Physical organic chemistry of supramolecular polymers

Pyromellitamide gelators: exponential rate of aggregation, hierarchical assembly, and their viscoelastic response to anions

The gelation and aggregation properties of a newly synthesized structurally simplified tetrahexyl pyromellitamide 2 have been studied and compared to the previously reported tetra(ethylhexanoate) pyromellitide 1, indicating that the ester groups in the latter significantly impede its aggregation. Morphology studies (AFM and TEM) on the aggregates formed by tetrahexyl pyromellitamide 2 in cyclohexane revealed highly uniform aggregates with different dimensions at different starting concentrations, suggesting that this molecule aggregates in a hierarchical fashion from a one-dimensional supramolecular polymer through hollow tubes or compressed helices to a network structure and then to a gel. This hypothesis is further supported by viscosity measurements that indicate a crossover point where individual supramolecular fibers get entangled at concentrations above ca. 3 mM in cyclohexane. Addition of 1 equiv of tetraalkylammonium salts of chloride or bromide, however, caused the viscosities of these pyromellitamide solutions to drop by a factor of 2-3 orders of magnitude, demonstrating the sensitivity of these aggregates to the presence of small anions. The sensitivity to anions does depend on the solubility of the salts used as small anion salts with little solubility in cyclohexane did not show this effect. Time-dependent viscosity studies showed that the aggregation of pyromellitamide 2 follows an exponential rate law, possibly related to the columnar rearrangements that are associated with the observed 6 angstroms contraction in d spacing in the XRD pattern of these gels. These results, particularly on the importance of kinetics of aggregation of self-assembled pyromellitamide gels, will be useful for future development of related materials for a number of applications, including tissue engineering and drug delivery.

Single-Molecule Force Spectroscopy of DNA-Based Reversible Polymer Bridges: Surface Robustness and Homogeneity

Single-molecule force spectroscopy, as implemented in an atomic force microscope, provides a rarely-used method by which to monitor dynamic processes that occur near surfaces. Here, a methodology is presented and characterized that facilitates the study of polymer bridging across nanometer-sized gaps. The model system employed is that of DNA-based reversible polymers, and an automated procedure is introduced that allows the AFM tip-surface contact point to be automatically determined, and the distance d between opposing surfaces to be actively controlled. Using this methodology, the importance of several experimental parameters was systematically studied, e.g. the frequency of repeated tip/surface contacts, the area of the substrate surface sampled by the AFM, and the use of multiple AFM tips and substrates. Experiments revealed the surfaces to be robust throughout pulling experiments, so that multiple touches and pulls could be carried out on a single spot with no measurable affect on the results. Differences in observed bridging probabilities were observed, both on different spots on the same surface and, more dramatically, from one day to another. Data normalization via a reference measurement allows data from multiple days to be directly compared.

Bridged coordination polymer multilayers with tunable properties

Coordination multilayers consisting of Pd(II) pincer-type complexes and poly(vinyl pyridine) were synthesized and characterized. Film properties were found to be dependent on and could be tuned by varying bath deposition concentrations, polymer molecular weight, and solution additives that compete with binding. Generally, smoother, thinner films were obtained with lower poly(vinyl pyridine) deposition bath concentrations. Likewise, film thickness and roughness could be reduced by employing a higher-molecular-weight poly(vinyl pyridine). Film properties could also be influenced by using acetonitrile as a solution additive, effectively driving the binding equilibrium slightly toward the free species.

Time and distance dependence of reversible polymer bridging followed by single-molecule force spectroscopy

Polymer bridging between surfaces plays an important role in a range of fundamental processes in the material and life sciences. Bridges formed by main-chain reversible polymers differ from their covalent analogs in that they can dynamically adjust their size and shape in response to external stimuli and have the potential to reform following bond scission. In this work, the time and distance dependence of main-chain reversible polymer bridge formation are studied using an atomic force microscope. The bridging process was studied using single-molecule force spectroscopy, and its dependence on the distance between surfaces and equilibration time was probed. The number of bridges formed decreases as the gap width increases, from approximately 2 bridges per 14 s equilibration at separations of 5-15 nm to approximately 0.5 bridges per 14 s equilibration at separations of 35-45 nm. The kinetics of bridge formation appear to be slightly faster at smaller separations.

Structure−Function Studies of Modular Aromatics That Form Molecular Organogels

Three urea-based aromatics 1-3, each with distinct steric and electronic characteristics, have been synthesized and their ability to undergo hierarchical assembly and gel organic solvents investigated. We have found that compound 1 promotes the sol-gel phase transition in primary alcohols (from CH3OH to C10H21OH; CGC < 15 mg/mL), while 2 and 3 do not. IR spectroscopy, X-ray powder diffraction (XRPD), and transmission electron microscopy (TEM) measurements show that 1 organizes into "cylinders" in primary alcohols, using three-centered hydrogen bonds and pi-pi aromatic interactions. The cylinders further organize into pairs through interdigitation of the methyl groups of the adjacent aromatic rings. Importantly, the lateral packing of the cylinders is enhanced as the bulk solvent polarity increases providing a mechanism for controlling the material's morphology via the solvophobic effect. Molecular mechanics (Amber) and semiempirical (AM1) calculations suggested similar conformational behavior but distinct electronic properties of 1-3. Thus, pi-deficient 2 without the methyl groups and pi-rich 3 without aromatic nitrogen fail to promote the sol-gel phase transition in alcohols due to their inability to undergo effective hierarchical assembly, which is necessary for the formation of a 3D fibrillar network. In addition, we have found that the ultrasonication of a supersaturated solution of 1 is necessary for the gelation. Presumably, a fast exchange of the aggregates of 1 is assisted with sonic waves to allow for the effective and "error free" assembly wherein an entangled net of fibers capable of encapsulating solvent molecules is formed.