Molecular recognition and self-assembly special feature: Supramolecular polymer formed by reversible self-assembly of tetrakisporphyrin

Self-Assembled Bis-Acridinium Tweezer Equilibria Controlled by Multi-Responsive Properties

Protonated and methylated bis-acridinium tweezers built around a 2,6-diphenylpyridyl and an electron enriched 2,6-di(p-anisyl)pyridyl spacer have been synthesized. These tweezers can self-assemble in their corresponding homodimers and the associated thermodynamic parameters have been probed in organic solvents. The switching properties of the tweezers have been exploited in biphasic transfer experiments showing the shift of the equilibria towards the homodimers. Moreover, the thermodynamic parameters of the formation of the reduced methylated homodimers investigated by electrochemical experiments revealed the dissociation of the dimers. Thus, in addition to solvent and temperature, the pH and redox responsiveness of the acridinium units of the tweezers make it possible to modulate to a larger extent the monomer-dimer equilibria.

Shape-Assisted Self-Assembly of Hexa-Substituted Carpyridines into 1D Supramolecular Polymers

The extent of the influence that molecular curvature plays on the self-assembly of supramolecular polymers remains an open question in the field. We began addressing this fundamental question with the introduction of "carpyridines", which are saddle-shaped monomers that can associate with one another through π-π interactions and in which the rotational and translational movements are restricted. The topography displayed by the monomers led, previously, to the assembly of highly ordered 2D materials even in the absence of strong directional interactions such as hydrogen bonding. Here, we introduce a simple strategy to gain control over the dimensionality of the formed structures yielding classical unidimensional polymers. These have been characterized using well-established protocols allowing us to determine and confirm the self-assembly mechanism of both fibers and sheets. The calculated interaction energies are significantly higher than expected for flexible self-assembling units lacking classical "strong" non-covalent interactions. The versatility of this supramolecular unit to assemble into either supramolecular fibers or 2D sheets with strong association energies highlights remarkably well the potential and importance of molecular shape for the design of supramolecular materials and the applications thereof.

Assembling Molecular Clips To Build π-Stacks

Nature utilizes an intimate stacking of aromatic motifs to construct functional structures, as demonstrated in protein folding and polynucleotide assembly. However, organized π-stacks of artificial molecules are difficult to build, primarily due to the weak, non-directional, and context-sensitive nature of van der Waals forces. To overcome these challenges, chemists have invented ingenious architectural designs to construct π-stacked supramolecular assemblies using clip-like molecules. This Concept article focuses on molecular clips that enable precise spatial control over assembly patterns, beyond the scope of simple host-guest chemistry. Different design strategies are analyzed and compared that leverage non-covalent interactions to create multi-layer π-stacks. Particular emphasis is placed on the choice of spine units as they play a crucial role in controlling the (i) spacing, (ii) orientation, and (iii) conformational pre-organization of linked aromatics to achieve long-range spatial ordering.

Porphyrin-based supramolecular polymers

Porphyrin derivatives are ubiquitous in bio-organisms and are associated with proteins that play important biological roles, such as oxygen transport, photosynthesis, and catalysis. Porphyrins are very fascinating research objects for chemists, physicists, and biologists owing to their versatile chemical and physical properties. Porphyrin derivatives are actively used in various fields, such as molecular recognition, energy conversion, sensors, biomedicine, and catalysts. Porphyrin derivatives can be used as building blocks for supramolecular polymers because their primitive structures have C₄ symmetry, which allows for the symmetrical introduction of self-assembling motifs. This review describes the fabrication of porphyrin-based supramolecular polymers and novel discoveries in supramolecular polymer growth. First, we summarise the (i) design concepts, (ii) growth mechanism and (iii) analytical methods of porphyrin-based supramolecular polymers. Then, the examples of porphyrin-based supramolecular polymers formed by (iv) hydrogen bonding, (v) metal coordination-based interaction, (vi) host-guest complex formation, and (vii) others are summarised. Finally, (viii) applications and perspectives are discussed. Although supramolecular polymers, in a broad sense, can include either two-dimensional (2D) networks or three-dimensional (3D) porous polymer structures; this review mainly focuses on one-dimensional (1D) fibrous supramolecular polymer structures.

Supramolecular chiral sensing by supramolecular helical polymers

A tetrakis(porphyrin) with branched side chains self-assembled to form supramolecular helical polymers both in solution and in the solid state. The helicity of the supramolecular polymers was determined by the chirality of solvent molecules, which permitted the polymer chains to be used in chiral sensing.

Phosphorescent Host-Guest Complexes on the Basis of Polyhedral Oligomeric Silsesquioxane-Functionalized Metallotweezers

Phosphorescent host-guest systems have attracted considerable attention because of their intriguing properties and diverse applications. In this study, a polyhedral oligomeric silsesquioxane-functionalized gold(III) tweezer receptor has been designed and synthesized. It is capable of sandwiching platinum(II) terpyridine compounds into its cavity with a high noncovalent binding affinity (association constants: ∼10⁵ M^-1 in chloroform). The resulting heterometallic host-guest complexes exhibit enhanced phosphorescent emission compared with those of the individual species in chloroform, thanks to the prevention of vibration and rotation upon noncovalent complexation. They can further assemble into nanospheres in chloroform/diethyl ether (1:9, v/v) owing to phase segregation between the metallotweezer/guest motif and the peripheral polyhedral oligomeric silsesquioxane unit. When terpyridine platinum(II) chloride serves as the complementary guest, the resulting noncovalent system displays an intraligand emission at the individual host-guest complexed state yet excimeric emission at the supramolecular assembled state, yielding the phosphorescent solvatochromic behaviors. Overall, the polyhedral oligomeric silsesquioxane-functionalized metallotweezer combines guest encapsulation and supramolecular assembly capabilities, which provides new avenues for color-tunable phosphorescent materials.

Self-Assembly-Directed Organization of a Fullerene-Bisporphyrin into Supramolecular Giant Donut Structures for Excited-State Charge Stabilization

Functional materials composed of spontaneously self-assembled electron donor and acceptor entities capable of generating long-lived charge-separated states upon photoillumination are in great demand as they are key in building the next generation of light energy harvesting devices. However, creating such well-defined architectures is challenging due to the intricate molecular design, multistep synthesis, and issues associated in demonstrating long-lived electron transfer. In this study, we have accomplished these tasks and report the synthesis of a new fullerene-bis-Zn-porphyrin e-bisadduct by tether-directed functionalization of C60 via a multistep synthetic protocol. Supramolecular oligomers were subsequently formed involving the two porphyrin-bearing arms embracing a fullerene cage of the vicinal molecule as confirmed by MALDI-TOF spectrometry and variable temperature NMR. In addition, the initially formed worm-like oligomers are shown to evolve to generate donut-like aggregates by AFM monitoring that was also supported by theoretical calculations. The final supramolecular donuts revealed an inner cavity size estimated as 23 nm, close to that observed in photosynthetic antenna systems. Upon systematic spectral, computational, and electrochemical studies, an energy level diagram was established to visualize the thermodynamic feasibility of electron transfer in these donor-acceptor constructs. Subsequently, transient pump-probe spectral studies covering the wide femtosecond-to-millisecond time scale were performed to confirm the formation of long-lived charge-separated states. The lifetime of the final charge-separated state was about 40 μs, thus highlighting the significance of the current approach of building giant self-organized donor-acceptor assemblies for light energy harvesting applications.

Organoplatinum(II)-Based Self-Complementary Molecular Tweezers with Guest-Induced Fluorochromic Behaviors

Cyclometalated organoplatinum(II) complexes have aroused tremendous interests due to their square-planar geometry and intriguing photophysics. To access multiplatinum systems with more than three cyclometalated organoplatinum(II) units, the traditional covalent synthetic approach suffers from tedious multistep reactions with low overall yield. In comparison, supramolecular assembly can be regarded as an effective strategy toward multiplatinum(II) architectures. Despite the progresses achieved, it is still challenging to fabricate well-ordered supramolecular assemblies with precise numbers of organoplatinum(II) units. Herein, self-complementary dimerized molecular tweezers with four cyclometalated platinum(II) units have been successfully constructed by taking advantage of dual roles of the incorporated 2,2':6',2''-terpyridine unit (serving as the rigid spacer and encapsulated guest). Furthermore, addition of electron-rich carbazoles leads to conversion of the self-complementary structure to molecular tweezer/guest complexes. Such a structural transformation gives rise to the concomitant luminescent color change. The unique guest-induced fluorochromic phenomena, which are seldom reported in the previous host-guest systems, would be promising as tunable luminescent and ratiometric sensing materials.

A dual redox-responsive supramolecular polymer driven by molecular recognition between bisporphyrin and trinitrofluorenone

A dual redox-responsive supramolecular polymer driven by molecular recognition between bisporphyrin (bisPor) and trinitrofluorenone (TNF) has been developed.

Hooking Together Sigmoidal Monomers into Supramolecular Polymers

Supramolecular polymers show great potential in the development of new materials because of their inherent recyclability and their self-healing and stimuli-responsive properties. Supramolecular conductive polymers are generally obtained by the assembly of individual aromatic molecules into columnar arrays that provide an optimal channel for electronic transport. A new approach is reported to prepare supramolecular polymers by hooking together sigmoidal monomers into 1D arrays of π-stacked anthracene and acridine units, which gives rise to micrometer-sized fibrils that show pseudoconductivities in line with other conducting materials. This approach paves the way for the design of new supramolecular polymers constituted by acene derivatives with enhanced excitonic and electronic transporting properties.

Self-complementary and narcissistic self-sorting of bis-acridinium tweezers

A molecular tweezer incorporating two acridinium moieties linked by a 1,3-dipyridylbenzene spacer was synthesized in three steps. The formation of its self-complementary dimer in water was demonstrated as a result of π-π stacking and hydrophobic interactions. Moreover, a 1 : 1 mixture of this bis-acridinium tweezer with one built on a 2,6-diphenylpyridyl spacer evidenced its narcissistic self-sorting behaviour in water.

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Design and synthesis of a 4-aminoquinoline-based molecular tweezer that recognizes protoporphyrin IX and iron(iii) protoporphyrin IX and its application as a supramolecular photosensitizer

We report on the design and synthesis of a new type of 4-aminoquinoline-based molecular tweezer 1 which forms a stable host-guest complex with protoporphyrin IX (PPIX) via multiple interactions in a DMSO and HEPES buffer (pH 7.4) mixed solvent system. The binding constant for the 1 : 1 complex (K 11) between 1 and PPIX is determined to be 4 × 106 M-1. Furthermore, 1 also forms a more stable complex with iron(iii) protoporphyrin IX (Fe(iii)PPIX), the K 11 value for which is one order of magnitude greater than that for PPIX, indicating that 1 could be used as a recognition unit of a synthetic heme sensor. On the other hand, the formation of the stable PPIX·1 complex (supramolecular photosensitizer) prompted us to apply it to photodynamic therapy (PDT). Cell staining experiments using the supramolecular photosensitizer and evaluations of its photocytotoxicity indicate that the PDT activity of PPIX is improved as the result of the formation of a complex with 1.

Entwined dimer formation from self-complementary bis-acridiniums

A dicationic tweezer incorporating two acridinium moieties linked by a 2,6-diphenylpyridine spacer was shown to self-assemble in an entwined dimer both in acetonitrile and water. The reaction was studied according to solvent polarity, temperature and concentration conditions. The entwined structure was confirmed in the solid state via single-crystal X-ray diffraction.

Donor-acceptor-type supramolecular polymers on the basis of preorganized molecular tweezers/guest complexation

The bottom-up self-assembly of donor-acceptor (D-A) units has received tremendous attention in recent years. Charge-transfer interactions, which are inherently embedded in D-A pairs, have suffered from some disadvantages such as erratic arrangements and weak binding affinity, thus hampering the precise arrangement of D-A units into long-range-ordered supramolecular polymers. To address this issue, a feasible protocol is to incorporate D-A units into molecular tweezers/guest recognition motifs, which concurrently feature high complexation directionality, strong binding affinity and stimuli-responsiveness. In this tutorial review, we have summarized the recent advances on the tweezering directed formation of D-A-type supramolecular polymers, with particular emphasis on the design principles of monomers and macroscopic behaviors of supramolecular polymers, together with future challenges in this research field.

Readily functionalized AAA-DDD triply hydrogen-bonded motifs

Herein we present a new, readily functionalized AAA-DDD hydrogen bond array. A novel AAA monomeric unit (3a-b) was obtained from a two-step synthetic procedure starting with 2-aminonicotinaldehyde via microwave radiation (overall yield of 52-66%). 1H NMR and fluorescence spectroscopy confirmed the complexation event with a calculated association constant of 1.57 × 107 M-1. Likewise, the usefulness of this triple hydrogen bond motif in supramolecular polymerization was demonstrated through viscosity measurements in a crosslinked supramolecular alternating copolymer.

Interior-filled self-assemblies of tyrosyl bolaamphiphiles regulated by hydrogen bonds

Bolaamphiphilic molecules with tyrosyl end groups formed interior-filled spherical self-assemblies, which are distinct from the vesicular or tubular structures of other similar peptidic bolaamphiphile assemblies reported in the literature. In this study, the self-assembly mechanism of these tyrosyl bolaamphiphiles was investigated taking into consideration the solvent effects on the molecular interaction forces using molecular modeling. The dissipative particle dynamics simulation of an aqueous tyrosyl bolaamphiphile solution suggested that the interior-filled assemblies were produced by a solvent-regulated assembly of small aggregates of bolaamphiphiles. These small aggregates were generated by hydrophobic interactions at an early stage, and then further assembled to form large spherical assemblies through intermolecular forces, including hydrogen bonds between the intermediate aggregates. Additional experiments and density functional theory calculations based on solvent variations proved that smaller assembled structures could be obtained by disrupting the hydrogen bonds between the intermediates. The assembly mechanism of these peptidic bolaamphiphiles afforded a facile way to create condensed supramolecular structures with controlled sizes.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions

A brief account has been presented on how the inter-heme interactions in μ-hydroxo diiron(iii) bisporphyrins and counter anions can induce significant change in the structure and properties including the iron spin state without affecting the overall topology.

Reorganization of porphyrin nanoparticle morphology driven by surface energetics

Organic nanoparticles (ONp) of an Fe(III) porphyrin appended with four [Formula: see text]-polyethyleneglyco-pyridinium moieties prepared in acetonitrile were deposited onto hydrophilic or hydrophobic Si surfaces. Self-organized by intermolecular interactions, ONp reorganize in response to environmental changes. Mechanisms for the control of nanoparticle morphologies and surface patterning by varying surface energies are discussed.

Structurally Flexible C₃-Symmetric Receptors for Molecular Recognition and Their Self-Assembly Properties

The bioinspired design and synthesis of building blocks and their assemblies by the supramolecular approach has ever fascinated scientists to utilize such artificial systems for numerous purposes. Flexibility is a basic feature of natural systems. However, in artificial systems this is difficult to control, especially if there is no preorganization of the component(s) of a system. We have designed and synthesized a series of C3 -symmetric N-bridged flexible receptors and successfully utilized them to selectively entrap the notorious and toxic nitrate anion in aqueous medium. This was the first report of highest binding affinity for the nitrate anion in aqueous medium. An impressive self-sorting phenomenon of reversibly formed hydrogen-bonded capsules, which self-assembled from flexible tripodal receptors having branches of similar size and bearing the same amide functionality, has been disclosed. Encapsulated nitrate anion has been further utilized for the photochemical [2+2] cycloaddition reaction for the synthesis of strained four-membered ring structures through dynamic self-assembly. In this Personal Account, we summarize these results showing the utility of naturally inspired flexibility in artificial systems.

Supramolecular nanoassemblies of an amphiphilic porphyrin-cyclodextrin conjugate and their morphological transition from vesicle to network

An amphiphilic compound, 5-(4'-dodecyloxyphenyl)-10,15,20-tri(permethyl-β-CD)-modified Zn(II)-porphyrin (1; β-CD = β-cyclodextrin), was synthesized by means of the click reaction of an alkylated Zn-porphyrin derivative with 6-deoxy-6-azidopermethyl-β-CD. The complexation between 1 and tetrasodium tetraphenylporphyrintetrasulfonate (5) with different molar ratios led to the formation of two distinctly different nanoarchitectures, which were proven to be vesicle and network aggregates, respectively, by using dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. On the basis of the results of the time-dependent TEM studies, fluorescence, and NMR spectroscopic measurements, we have determined that the mechanism of the morphology transition from vesicles to networks is controlled by the stepwise complexation of 1 with 5. Furthermore, these supramolecular nanoarchitectures show the controlled- release property of doxorubicin as potential candidates for drug delivery.

Intermolecular Interactions of CpFePPh3(CO)CO(CH2)5CH3: From a Crystalline Solid to a Supramolecular "Iron-Truss" Polymer

PPh₃CpFe(CO)(CO)(CH₂)₅CH₃ (FpC6) spontaneously forms supramolecular polymers in the solid state. The polymers crystallize slowly over a period of one month and can be recovered by melting the crystals at 65 °C. The rheological profile of FpC6 fits the Maxwell model indicating the presence of chain entanglement. Crystal analysis reveals that FpC6 is able to assemble, via cooperative π-π interactions and weak C-H···O hydrogen bonding, into a duplex chain structure with truss arrangement of iron atoms. Powder X-ray diffraction (PXRD) of the polymers shows a double-peak pattern, characteristic for duplex ladder polymers. FTIR/ATR analysis further supports that carbonyl groups are involved in C-H···O hydrogen bonding responsible for the self-assembly. This discovery opens up new design motifs for organometallic supramolecular polymers.

Self-association and nitroaromatic-induced deaggregation of pyrene substituted pyridine amides

The self-assembly features of the bis-pyrene methyl amide functionalized pyridine and benzene "tweezers" 1 and 2 were studied in organic solution and in the solid state. These systems were found to display remarkably different self-association features and optical properties, which was rationalized by control experiments using compounds bearing pyrenemethyl esters, alkyl groups, or a single pyrene substituent (3-6). As dilute solutions in chloroform, tweezers 1 displays both pyrene monomer and excimer emission features reflecting intramolecular contacts between the pyrene subunits. At higher concentrations in chloroform, as well as in the solid state, tweezers 1 self-assembles to form a linear supramolecular polymer. In contrast, tweezers 2 does not interact in an intermolecular fashion and photoexcitation produces emission features characteristic of a pyrene monomer. DFT (density functional theory) and TDDFT (time dependent density functional theory) calculations revealed that the lowest vertical transitions are forbidden and that S1 of 1 is an emissive state. In contrast to 1 and 2, both pyrene-free control systems 5 and 6 were found to form linearly self-assembled supramolecular arrays in the solid state, albeit of differing structure. Upon exposure to trinitrobenzene (TNB), the self-assembled structures formed from 1 undergo deaggregation to form TNB complexes. This change is reflected in both an easily discernible color change and a quenching of the fluorescence emission intensity. Changes in the optical features were also seen in the case of 2. However, notable differences between these two ostensibly similar systems were seen.