Yuji Suzaki

Physical gels based on supramolecular gelators, including host–guest complexes and pseudorotaxanes

This article reviews recent studies on the gels formed by host–guest complexes, rotaxane and pseudorotaxanes. Pseudorotaxanes of crown ethers and rotaxanes of cyclic viologens with organic axle molecules form organogels through bonding of the functional end groups of the axle component. Cyclodextrins and cucurbiturils form gels in organic media and in aqueous mineral acids, respectively. The host–guest complexes and the pseudorotaxane of these macrocyclic compounds form hydrogels. These physical gels can be changed to sols by heating or addition of competing guests. Precise control of the gelation was enabled by using these supramolecules as the hydrogelator. Amphiphilic N-alkylpyridiniums and α-cyclodextrin form the hydrogel composed of their pseudorotaxanes. The mechanism of the gelation is described.

Rotaxanes of a macrocyclic ferrocenophane with dialkylammonium axle components

Octaoxa[22]ferrocenophane, 1, was synthesized and employed as the macrocyclic component of [2]rotaxanes. [2]Pseudorotaxanes composed of macrocyclic molecule 1 and dialkylammonium derivatives with a terminal vinyl group undergo end-capping via cross-metathesis of the terminal group with bulky acrylates. The [2]rotaxanes of 1 with axle components having bulky terminal groups, such as 3,5-dimethylphenyl, 9-anthryl, and ferrocenyl groups, maintain an interlocked structure in CDCl(3) solution, but they are gradually converted into a mixture of the individual components via dethreading of the end groups in polar solvents such as CD(3)CN and dmso-d(6). The reaction rate varies depending on the end group and solvent. The cationic rotaxane with an anthryl end group of the axle component, [(){AnCH(2)NH(2)CH(2)C(6)H(4)-4-OCH(2)CH(2)CH[double bond, length as m-dash]CHCOOC(6)H(4)-4-C(C(6)H(4)-4-tBu)(3)}](BAr(F)) (An = 9-anthryl, BAr(F) = B{C(6)H(3)-3,5-(CF(3))(2)}(4)) shows weak emission upon excitation of the anthryl group (12b, lambda(em) = 419 nm, quantum yield, phi = 0.012). The quantum yield is lower than that of the neutral rotaxane 13b(phi = 0.030) formed by N-acetylation of 12b and a physical mixture of the corresponding free axle molecule, AnCH(2)N(Ac)CH(2)C(6)H(4)-4-OCH(2)CH(2)CH=CHCOOC(6)H(4)-4-C(C(6)H(4)-4-tBu)(3) (8), and 1 (phi = 0.34). The efficiency of the quenching caused by the ferrocenylene group caused by energy transfer is affected significantly by the relative positions of the anthryl and ferrocenylene groups in the rotaxane. The rotaxane with axles having a secondary ammonium moiety has a redox potential E(1/2) = -0.03-0.02 V (vs. Ag(+)/Ag), which is lower those of than compound 1 (E(1/2) = -0.10 V) and the neutral [2]rotaxanes with the N-acetylated axle components (E(1/2) = -0.11 and -0.22 V).

Synthesis and dethreading reaction of a rotaxane-like complex of an octaoxa[22]ferrocenophane with dialkylammonium

An octaoxa[22]ferrocenophane 1 composed of 1,1′-ferrocenylene and C6H4-1,2-{(OCH2CH2)3O}2 groups was employed as macrocyclic component of a rotaxane-like complex with dialkylammonium. Ru-catalysed cross-metathesis reaction of [{C6H3-3,5-(OMe)2}CH2NH2CH2C6H4-4-OCH2CH2CH = CH2]BARF (BARF = B{C6H3-3,5-(CF3)2}4) with CH2 = CHCOO(C6H4-4-C(C6H4-4-tBu)3) in the presence of 1 yielded rotaxane-like complex [(1){(C6H3-3,5-(OMe)2)CH2NH2CH2C6H4-4-OCH2CH2CH = CHCOO(C6H4-4-C(C6H4-4-tBu)3)}]BARF. It kept the interlocked structure in CDCl3, benzene-d 6 and toluene-d 8. Dissolution of the rotaxane-like complex in polar solvents, such as CD3CN and DMSO-d 6, caused the dethreading reaction to form the component molecules. A rotaxane having dibenzo[24]crown-8 as the macrocyclic component did not undergo the dethreading reaction in the solution.