Highly Defined, Colloid-Like Ionic Clusters in Solution

Many societal challenges at the beginning of the 21st century lead to an apparent and growing need for functional materials and novel ways of materials synthesis and assembly. Rising to the challenge, the utilization of small, self-assembling building blocks for the bottom-up construction of new types of polymers and nanostructures has enjoyed increasing popularity among materials researchers in the recent past. Supramolecular materials like foldamers, surface films, nanoparticles, etc. are created by exploiting noncovalent forces leading to an ordered arrangement of nanoscale building blocks. In the search for new polymers based on noncovalent molecular forces, we are motivated by the idea of supramolecular or even polymer-like structures by self-assembly of small ionic monomers, merely formed from electrostatic and solvent interactions. We, in particular, focus on applying intrinsically nondirected electrostatic interactions to create nonetheless wellorganized supramolecular structures. To achieve this aim, we here make use of a recently highlighted, multicationic molecular box (1 ; Figure 1a), developed by Sessler and co-workers, which has already been applied in self-assembly applications of metal–organic rotaxane frameworks. The molecular box 1 seems to facilitate structure generation through selfassembly, indicated by, for example, the surprising ability to build supramolecular necklaces through incorporation of electron-rich aromatic guest molecules. However, not focused on short-ranged interactions but on new applications of longranged electrostatic forces at the nanoscale, we combine 1 with small di-anionic salts, (KSO3)2CH2 (2 2 ) and (KSO3)2NO (3 ), to generate a defined assembly of cations and anions in solution (Figure 1a). However, self-assembly due to electrostatic attraction is complex to describe qualitatively as well as quantitatively, because not only Coulomb forces but also entropy changes due to counterion release, solvation of ions, depletion forces, etc. , contribute to the total free energy of a system. A central question in any application of solutionbased ionic self-assembly therefore is : does any kind of ordered arrangement of the ionic building blocks take place? Intuitively one would assume the distribution of ions in solution to be of random, rather homogeneous nature. In the following, we show dynamic light scattering (DLS) data, which clearly illustrate that low-concentrated mixtures of anionic 2 and cationic 1 self-assemble in solution to highly defined, monodisperse ionic clusters. In this special case, the distribution of ions in solution is therefore not at all random but well-ordered due to long-range electrostatic correlations. Performing molecular dynamics (MD) and Monte Carlo (MC) simulations, we further elucidate the internal constitution of the ionic clusters and show that a long-ranged order may be induced by electrostatic correlations of the cationic macrocycles. The simulations indicate that the monodisperse objects are constituted of relatively loosely bound ions that, however, occupy certain fixed coordinates in the clusters for at least orders of nanoseconds. The reliability of the computational data is additionally supported by double electron–electron resonance (DEER) spectroscopy data. Taking long-ranged (DLS, MC) and molecular (MD, DEER) insights together, we demonstrate, to the best of our knowledge for the first time, that size-controlled self-assembly through electrostatic interaction between small ions in solution is possible. The so formed objects can be regarded as loosely bound ion-based colloids, thus we propose the name ionoids. These ionoids are peculiar as they result from ion–ion correlations of moderately charged constituents at low concentrations, while long-range electrostatic correlations have so far only been discussed in the context of macroions. DLS experiments were performed on a system containing a mixture of 2 /1 at an approximate ratio of 3 mm :1 mm in DMSO/88% aqueous glycerol (1:1). Note that the exact concentration of 2 within the light-scattering sample is difficult to determine, as the solubility of the pure component 2 , in contrast to 3 , is only 3 mm in our solvent mixture. The measurements revealed monodisperse small objects in the solution, after an initial period of several days where no correlation could be detected. The fit of one correlation function measured at a scattering angle of 90 8 is depicted in Figure 1b. Note the monoexponential nature of this fit, which indicates monodispersity of the aggregates in the solution. The intercept of the correlation function is unusually low, due to the small signal-to-noise ratio of <3, that is, the scattered intensity from the pure solvent mixture is nearly 40% of the overall scattered intensity. As, in the absence of 2 , we found no correlation at all but only the DLS signature of the pure solvent mixture, it is at hand to deduce that these small objects stem from selfassembly of 2 –1 clusters. The hydrodynamic radius of the self-assembled 2 –1 cluster is quite small, that is, only 6.2 nm 0.2 nm can be observed 10 days after sample preparation, thus, growing very slow (+2 nm after 140 days; see Figure 1c) and fostering an analogy with charge-stabilized col[a] D. Kurzbach, Dr. D. R. Kattnig, Dr. D. Hinderberger Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz (Germany) E-mail : [email protected] [b] N. Pfaffenberger, Dr. W. Sch rtl Institute for Physical Chemistry, Johannes Gutenberg-University Mainz Jakob-Welder-Weg 11, 55099 Mainz (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/open.201200025. 2012 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.