Li-Jun Chen

Light-triggered reversible supramolecular transformations of multi-bisthienylethene hexagons.

It is very challenging to realize well-controlled structural transformations in artificial supramolecules. Herein we report the construction of a novel family of multi-bisthienylethene hexagons with precise control of the shape and size as well as the specific number of photochromic units via coordination-driven self-assembly. These newly developed multi-bisthienylethene hexagons are highly sensitive and responsive to photostimuli, especially allowing for quantitative reversible supramolecular transformations triggered by light irradiation.

Construction of Smart Supramolecular Polymeric Hydrogels Cross-linked by Discrete Organoplatinum(II) Metallacycles via Post-Assembly Polymerization

Postassembly modification strategy has been successfully employed in the construction of discrete metallosupramolecular assemblies. However, the most known reports have been limited to the simple structural conversion through the easy covalent reactions, thus hindering the development of organometallic functional materials. In this study, we first combined coordination-driven self-assembly and postassembly reversible addition-fragmentation chain-transfer (RAFT) polymerization to produce a new family of star supramolecular polymers containing well-defined metallacycles as cores, which featured typical lower critical solution temperature (LCST) behavior in water because of the existence of poly(N-isopropylacrylamide) (PNIPAAM) moieties. Moreover, the obtained star polymers could further form supramolecular hydrogels cross-linked by discrete hexagonal metallacycles at room temperature without heating-cooling process. Interestingly, the resultant polymeric hydrogels exhibited stimuli-responsive behavior toward temperature and bromide anion as well as self-healing property. We demonstrated that the dynamic nature of Pt-N bonds in the hexagonal metallacycles played an important role in determining the stimuli-responsive and self-healing property of the final soft matters. Thus, merging coordination-driven self-assembly and postassembly polymerization provided a new avenue to the preparation of functional materials containing well-defined, discrete metal-organic assemblies as main scaffolds.

Vapochromic Behavior of a Chair-Shaped Supramolecular Metallacycle with Ultra-Stability

A new discrete supramolecular metallacycle functionalized with an alkynylplatinum(II) bzimpy moiety was successfully prepared via coordination-driven self-assembly, and it displayed a reversible color change in the solid state between yellow and red, triggered by CH2Cl2 vapor or mechanical grinding. Notably, unlike many known vapochromic systems, the obtained vapochromic metallacycle exhibits ultra-stability, with the red color remaining unchanged in air for several months at room temperature or even under vacuum for >1 week. Further investigation revealed that the chair conformation of the metallacyclic scaffold, which was thought to prevent intermolecular steric repulsion between the alkyl chain and triethylphosphine, favored close molecular stacking through intermolecular Pt···Pt and π-π stacking interactions, thus allowing such vapochromic behavior with ultra-stability.

From Ring-in-Ring to Sphere-in-Sphere: Self-Assembly of Discrete 2D and 3D Architectures with Increasing Stability

Directed by increasing the density of coordination sites (DOCS) to increase the stability of assemblies, discrete 2D ring-in-rings and 3D sphere-in-sphere were designed and self-assembled by one tetratopic pyridyl-based ligand with 180° diplatinum(II) acceptors and naked Pd(II), respectively. The high DOCS resulted by multitopic ligand provided more geometric constraints to form discrete structures with high stability. Compared to reported supramolecular hexagons and polyhedra by ditotpic ligands, the self-assembly of such giant architectures using multitopic ligands with all rigid backbone emphasized the structural integrity with precise preorganization of entire architecture, and required elaborate synthetic operations for ligand preparation. In-depth structural characterization was conducted to support desired structures, including multinuclear NMR ((1)H, (31)P, and (13)C) analysis, 2D NMR spectroscopy (COSY and NOESY), diffusion-ordered NMR spectroscopy (DOSY), multidimensional mass spectrometry, TEM and AFM. Furthermore, a quantitative definition of DOCS was proposed to compare 2D and 3D structures and correlate the DOCS and stability of assemblies in a quantitative manner. Finally, ring-in-rings in DMSO or DMF could undergo hierarchical self-assembly into the ordered nanostructures and generated translucent supramolecular metallogels.

The construction of complex multicomponent supramolecular systems via the combination of orthogonal self-assembly and the self-sorting approach

The realization of controllable and well-organized self-assembly within multicomponent supramolecular systems (MSSs) is still a great challenge. Herein, we present the construction of multicomponent supramolecular systems with high-level complexity through the combination of orthogonal self-assembly and the self-sorting approach. Driven by the orthogonality of metal–ligand coordination and host–guest interactions in the orthogonal self-assembly as well as directed by multiple molecular codes in the self-sorting process, five types of simple components (up to eighteen precursors) were successfully self-assembled into two novel tris[2]pseudorotaxanes in one pot through a highly selective manner, which were well-characterized by one-dimensional (1-D) and two-dimensional (2-D) multinuclear NMR as well as ESI-TOF-MS.

Stimuli‐Responsive Supramolecular Gels through Hierarchical Self‐Assembly of Discrete Rhomboidal Metallacycles

A new class of supramolecular organometallic gels with a discrete rhomboidal metallacycle as the main skeleton was fabricated through hierarchical self-assembly (see figure). More importantly, by taking advantage of the dynamic nature of metal-ligand bonds, the stimuli-responsive gel-sol transition of the obtained gels was realized by disassembly and reassembly of the rhomboidal scaffold as controlled by bromide anions.

Organometallic rotaxane dendrimers with fourth-generation mechanically interlocked branches

Significance In this study, the preparation of organometallic rotaxane dendrimers with a well-defined topological structure and enhanced rigidity was developed. Starting from a simple rotaxane building block, high-generation rotaxane branched dendrimers were synthesized and characterized. The fourth-generation structure described is among the highest-generation organometallic rotaxane dendrimers reported to date. The introduction of pillar[5]arene rotaxane units activates dynamic features in the dendrimer and enhances the rigidity of each branch of the supermolecules. This research offers a facile approach to the construction of high-generation rotaxane branched dendrimer, which not only enriches the library of rotaxne dendrimer but also provides the further insight into their applications as supramolecular dynamic materials. Mechanically interlocked molecules, such as catenanes, rotaxanes, and knots, have applications in information storage, switching devices, and chemical catalysis. Rotaxanes are dumbbell-shaped molecules that are threaded through a large ring, and the relative motion of the two components along each other can respond to external stimuli. Multiple rotaxane units can amplify responsiveness, and repetitively branched molecules—dendrimers—can serve as vehicles for assembly of many rotaxanes on single, monodisperse compounds. Here, we report the synthesis of higher-generation rotaxane dendrimers by a divergent approach. Linkages were introduced as spacer elements to reduce crowding and to facilitate rotaxane motion, even at the congested periphery of the compounds up to the fourth generation. The structures were characterized by 1D multinuclear (1H, 13C, and 31P) and 2D NMR spectroscopy, MALDI-TOF-MS, gel permeation chromatography (GPC), and microscopy-based methods including atomic force microscopy (AFM) and transmission electron microscopy (TEM). AFM and TEM studies of rotaxane dendrimers vs. model dendrimers show that the rotaxane units enhance the rigidity and reduce the tendency of these assemblies to collapse by self-folding. Surface functionalization of the dendrimers with ferrocenes as termini produced electrochemically active assemblies. The preparation of dendrimers with a well-defined topological structure, enhanced rigidity, and diverse functional groups opens previously unidentified avenues for the application of these materials in molecular electronics and materials science.

Design and Construction of Endo-Functionalized Multiferrocenyl Hexagons via Coordination-Driven Self-Assembly and Their Electrochemistry

The construction of a new family of endo-functionalized multiferrocenyl hexagons with various sizes via coordination-driven self-assembly is described. The structures of these novel metallacycles, containing several ferrocenyl moieties at their interior surface, are characterized by multinuclear NMR ((31)P and (1)H) spectroscopy, cold-spray ionization mass spectrometry (CSI-TOF-MS), elemental analysis, and molecular modeling. Insight into the structural and electrochemical properties of these endo-functionalized multiferrocenyl hexagons was obtained through cyclic voltammetry investigation.

Design and synthesis of branched platinum–acetylide complexes possessing a porphyrin core and their self-assembly behaviour

A series of branched platinum-acetylide porphyrin derivatives were successfully prepared via a modified method and their self-assembly behaviour both in solution and on the surface was explored.

A novel pyrene-containing fluorescent organogel derived from a quinoline-based fluorescent porbe: synthesis, sensing properties, and its aggregation behavior

A new family of pyrene-containing compounds 2–4 derived from aminoquinoline-containing fluorescent probe 1 were successfully synthesized and well characterized. The investigation of the absorption and emission spectra of these compounds revealed that the photophysical properties were significantly affected by the substitution of pyrene. Moreover, these compounds exhibited selective fluorescence behaviour towards Zn2+ in aqueous solution. The gelation properties of these compounds were investigated by the “stable to inversion of a test tube” method. Interestingly, 1,8-bis-substituted compound 4 displayed stable gel-formation properties in acetone, dioxane, tetrahydrofuran, ethyl acetate, chloroform, and dichloromethane. The morphologies of the xerogels were investigated by scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The concentration- and temperature-dependent emission properties, and concentration- and temperature-variable 1H NMR spectroscopy of compound 4 were investigated, which suggested that both π–π stacking interaction and hydrogen bonding were the driving forces for the process of self-aggregation and the gel formation. In addition, studies on the luminescence properties indicated that 4 had the ability to form a fluorescent organogel with interesting fluorescence behaviour. More importantly, it was found that compound 4 could form a stimuli-responsive gel that had a sensitive gel-to-sol transition response to heating or adding Zn2+.