Self-Assembly and Gelation Behavior of Tris(phenylisoxazolyl)benzenes

• Published in: Journal of organic chemistry 2011-06, Vol.76 (12), p.5082-5091
• Main Authors: Tanaka, Masahiro, Ikeda, Toshiaki, Mack, John, Kobayashi, Nagao, Haino, Takeharu
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; General and physical chemistry; Heterocyclic compounds; Heterocyclic compounds with n hetero atom and also o and/or s, se, te hetero atoms; Materials science; Methods of nanofabrication; Noncondensed benzenic compounds; Organic chemistry; Physics; Preparations and properties; Supramolecular and biochemical assembly
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo200766u

Low-molecular-mass organic gelators (LMOG), tris(phenylisoxazolyl)benzenes, were synthesized, and their self-assembling behavior was examined using 1H NMR and UV–vis absorption spectroscopies. They turned into a gel in both nonpolar and highly polar solvents such as methylcyclohexane, ether, acetone, dimethylsulfoxide, etc. Field emission scanning electron microscopy (FESEM) observation of the xerogels of 1 and 3 possessing the saturated alkyl chains revealed that well-developed straight fibers were formed, whereas the unsaturated termini of the alkyl chains of 2 promoted the formation of both the right- and left-handed helical fibers. The self-association behavior of 1, 2, and 5 in solution were investigated using 1H NMR and UV–vis spectroscopies. The flat aromatic compound 1 stacked in a columnar fashion along its C 3 axis via π–π stacking interactions. The assemblies were regulated by the peripheral alkyl substituents; the saturated alkyl groups facilitated the assemblies while terminal double bonds impeded the intermolecular association, and the branched substituents obviously interfered in the formation of the stacks, probably due to steric requirements. Theoretical calculations suggest that the three dipoles of the isoxazole groups adopt the circular array. The conformational search of the hexameric stacks of 4 using MacroModel V9.1 gave rise to two major conformers: one is nonhelical and the other is helical. Further detailed structural analysis of the assemblies of chiral 5 using circular dichroism (CD) measurements indicated that their assemblies adopt helical structures in solution. CD spectra and DFT calculations revealed that R-5 forms a left-handed supramolecular helicate. The coassembly of R- and S-5 displayed chiral amplification, since the chiral information from 5 was transferred to the supramolecular chirality of the helical assemblies of 1. A small amount of optically active 5 provided enough chiral stimulus to produce a remarkable chiral response and supramolecular helical structures of 1.