61st Annual Report on Research 2016

Self-assembly of three-dimensional (3D) metallo-supramolecular cages have received considerable attention in diverse fields, such as host-guest chemistry, molecular recognition, reactivity modulation, catalysis, gas storage, template synthesis and biology. In our study, we proposed a series of discrete 3D architectures using adamantane based terpyrdine organic ligands as corners and naked metal ions as edges, including trigonal bipyramidal-like dimer, tetrahedron, cube and dodecahedron. In our design, adamantane, which is the simplest form of diamondoids and exists naturally in petroleum deposits, possesses ~109.5^oangle at its points of bisection as excellent directing unit in the corners of 3D structures for proposed 3D self-assembly. If the first level assembly is assumed as the spontaneous formation of metal−ligand bonds to generate discrete cores, hierarchical self-assembly should be driven by multiple intermolecular interactions (e.g., π−π stacking, CH−π interactions, and hydrogen bonds) in the second level to deliver complex materials. Such hierarchically formed architectures may exhibit unique properties and functions that are not displayed by their individual components. For example, many of our 2D and 3D supramolecules could hierarchically self-assemble into higher order nanostructures, i.e., supramolecular metallo-gels by taking advantage of their precisely controlled shapes and sizes even without modification. Among the supramolecular cages we assembled, only cube was observed to form metallo-supramolecular gels with remarkable self-healing properties via hierarchical self-assembly in solution.

Note that the gelation behavior was not observed for the other two supramolecules with bipyramidal-like dimer and tetrahedron structures synthesized by us. We speculated that the formation of supramolecular gel should be driven by the intermolecular interactions, such as hydrophobic forces and π−π stacking. The cubic cage structure also enhanced the hierarchical self-assembly to form gel, because with such rigid and uniform geometry the supramolecules could be well arranged along three-dimensional directions. Such supramolecular cubes showed a reversible sol−gel phase transition at 50 °C above which it became a homogeneous solution. The gel to sol transition could be explained by the disassembly of supramolecules, as well as the dissociation of metal−ligand bonds at high temperature. More importantly, the assembly or disassembly, association or dissociation behaviors are dynamic processes due to relatively weak noncovalent intermolecular interactions and moderate bond energy (15−50 kcal mol−1) of metal−ligand bonds. The dynamic nature endows the supramolecular gel good self-healing property at room temperature. Figure 1c shows the schematic illustration of self-healing behavior of supramolecular cubes based on the combination of dynamic intermolecular interaction and coordination drives the reassembly of supramolecules at the crack, thus self-healing small cracks within tens of seconds and big cracks in minutes. We also developed for a hybrid material based on this supramolecular gel and polypyrrole. The hybrid gel combines the high conductivity of polypyrrole aerogel and self-healing property of supramolecular cube gel and exhibits enhanced mechanical strength and excellent elasticity due to its unique binary network structure. Supported by these unique features, thin films with good uniformity and flexibility of hybrid gel were successfully fabricated and a self-healable electronic circuit based on hybrid gel films was demonstrated. This conductive, room-temperature self-healing gel material takes unique advantage of supramolecular chemistry and polymer nanoscience and shows its great potential in various fields such as self-healing electronics, artificial skins, soft robotics, biomimetic prostheses, and energy storage. This study suggested precise control over the size and shape of supramolecule through self-assembly will open new avenue for new materials design and development.

Our self-healing hybrid gel published on Nano Letters (2015, 15, 6276–6281) were reported by many media and generated broader impact in our society in seeking high performance materials. Note that in this collaborative project with Prof. Guihua Yu at University of Texas – Austin, our group designed and synthesized all the supramolecular gel materials with remarkable self-healing ability.

KUT News

http://kut.org/post/have-you-seen-gel-self-healing-substance-could-help-smartphones-repair-themselves

Space Daily

http://www.spacedaily.com/reports/New_self_healing_gel_makes_electronics_more_flexible_999.html

Network World

http://www.networkworld.com/article/3011932/data-center/self-healing-gel-breakthrough-could-lead-to-flexible-electronics.html

Engineering360

http://insights.globalspec.com/article/1782/self-healing-gel-makes-electronics-more-flexible

Austin Statesman as Front-page news

ScienceDaily

https://www.sciencedaily.com/releases/2015/11/151125094742.htm

gizmag

http://www.gizmag.com/self-healing-gel-repairs-electronic-circuits/40601/

DesignNews.

http://www.designnews.com/author.asp?section_id=1386&doc_id=279263&dfpPParams=ind_183,industry_consumer,industry_medical,kw_33,aid_279263&dfpLayout=blog

Austin Statesman

http://www.mystatesman.com/news/news/ut-research-leads-to-revolutionary-self-healing-ge/np3JY/?icmp=statesman_internallink_referralbox_free-to-premium-referral

Fox News

http://www.fox7austin.com/news/local-news/78999189-story

Nanowerk

http://www.nanowerk.com/spotlight/spotid=41131.php

Phys.Org.

http://phys.org/news/2015-08-electrical-circuit-gel.html

In addition to supramolecular assembly using adamantane as directing unit, some of our ligands were also employed for other surpramolecular architectures, for instance, concentric hexagons. Directed by increasing density of coordination sites, we designed and synthesized a series of multitopic terpyridine ligands for the self-assembly of discrete 2D supramolecular architectures using direct self-assembly. In the project of concentric hexagons, a series of giant hexagons were assembled using tetratopic terpyridine (tpy) ligands. In preparation of tetratopic ligand, pyrylium and pyridinium salts chemistry significantly facilitated synthesis. These discrete metallo-supramolecular concentric hexagons were fully characterized by NMR, ESI-MS, ion mobility-mass spectrometry (IM-MS) and TEM, establishing their hexagon-in-hexagon architectures. Furthermore, these concentric hexagon supramolecules with precisely controlled shapes and sizes were utilized as building blocks to hierarchically self-assemble supramolecular metal-organic nanoribbons (SMON) at solid-liquid interfaces. Ambient STM imaging showed the formation of long 1D SMON rather than 2D assembly on the basal plane of highly oriented pyrolytic graphite (HOPG) surface after simple dropcasting the solution of pre-assembled concentric hexagons onto a freshly cleaved surface of HOPG. This wet chemical method based on self-assembly may offer simple, economical, and scalable routes to deliver complex materials. The manuscript has been published on the Journal of American Chemical Society (2016, 138, 9258–9268). More recently, we successfully synthesized hexatopic terpyridine ligands for generation 3 concentric hexagon, which has been assembled using Cd(II) and exhibited enhanced stability. We are preparing manuscript for publication.