Controlled morphology and photoreduction characteristics of polyoxometalate(POM)/lipid complexes and the effect of hydrogen bonding at molecular interfaces

Abstract

Supramolecular complexes consisting of anionic polyoxometalate (POM) and chiral, cationic lipids are newly developed. They give nanofibers, helical ribbons, and nanotapes in organic media depending on the chemical structure of lipid molecules. Lipid ammonium groups exert significant influence on their photoreduction characteristics.

Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self‐Assembly Pathways

“…[2,[14][15][16] They spontaneously form vesicles in aqueous-organic solvent mixtures, [14,16] or micelles [15] in aqueous media. However, the site-selective modification approach requires considerable synthetic effort and it inevitably loses the intrinsic electronic properties of POMs.A dditionally,s ymmetric introduction of hydrophobic units [14,16] is not as uitable molecular design for aqueous selfassembly.Meanwhile,the third approach utilizes electrostatic interactions between anionic POMs and cationic amphiphiles, [17][18][19][20][21][22][23][24][25][26] similar to that developed for 1D metal complexes. [27,28] These lipophilic amphiphile-POM complexes are dispersible in nonpolar organic solvents,g iving aggregates with globular,corn-shaped, fibrous,orhelical superstructures, depending on the constituents.…”

“…[27,28] These lipophilic amphiphile-POM complexes are dispersible in nonpolar organic solvents,g iving aggregates with globular,corn-shaped, fibrous,orhelical superstructures, depending on the constituents. [17][18][19][20][21][22][23][24][25][26] All these approaches, however, provide unpredictable 3D curved aggregate morphologies that are difficult to control and not suitable for technological applications.O ne of the most valuable nanoarchitectures is an extended 2D nanosheet, which can be accumulated into multilayered films.A lthough this type of nanostructure is greatly anticipated, no general method for self-assembly of spherical POMs into extended nanosheets exists to date.T oachieve this goal, the energy landscapes and pathways in self-assembly need to be controlled at the molecular level.…”

“…We expected that the three ether-c ontaining chains of (1-H) + would flexibly tune their conformation to accommodate 2D alignment of the rigid POMs.W er ecently reported that cationic azobenzene amphiphiles covalently quarternized with 1,s howing regular packing because of the flexible alignment of the ether-containing alkyl chains. [29] Thes hort molecular length of the C 1 OC 2 OC 2 -u nit in 1 contrasts remarkably with conventional surfactants with long alkyl chains, [17][18][19][20][21][22][23][24][25][26] which may contribute to the minimization of electric resistance caused by the long alkyl chains.A s ac ontrol, triheptyl ammonium hydrochloride (2-H) + was employed as acounterion ( Figure 1).…”

“…On the other hand, when aqueous (1-H) + (15 mm)a nd H 3 [PM 12 O 40 ]( 5mm ;M= W, Mo) were mixed in a3:1 molar ratio,precipitates with irregular 3D morphology were immediately formed (Supporting Information, Figure S8). The stoichiometry of this irregular complex was determined as (1-H) + :[PM 12 O 40 ] 3À = 3:1, which is ac omposition commonly observed for the surfactant-POM complexes, [21][22][23][24] which contain no protonated POMs.T he PXRD pattern observed for (1-H) 3 [PW 12 O 40 ]i sd istinct from that observed for (1-H) 2 H[PW 12 O 40 ]n anosheets (Supporting Information, Figure S7 b). Apparently,t he molecular alignment of POMs in 2D nanosheets is different from that in the 3D microparticles.…”

Self-assembly of Oligo(ethylene oxide)-linked Diammonium Ions with Polyoxometalates into Ordered Polyhedron Nanocrystals in Aqueous Media

“…14 We also reported the introduction of chiral ammonium ions and consequent chiral nanoassembly of POMs. 15,16 The electrostatic complexation of POMs with monocationic lipids provides lipophilic complexes, in which the long alkyl chains of lipid molecules inevitably serve as insulators, which are unfavorable from the perspective of maximizing electronic interactions among the accumulated POMs.…”