A complete, continuous transition from discrete macroions to blackberry structures, and then back to discrete macroions, is reported for the first time in the system of {Mo132}/water/acetone, with {Mo132} (full formula (NH4)42[Mo132O372(CH3COO)30(H2O)72].ca.300H2O.ca.10CH3COONH4) as the C60-like anionic polyoxomolybdate molecular clusters. Laser light scattering studies reveal the presence of the self-assembled {Mo132} blackberry structures in water/acetone mixed solvents containing 3 vol % to 70 vol % acetone, with the average hydrodynamic radius (Rh) of blackberries ranging from 45 to 100 nm with increasing acetone content. Only discrete {Mo132} clusters are found in solutions containing <3 vol % and >70 vol % acetone. The complete discrete macroion (cluster)-blackberry-discrete macroion transition helps to identify the driving forces behind the blackberry formation, a new type of self-assembly process. The charge density on the macroions is found to greatly affect the blackberry formation and dissociation, as the counterion association is very dominant around blackberries. The transitions between single {Mo132} clusters and blackberries, and between the blackberries with different sizes, are achieved by only changing the solvent quality.

We show that the equilibrium size of single-layer shells composed of polyoxometalate macroions is inversely proportional to the dielectric constant of the medium in which they are dispersed. This behavior is consistent with a stabilization mechanism based on Coulomb repulsion combined with charge regulation. We estimate the cohesive energy per bond between macroions on the shells to be approximately ÿ6kT. This number is extracted from analysis based on a charge regulation model in combination with a model for defects on a sphere. The value of the cohesive bond energy is in agreement with the model-independent critical aggregate concentration. This observation points to a new class of thermodynamically stable shell-like objects. We point out the possible relevance our findings have for certain surfactant systems.

Dedicated to Professor Peter Day on the occasion of his 70th birthdayIn the field of polyoxometalate chemistry, porous spherical molybdenum oxide-based clusters of the type {(Mo VI )Mo VI 5 } 12 (linker) 30 , [1] called Keplerates, [2a,b] are notable not only from an aesthetic point of view [2c] but also because they show properties of interest for different areas of science. Some of these clusters can act as artificial cells exhibiting gated pores while interacting specifically with their environments; others are of interest for several aspects of materials science. [3][4][5] In detail, of interest are a) solution properties in connection with a new type of assembly process leading to vesicles, including magnetic ones, [6a,b] b) the option to employ the characteristic interactions with amphiphiles for the generation of monolayers and Langmuir-Blodgett films [6c] as well as highly ordered honeycomb nanostructures at airwater interfaces, [6d] c) the integration into sol-gel-derived silica to obtain unprecedented hybrids, [6e] and d) the discovery of novel magnetic properties, [6f]

Growing blackberries: The association of counterions around {Mo72V30} polyoxometalate macroanions increases with increasing macroion concentration and decreasing solvent polarity. The counterion distribution extends about 2–9 Å from the macroion surface with the highest probability at 2–3 Å from the macroion surface. A close connection between counterion association and the self‐assembly of {Mo72V30} into “blackberry” structures is observed.

Dedicated to Professor Peter Day on the occasion of his 70th birthdayIn the field of polyoxometalate chemistry, porous spherical molybdenum oxide-based clusters of the type {(Mo VI )Mo VI 5 } 12 (linker) 30 , [1] called Keplerates, [2a,b] are notable not only from an aesthetic point of view [2c] but also because they show properties of interest for different areas of science. Some of these clusters can act as artificial cells exhibiting gated pores while interacting specifically with their environments; others are of interest for several aspects of materials science. [3][4][5] In detail, of interest are a) solution properties in connection with a new type of assembly process leading to vesicles, including magnetic ones, [6a,b] b) the option to employ the characteristic interactions with amphiphiles for the generation of monolayers and Langmuir-Blodgett films [6c] as well as highly ordered honeycomb nanostructures at airwater interfaces, [6d] c) the integration into sol-gel-derived silica to obtain unprecedented hybrids, [6e] and d) the discovery of novel magnetic properties, [6f]

We report the self-assembly processes in solution of three Keplerate-type molybdenum-oxide based clusters {Mo72V30}, {Mo72Cr30} and {Mo72Fe30} (all with diameters of approximately 2.5 nm). These clusters behave as unique weak polyprotic acids owing to the external water ligands attached to the non-Mo metal centers. Whereas the Cr and Fe clusters have 30 water ligands attached at the 30 M3+ centers pointing outside, {Mo72V30} has 20 water ligands coordinated to vanadium atoms, of which only 10 are pointing outside. The self-assembly processes of the Keplerates leading to supramolecular blackberry-type structures are influenced by the effective charge densities on the cluster surfaces, which can be tuned by the pH values and solvent properties. As expected, {Mo72Cr30} and {Mo72Fe30} behave similarly in aqueous solution due to their analogous structures and in both cases the self-assembly follows the partial deprotonation of the external water ligands attached to the non-Mo metal centers. However, the M-OH2 functionalities differ not only in acidity but also lability, i.e. in different residence times of the H2O ligands. In contrast to {Mo72Cr30} and {Mo72Fe30}, the {Mo72V30} clusters carry a rather large number of negative charges so that their solution properties are different. They exist as discrete macroions in dilute aqueous solution, and form only in mixed water/organic solvent (like acetone) blackberry-type structures whose size increases with acetone content. The comparison of the properties of the clusters allows more general information about the interesting self-assembly phenomenon to be unveiled.

Die Anlagerung von Gegenionen an makroionische {Mo72V30}‐Polyoxometallate nimmt mit steigender Makroionen‐Konzentration und sinkender Solvenspolarität zu. Die Gegenionen verteilen sich im Umkreis von 2–9 Å um das Makroion, mit der höchsten Dichte bei 2–3 Å Entfernung von der Makroionenoberfläche (siehe Bild). Ein enger Zusammenhang zwischen der Anlagerung der Gegenionen und der Selbstorganistion der {Mo72V30}‐Moleküle in brombeerartige Strukturen wird beobachtet.

We report an approach of exploring the interaction between cationic surfactants and a type of structurally well-defined, spherical "Keplerate" polyoxometalate (POM) macroanionic molecular clusters, {Mo72V30}, in aqueous solution. The effectiveness of the interaction can be determined by monitoring the size change of the "blackberry" supramolecular structures formed by the self-assembly of {Mo72V30} macroions, which is determined by the effective charge density on the macroions. Long-chain surfactants (CTAB and CTAT) can interact with {Mo72V30} macroions stoichiometrically and lower their charge density. Consequently, the blackberry size decreases continuously with increasing surfactant concentration in solution. On the other hand, for short-chain surfactants (e.g., OTAB), a larger fraction of surfactants exist as discrete chains in solution and do not strongly interact with the macroions. This approach shows that a controllable amount of suitable surfactants can accurately tune the charge on large molecular clusters.