Supramolecular polymers represent a family of polymeric materials that are held together with dynamic, noncovalent interactions. In contrast to conventional functional polymers that usually have high melt-viscosity due to their covalent nature and chain entanglement, supramolecular polymers combine excellent physical properties with low melt-viscosity, allowing for less energy-intensive processability and recyclability. Dynamic bonding with multiple binding sites, such as multiple hydrogen bonding or multiple ionic bonding, exhibits much stronger binding strength compared to the counterparts containing only a single binding site, thereby allowing for enhanced mechanical integrity to the polymers and facilitate self-assembly. This dissertation focuses on the design of novel supramolecular polymers building from the doubly-charged or quadruple hydrogen bonding (QHB) scaffolds utilizing chain-growth polymerization or step-growth polymerization, as well as elucidate the structure-property-morphology relationships of the polymers. A 2-step nucleophilic substitution reaction afforded a series of 1,4-diazabicyclo[2.2.2]octane (DABCO)-based styrenic monomers with two pairs of charged groups. An optimized 2-step reversible-addition-fragmentation chain-transfer (RAFT) polymerization synthesized ABA triblock thermoplastic elastomers (TPEs) with a low Tg poly (n-butyl acrylate) central block and a high Tg external charged blocks. Strong ionic interactions between doubly-charged units drove molecular self-assembly to form densely packed, hierarchical microstructures, which contributed to a robust, crosslinked physical network that allows the polymer to retain thermomechanical integrity until degradation. High-resolution single-crystal X-ray diffraction (SCXRD) coupled with powder X-ray diffraction (PXRD) further disclosed a detailed 3-D structural information of molecular arrangement and ion distribution within the charged phase through comparing DABCO-salt monomer single-crystal structure and the corresponding homopolymer XRD pattern. It was found that the physical properties of the DABCO-salt copolymers not only relied on their charge content and architectures but also dependent on their electrostatically-bonded counterions. The size and structure of the counterion determined the strength of dipole-dipole interaction, which significantly impact on thermal property, (thermo)mechanical performance, water affinity, and microstructure. A cytosine-functionalized monomer, cytosine acrylate (CyA), allowed the synthesis of acrylic ABA triblock TPEs with pendant nucleobase moieties in the external blocks and a low Tg central polymer matrix through RAFT polymerization. Post-functionalization of cytosine (Cyt) bidentate hydrogen bonding sites with alkyl isocyanate, allowed the formation of ureido-cytosine (UCyt) groups in the external block that were readily dimerized through QHB interactions. The UCyt units in the external block enhanced mechanical strength and induced stronger phase-separation of the block copolymers compared to the corresponding Cyt-containing TPE analogs. Facile conventional free-radical polymerization using CyA and subsequent post-functionalization enabled accessibility to random copolymers containing pendant UCyt QHB moieties in the soft polymer matrix. The synergy of the flexible polymer matrix and the dynamic character of QHB groups contributed to the ultra-high elasticity of the polymer and rapid self-healing properties. QHB interactions enabled efficient mechanical recovery upon deformation by facilitating elastic chain retraction to regenerate the original physical network. Finally, one-pot step-growth polymerization through chain extending a novel bis-Cyt monomer and a commercially available polyether diamine using a di-isocyanate extender afforded segmented polyurea series for extrusion additive manufacturing. The molecular design of the polyureas featured soft segments containing flexible polyether chain and a relatively weak urea hydrogen bonding sites in the soft segment and rigid UCyt hydrogen bonding groups in the hard segment. The reversible characteristics of QHB enabled low viscosity at the processing temperature while providing mechanical integrity after processing and reinforced bonding between the interlayers, which contributed to the remarkable strength, elasticity, toughness, and interlayer adhesion of the printed parts. / Doctor of Philosophy / This dissertation focuses on designing supramolecular thermoplastic elastomers containing strong noncovalent interactions, i.e., quadruple hydrogen bonds or double ionic bonds. Inspired from noncovalent interactions in our mother nature, a series of bio-inspired monomers functionalized with nucleobase or ionic units were synthesized through scalable reactions with minimal purification steps. Polymerization of the functional monomers through step-growth or chain-growth polymerization techniques affords a variety of supramolecular thermoplastic elastomers with well-defined structures and architectures. These thermoplastic elastomers comprise soft and hard constituents; the former contains low glass transition polymer chains that provide elasticity while the latter contains strong noncovalent units to impart mechanical strength. Varying the soft/hard component ratios enables polymers with tunable physical properties to address different needs. Systematic characterizations of these supramolecular polymers revealed their distinct properties from the polymers containing the covalent or weak noncovalent interactions and facilitate molecular-level understanding of the polymers. Generally, incorporating strong noncovalent interactions increases the temperature for polymer segmental motion and extends thermomechanical plateau windows. Additionally, the strong association strength of those non-covalent interactions promotes microphase separation and self-assembly, contributing to a high degree of structural ordering of the polymers. Moreover, the dynamic characteristics of the noncovalent interactions offer the polymers with reversible properties, which not only enables melt-processability and recyclability of the polymer but also contributes to a series of smart properties, including self-healing, shape-memory, and recoverability. Thus, the molecular design using supramolecular chemistry provides promising avenues to developing functional materials with enhanced mechanical properties, processability, and stimuli-responsiveness for emerging applications.

Design and preparation of novel host/guest systems, such as pseudorotaxanes, semirotaxanes, rotaxanes and catenanes, with high association constants, enhanced yields and the abilities to respond to external stimuli are of great importance and significance due to their topological novelty and potential application. The convergence of supramolecular chemistry with polymer science provides an important way to extend the scope of polymer and material sciences by incorporating designed host/guest systems into polymers, and the resulting non-covalently linked supramolecular polymers are expected to have unusual properties due to their unique architectures compared with traditional polymers. After discovery of bis(meta-phenylene)-32-crown-10 (BMP32C10) derivative/paraquat complexes, for about a quarter century only “taco”-shaped complexes were observed by X-ray crystallography. Here, by the self-assembly of a BMP32C10 bearing two electron-donating groups (carbazoles) with electron-accepting paraquat derivatives, the first [2]pseudorotaxane and the first pseudocryptand-type poly[2]pseudorotaxane based on BMP32C10 were isolated as crystalline solids as shown by X-ray analyses. The first dual component pseudocryptand-type [2]pseudorotaxanes were designed and prepared via the self-assembly of synthetically easily accessible BMP32C10 pyridyl, quinolyl and naphthyridyl derivatives with paraquat. The formation of the pseudocryptand structures in the complexes remarkably improved the association constants by forming the third pseudo-bridge via H-bonding with the guest and π-stacking of the heterocyclic units. A pseudocryptand-type [2]pseudorotaxane was formed via the self-assembly of a dipyridyl BMP32C10 derivative and a paraquat derivative. Due to the basicity of the pyridyl group, which forms the third pseudo-bridge of the pseudocryptand, this pseudorotaxane represents the first system with acid-base adjustable association constants, i. e., finite both under acidic and neutral conditions. The first pseudocryptand-type supramolecular [3]pseudorotaxane was designed and prepared via the self-assembly of a bispicolinate BMP32C10 derivative and a bisparaquat. The complexation behavior was cooperative. In addition, the complex comprised of the BMP32C10 derivative and a cyclic bisparaquat demonstrated strong binding; interestingly, a poly[2]pseudocatenane structure was formed in the solid state for the first time. Two novel BMP32C10 cryptands, bearing covalent and metal complex linkages, were designed and prepared. By employing the self-assembly of these biscryptands, which can be viewed as AA monomers, and a bisparaquat, which can be viewed as a BB monomer, the first AA/BB-type linear supramolecular polymers with relatively high molecular weights were successfully prepared. Via the self-assembly of two BMP32C10-based cryptands, bearing covalent and metal complex (ferrocene) linkages, with dimethyl paraquat, novel [3]pseudorotaxanes were formed statistically and anticooperatively, respectively. From a hydroxyl-functionalized secondary ammonium salt a [2]semirotaxane and a [2]rotaxane were prepared successfully with dibenzo-24-crown-8 (DB24C8). X-ray analysis of a single crystal of the [2]semirotaxane confirmed its semirotaxane nature. In addition, the formation of the [2]semirotaxane can be reversibly controlled by adding KPF6 and 18C6 sequentially. This system affords a way to prepare novel supramolecular polymers. Dibenzo-30-crown-10 (DB30C10) derivatives and pyridine-based DB30C10 cryptands were prepared by employing the templating method established by our group. A [2]pseudorotaxane was prepared based on DB30C10 diol and paraquat diol. The [3]pseudorotaxane formed via the self-assembly between DB30C10 cryptand and bisparaquat diol occurred in a cooperative manner. In addition, a bromo-functionalized DB30C10 cryptand was successfully designed and prepared. An alkyne-functionalized DB30C10 cryptand was designed and is under preparation; its precursors have been prepared successfully. In the future, based on these functionalized cryptands and paraquat salts, AA and AB type monomers will be prepared. Via the self-assembly between these monomers, non-covalent supramolecular polymers with high molecular weight will be afforded. A novel DB30C10 cryptand bearing an organometallic bridge, ferrocene, was prepared via 1-(3'-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) coupling of the crown ether diol with ferrocene dicarboxylic acid. The cryptand is dimerized in the solid state via π, π-stacking and hydrogen bonds. The ferrocene-based cryptand formed novel [2]pseudorotaxanes with paraquat and diquat PF₆ salts with association constants (Kₐ) of 1.7 ± 0.1 x 10³ and 4.2 ± 0.3 x 10⁴ M⁻¹ in acetone-d₆, respectively. In order to prepare linear polycatenanes, the preparation of which represent a real synthetic challenge, a series phenanthroline derivatives were designed and prepared. A “U” shaped monomer was successfully prepared in relative high yield with good solubility. In the future, real linear polycatenanes will be prepared. In addition, a novel diphenanthroline-based BMP32C10 derivative was prepared in high yield and the complexation behavior between it and dimethyl paraquat was studied. / Ph. D.

Over the past few decades, supramolecular self-assembly has become an alternative synthetic tool for constructing targeted discrete molecular architectures. Among various interactions, metal-ligand coordination has attracted great attention owing to high bond enthalpy (15−50 Kcal/mol) and predictable directionality. The basic principle of metal-ligand directed self-assembly relies on the proper designing of information encoded rigid complementary building units (a transition metal based acceptor and a multidentate organic donor) that self-recognize themselves in a chemically reasonable way (depends on their bite angle and symmetry) during self-assembly process. As far as acceptor units are concerned, Pd(II) and Pt(II) metal-based cis-blocked 90° acceptors have so far been used greatly for the construction of a library of 2D/3D discrete supramolecular architectures due to their rigid square planar geometry and kinetic lability. However, in some cases the efforts to design finite supramolecular architectures using a cis-blocked 90° acceptor in combination with a bulky donor ligand were unsuccessful, which may be due to the steric demands of donor ligand. Moreover, the resulted assemblies from such cis-blocked 90° building unit are mostly non-fluorescent in nature and limit the possibility of using them as chemosensors for various practical applications. Unlike that of rigid square-planar Pt(II) and Pd(II)-metal based building blocks, the use of other transition metal-based building units for the construction of discrete nanoscopic molecular architectures are known to lesser extent, mainly because of their versatile coordination geometries. However, some of the half-sandwiched piano-stool complexes of late transition metals like Ru, Os, Ir and Rh are known to maintain the stable octahedral geometry under various reaction conditions. Moreover, the self-assembly using redox active transition metal-based building units may lead to redox active assemblies. On the other hand, symmetrical rigid donors have been widely used as the favorite choices for the purpose of constructing desired product mainly due to their predictable directionality. Flexible linkers are not predictable in their directionality during self-assembly process and thus results mostly in undesired polymeric products. Furthermore, metal-ligand directed self-assembly provides opportunity to introduce multifunctionality in a single step within/onto the final supramolecular architectures. Among various functional groups, the incorporation of unsaturated ethynyl functionality is expected to enrich the final assemblies to be π-electron-rich and the attachment of ethynyl functionality with heavy transition metal ions are known to be luminescent in nature due to the facile metal to ligand charge transfer (MLCT). Hence, the final supramolecular complexes can be used as potential fluorescence sensors for electron-deficient nitroaromatics, which are the chemical signature of most of the commercially available explosives. The main thrust of the present investigation is focused on the judicious design and syntheses of multifaceted 2D/3D supramolecular architectures of finite shapes, sizes and functionality using Pt(II)/Ru(II) based “shape-selective” organometallic building blocks and investigation of their application as chemosensors. CHAPTER 1 of the thesis presents a general review on the core concepts of self-assembly and supramolecular chemistry. In particular, it underlines the importance of metal-ligand directional bonding approach for designing a vast plethora of discrete 2D/3D supramolecular architectures with tremendous variation in topology. CHAPTER 2 describes the design and syntheses of a series of 2D metallamacrocycles using carbazole-functionalized shape-selective 90° building units. A new Pt2II organometallic 90° acceptor 3,6-bis[trans-Pt(PEt3)2(NO3)(ethynyl)]carbazole (M1) containing ethynyl functionality is synthesized via Sonagashira coupling reaction and characterized. The combination of M1 with three different flexible ditopic donors (L1−L3) afforded [2 + 2] self-assembled molecular squares (1−3), respectively [where L1 = 1,3-bis(4-pyridyl)isophthalamide; L2 = 1,3-bis(3-pyridyl)isophthalamide; L3 = 1,2-bis(4-pyridyl)ethane] (Scheme 1). Scheme 1: Schematic presentation of the formation of a series of [2 + 2] self-assembled molecular squares. An equimolar (1:1) combination of same acceptor M1 with rigid linear ditopic donors (L4-L5) yielded [4 + 4] self-assembled octanuclear molecular squares 4 and 5, respectively [L4 = 4,4’-bipyridine; L5 = trans-1,2-bis(4-pyridyl)ethylene]. Conversely, a similar reaction of M1 with an amide-based unsymmetrical linear flexible ditopic donor L6 resulted in the formation a [2 + 2] self-sorted molecular rhomboid (6a) as a single product [L6 = N-(4-pyridyl)isonicotinamide]. Despite the possibility of several linkage isomeric macrocycles (rhomboids, triangles and squares) due to different connectivity of the ambidentate linker, the formation of a single and symmetrical molecular rhomboid 6a as an exclusive product is an interesting observation. This chapter also presents the synthesis and characterization of a complementary 90° dipyridyl donor 3,6-bis(4-pyridylethynyl)carbazole (L7). Stoichiometric combination of L7 with several PdII/PtII-based 90° acceptors (M2−M4) yielded [2 + 2] self-assembled molecular “bowl” shaped macrocycles (7−9) respectively, in good yields [M2 = cis-(dppf)Pd(CF3SO3)2; M3 = cis-(dppf)Pt(CF3SO3)2; M4 = cis-(tmen)Pd(NO3)2]. All these newly synthesized macrocycles were characterized by various spectroscopic techniques and molecular structures of some of them were confirmed by single crystal X-ray diffraction analysis. In addition to their syntheses and characterization, fluorescence chemosensing ability for various analytes was investigated. Macrocycle 1 is a system composed of amide-based receptor units and carbazole-based fluorophore moieties. The fluorescence study of 1 elicited a dramatic enhancement in the fluorescence intensity upon gradual addition of P2O74- anion in DMF/H2O solvent mixture, whereas similar titration under identical condition with other anions like F-, ClO4-, and H2PO4- did not show such change. Hence, molecular square 1 can be used as selective fluorescence sensor for pyrophosphate (P2O74-) anion. Due to their extended π-conjugation, macrocycles 3-4 were used as fluorescence sensors for electron-deficient nitroaromatics, which are the chemical signatures of many commercially available explosives. The fluorescence study showed a marked quenching of initial fluorescence intensity of the macrocycles(3-4) upon gradual addition of picric acid (PA) and they exhibited large fluorescence quenching responses with high selectivity for nitroaromatics among various other electron deficient aromatic compounds tested. As macrocycle 7 has large concave aromatic surface, it was utilized as a suitable host for large convex guest such as fullerene C60. The fluorescence quenching titration study suggested that macrocycle 7 forms a stable ~1:1 host-guest complex with C60 and the calculated association constant (KSV) is 1.0 × 105 M-1. CHAPTER 3 presents two-component coordination-driven self-assembly of a series of [2 + 2] molecular rectangles and a [2 + 4] self-assembled molecular tetragonal prism. An equimolar combination of pre-designed linear PtII2-acceptors M5−M6 separately with three different “clip” donors (L2, L8−L9) led to the formation of [2 + 2] self-assembled tetranuclear cationic molecular rectangles (10−15), respectively [M5 = 1,4-bis[trans-Pt(PEt3)2(NO3)(ethynyl)] benzene; M6 = 4,4’-bis[trans-Pt(PEt3)2(CF3SO3)(ethynyl)]biphenyl; L8 = 1,3-bis(3-pyridyl)ethynylbenzene; L9 = 1,8-bis(4-pyridyl)ethynylanthracene]. Rectangles 10-15 showed strong fluorescence in solution owing to their extended π-conjugation. Amide-functionalized rectangle 10 was used as a macrocyclic receptor for dicarboxylic acids. Solution state fluorescence study showed that rectangle 10 selectively binds (KSV = 1.4 × 104 M-1) with maleic acid by subsequent enhancement in emission intensity and addition of other analogous aliphatic dicarboxylic acids such as fumaric, succinic, adipic, mesaconic and itaconic acids causes no change in the emission spectra; thereby demonstrated its potential use as macrocyclic receptor in sensor applications. Since rectangle 15 is enriched with π-conjugation, it was examined as a fluorescence sensor for electron-deficient nitroaromatics such as picric acid, which is often considered as a secondary chemical explosive. The fluorescence study of 15 showed a significant quenching of initial emission intensity upon titrating with picric acid (PA) and it exhibited the largest fluorescence quenching response with high selectivity for picric acid. Scheme 2: Schematic representation of formation of [2 + 4] self-assembled of molecular tetragonal prism. This chapter also describes two-component coordination [2 + 4] self-assembly of a pyrene-based PtII8 tetragonal prism (16) as shown in Scheme 2, using a newly designed tetratopic organometallic acceptor (M7; 1,3,6,8-tetrakis[trans-Pt(PEt3)2(NO3)(ethynyl)]pyrene) in combination with an amide-based “clip” donor (L2) and propensity of this prism (16) as a selective fluorescence sensor for nitroaromatic explosives has been examined both in solution as well as in thin-film. CHAPTER 4 reports the synthesis and structural characterization of a series of Ru(II)-based bi-and tetra-nuclear metallamacrocycles and hexanuclear trigonal prismatic cages. In principle, the self-assembly of a “clip” acceptor with an asymmetrical ditopic donor is expected to give two different linkage isomeric (head-to-tail and head-to-head) molecular rectangles because of different bond connectivity of the donor. However, the equimolar combination of half-sandwiched p-cymene binuclear Ru(II)-based “clip” acceptors (M8−M9) and an amide-based ambidentate donor (L6) resulted in the self-sorting of single linkage (head-to-tail) isomeric rectangles 17−18 as only products, respectively [M8 = [Ru2(μ-η4-C2O4)(MeOH)2(η 6-p-cymene)2](CF3SO3)2; M9 = [Ru2(μ- η4-C6H2O4)(MeOH)2(η 6-p-cymene)2](CF3SO3)2]. Molecular structures of these head-to-tail linkage isomeric rectangles were unambiguously proved by single crystal X-ray diffraction analysis. Likewise, the self-assembly of oxalato-bridged Ru(II) acceptor M8 with a rigid dipyridyl “clip” donor L8 yielded a tetranuclear cationic pincer complex 19, while a similar reaction of M8 with an anthracene-functionalized “clip” donor L9 having shorter distance (between their reactive sites) compared to L8 led to the formation of [1 + 1] self-assembled macrocycle 20. This chapter also represents the design and synthesis of two hexanuclear trigonal prismatic cages (21−22) from the self-assembly of a π-electron rich tripyridyl donor (L10; 1,3,5-tris(4-pyridylethynyl)benzene) in combination with binuclear acceptors M8 and M9, respectively (Scheme 3). Formation of these prismatic cages was initially characterized using various spectroscopic techniques and the molecular structure of oxalato-bridged prism 21 was confirmed by single crystal X-ray diffraction analysis. In addition to the structural characterization, the pincer complex 19 and trigonal prismatic cages 21−22 were used as fluorescence sensors for nitroaromatic explosives owing to their large internal porosity and their π-electron rich nature. Scheme 3: Schematic representation of the formation of [3 + 2] self-assembled trigonal prismatic cage. CHAPTER 5 covers the syntheses of a few discrete metallamacrocycles using flexible imidazole/carboxylate based donors instead of much widely employed polypyridyl donors. The metal-ligand directed self-assembly of oxalato-bridged acceptor M8 and an imidazole-based tetratopic donor (L11; 1,2,4,5-tetrakis(imidazol-1-yl)benzene) in methanol afforded [2 + 1] self-assembled tetranuclear macrocycle 23. Conversely, the similar combination of L11 with 2,5-dihydroxy-1,4-benzoquinonato-bridged binuclear complex (M9) in 1:2 molar ratio in methanol resulted in an octanuclear cage 24. Both the complexes (23−24) were isolated as their triflate salts in high yields and were characterized by various spectroscopic methods including single crystal X-ray diffraction analysis. Scheme 4: Schematic representation of formation of an octanuclear incomplete Ru(II) open prism via ruthenium-oxygen coordination driven self-assembly. This chapter also explains the self-sorting of an unusual octanuclear incomplete prism [Ru8(η6-p-cymene)8(tma)2(μ-η4-C2O4)2(OMe)4](CF3SO3)2 (25) via ruthenium-oxygen coordination driven self-assembly of building block M8 and sodium benzene-1,3,5-tricarboxylate (L12) (Scheme 4). Electronic absorption study indicated that prism 25 exhibited a remarkable shape-selective binding affinity for 1,3,5-trihydroxybenzene (phluoroglucinol) via multiple hydrogen bonding interactions and such shape-selective binding was confirmed by single crystal X-ray diffraction analysis. (For figures pl see the abstract file)