the fibrous aggregates (Figure 1). In this paper, we report multicomponent supramolecular gelformation involving increase in gelation ability by addition of trace amounts of molecule(s). Previously, we found that C 3 -symmetric tris-urea molecule 1 and deriva- tives act as LMWGs for a variety of organic solvents. 14 For the purpose of making part of a multicomponent supramolecular gel, we designed tris-urea 2 with 4-pyridyl groups as the ligand or hydrogen-bonding unit (Figure 2). The gelation ability of tris-urea 1 was dramatically improved by the addition of a trace amount of 2 with or without metal salt or dicarboxylic acid. Results and Discussion Synthesisof Pyridyl-Substituted Tris-Urea 2. Tris-urea 2 was synthesized by a condensation reaction of triamine 3 and 4-pyridylisocyanate (Scheme 1). Triamine 3 was prepared from 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene by etheri- fication with 3-nitrophenol and reduction of the nitro groups in the presence of tin(II) chloride. 14 The obtained triamine 3 was reacted with in situ prepared 4-pyridyl isocyanate; 64% yieldof the desired tris-urea 2 being obtained after purification. The structure of tris-urea 2 was confirmed by 1 H and 13 C NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. Three-Component LMWG Systems Using Metal-Ligand Interaction. Tris-urea 1 gelled acetone upon ultrasound irradiation, the critical gelation concentration (CGC) being 1.5 wt % (Table 1, Entry 1 and Figure 3a). The gelation of acetone was not complete at a lower concentration of 1 than the CGC. For example, a 0.75 wt % mixture of 1 in acetone gave a partial gel (Table 1, Entry 2 and Figure 3b). Scanning electron microscopy (SEM) images of xerogels prepared from the acetone gel of 1 and the partial gel of acetone and 1 showed structures resembling nanosized fibers (Figures 3a and 3b). These results indicate that conduction ofintertwining of the © 2010 The Chemical Society of Japan Published on the web August 26, 2010; doi:10.1246/bcsj.20100136 Bull. Chem. Soc. Jpn. Vol. 83, No. 9, 1127–1131 (2010) 1127