Directing the Self-Assembly Behaviour of Porphyrin-Based Supramolecular Systems

Porphyrin Aggregation under Homogeneous Conditions Inhibits Electrocatalysis: A Case Study on CO2 Reduction

Metalloporphyrins are widely used as homogeneous electrocatalysts for transformations relevant to clean energy and sustainable organic synthesis. Metalloporphyrins are well-known to aggregate due to π-π stacking, but surprisingly, the influence of aggregation on homogeneous electrocatalytic performance has not been investigated previously. Herein, we present three structurally related iron meso-phenylporphyrins whose aggregation properties are different in commonly used N,N-dimethylformamide (DMF) electrolyte. Both spectroscopy and light scattering provide evidence of extensive porphyrin aggregation under conventional electrocatalytic conditions. Using the electrocatalytic reduction of CO2 to CO as a test reaction, cyclic voltammetry reveals an inverse dependence of the kinetics on the catalyst concentration. The inhibition extends to bulk performance, where up to 75% of the catalyst at 1 mM is inactive compared to at 0.25 mM. We additionally report how aggregation is perturbed by organic additives, axial ligands, and redox state. Periodic boundary calculations provide additional insights into aggregate stability as a function of metalloporphyrin structure. Finally, we generalize the aggregation phenomenon by surveying metalloporphyrins with different metals and substituents. This study demonstrates that homogeneous metalloporphyrins can aggregate severely in well-solubilizing organic electrolytes, that aggregation can be easily modulated through experimental conditions, and that the extent of aggregation must be considered for accurate catalytic benchmarking.

Cascade Radical Pathway-Enabled Nitrogen-Sulfur Coupling: Access to Isothiazolo[3,4-b]-meso-tetraarylporphyrins

A catalyst-free radical-mediated domino strategy for the construction of isothiazolo[3,4-b]-meso-tetraarylporphyrins was developed. During the course of the reaction, 2-benzothioylamino-3-thioformyl-meso-tetraarylporphyrins generated in situ after the addition of Lawesson's reagent to a solution of 2-benzoylamino-3-formyl-meso-tetraarylporphyrins in refluxing toluene underwent a homolytic cleavage to produce nitrogen-sulfur radicals. Subsequently, the formation of a new N-S bond through an intramolecular cascade radical coupling provided direct access to novel β-isothiazole-fused porphyrins.

A divergent one-pot thiol-Michael strategy to create β-thiophene-fused porphyrins

A divergent one-pot domino strategy for the synthesis of nickel(II) and copper(II) β-thiophene-fused 5,10,15,20-tetraarylporphyrins was developed through a thiol-Michael addition of thioglycolic/thiolactic acid to the corresponding 2-iminoporphyrins, formed in situ after the reaction of nickel(II) and copper(II) 2-formyl-5,10,15,20-tetraarylporphyrins with sterically hindered tert-butylamine in 1,2-dichloroethane at 80 °C. Interestingly, the reaction of 2-formylporphyrins with comparatively less sterically hindered primary amines and thioglycolic acid afforded a mixture of β-substituted porphyrinic thiazolidinones and β-thiophene-fused porphyrins. A similar one-pot thiol-Michael protocol was applied to construct a novel free-base thieno[2,3-b]-meso-tetrakis(4-methoxyphenyl)porphyrin, which underwent zinc insertion by using zinc acetate in a CHCl3-MeOH mixture and afforded zinc(II) β-thiophene-fused meso-tetrakis(4-methoxyphenyl)porphyrin in an appreciable isolated yield. On photophysical evaluation, these new porphyrins displayed a modest bathochromically shifted electronic absorption in contrast to meso-tetraarylporphyrin building blocks.

Consequences of Vibrational Strong Coupling on Supramolecular Polymerization of Porphyrins

Supramolecular polymers display interesting optoelectronic properties and, thus, deploy multiple applications based on their molecular arrangement. However, controlling supramolecular interactions to achieve a desirable molecular organization is not straightforward. Over the past decade, light-matter strong coupling has emerged as a new tool for modifying chemical and material properties. This novel approach has also been shown to alter the morphology of supramolecular organization by coupling the vibrational bands of solute and solvent to the optical modes of a Fabry-Perot cavity (vibrational strong coupling, VSC). Here, we study the effect of VSC on the supramolecular polymerization of chiral zinc-porphyrins (S-Zn) via a cooperative effect. Electronic circular dichroism (ECD) measurements indicate that the elongation temperature (Te) of the supramolecular polymerization is lowered by ∼10 °C under VSC. We have also generalized this effect by exploring other supramolecular systems under strong coupling conditions. The results indicate that the solute-solvent interactions are modified under VSC, which destabilizes the nuclei of the supramolecular polymer at higher temperatures. These findings demonstrate that the VSC can indeed be used as a tool to control the energy landscape of supramolecular polymerization. Furthermore, we use this unique approach to switch between the states formed under ON- and OFF-resonance conditions, achieved by simply tuning the optical cavity in and out of resonance.

A Nanoscale Trans-Platinum(II)-Based Supramolecular Coordination Self-Assembly with a Distinct Anticancer Mechanism

Drug resistance significantly hampers the clinical application of existing platinum-based anticancer drugs. New platinum medications that possess distinct mechanisms of action are highly desired for the treatment of Pt-resistant cancers. Herein, a nanoscale trans-platinum(II)-based supramolecular coordination self-assembly (Pt-TCPP-BA) is prepared via using trans-[PtCl2(pyridine)(NH3)] (transpyroplatin), tetracarboxylporphyrin (TCPP), and benzoic acid (BA) as building blocks to combat drug resistance in platinum-based chemotherapy. Mechanistic studies indicate that Pt-TCPP-BA shows a hydrogen-peroxide-responsive dissociation behavior along with the generation of bioactive trans-Pt(II) and TCPP-Pt species. Different from cisplatin, these degradation products interact with DNA via interstrand cross-links and small groove binding, and induce significant upregulation of cell-death-related proteins such as p53, cleaved caspase 3, p21, and phosphorylated H2A histone family member X in cisplatin-resistant cancer cells. As a result, Pt-TCPP-BA exhibits potent killing effects against Pt-resistant tumors both in vitro and in vivo. Overall, this work not only provides a new platinum drug for combating drug-resistant cancer but also offers a new paradigm for the development of platinum-based supramolecular anticancer drugs.

Supramolecular copolymerization of hydrophobic and hydrophilic monomers in liquid crystalline media

In the case of covalent polymers, immiscible polymers can be integrated by covalently linking them together, but such a strategy is not possible in supramolecular polymers. Here we report the supramolecular copolymerization of two porphyrin-based monomers, C10P2H and TEGPCu with side chains bearing cyanobiphenyl (CB) groups at the ends of hydrophobic alkyl or hydrophilic tetraethylene glycol chains, respectively. These monomers undergo self-sorting supramolecular polymerization in highly diluted solutions ([monomer] = 3.4 × 10-9 mol% (2.0 × 10-8 mol L-1)) in nonpolar media due to the incompatibility of the side chains. Surprisingly, these monomers undergo supramolecular copolymerization under high concentration conditions ([monomer] = 7.7 mol%) in the medium of 4-cyano-4'-pentyloxybiphenyl (5OCB) to form a columnar liquid crystalline phase under thermodynamic conditions, where the individual columns are composed of supramolecular block copolymers. The combination of CB ends of both monomers and the 5OCB medium is essential for the two monomers to form an integrated structure in a condensed system without phase separation.

Synthesis and Direct Observation of Chiral Supramolecular Polymer of Porphyrin Having Cholesteryl Groups

We report the synthesis and microscopic investigations of two chiral helical porphyrin supramolecular polymers with different coordinating metals that are expected to be capable of serving as synthetic macromolecular motors driven by thermal fluctuations. Furthermore, based on their microscopic images, we propose a stepwise process for the formation of higher-order structures. These porphyrins formed completely different association states, and this was reflected in the marked differences in the shapes of the supramolecular polymers. The Cu-TChOAlaCPP supramolecular polymers formed H-aggregate rods in diisopropyl ether, then grew into superhelices and then into ribbons. On the other hand, Zn-TChOAlaCPP supramolecular polymers formed aggregates based on van der Waals interactions in diethyl ether, then grew into fibers and then grew into multiple-helices and ribbons. In addition, we imaged the interaction between long and short chains of the Cu-TChOAlaCPP supramolecular polymer by fast-scanning atomic force microscopy, and we indicated the availability as a macromolecular motor driven by thermal fluctuations.

Exploring Linker-Group-Guided Self-Assembly of Ultrathin 2D Supramolecular Nanosheets in Water for Synergistic Cancer Phototherapy

Water is ubiquitous in natural systems where it builds an essential environment supporting biological supramolecular polymers to function, transport, and exchange. However, this extreme polar environment becomes a hindrance for the superhydrophobic functional π-conjugated molecules, causing significant negative impacts on regulating their aggregation pathways, structures, and properties of the subsequently assembled nanomaterials. It especially makes the self-assembly of ultrathin two-dimensional (2D) functional nanomaterials by π-conjugated molecules a grand challenge in water, although ultrathin 2D functional nanomaterials have exhibited unique and superior properties. Herein, we demonstrate the organic solvent-free self-assembly of one-molecule-thick 2D nanosheets based on exploring how side chain modifications rule the aggregation behaviors of π-conjugated macrocycles in water. Through an in-depth understanding of the roles of linking groups for side chains on affecting the aggregation behaviors of porphyrins in water, the regulation of molecular arrangement in the aggregated state (H- or J-type aggregation) was attained. Moreover, by arranging ionic porphyrins into 2D single layers through J-aggregation, the ultrathin nanosheets (thickness ≈ 2 nm) with excellent solubility and stability were self-assembled in pure water, which demonstrated both outstanding 1O2 generation and photothermal capability. The ultrathin nanosheets were further investigated as metal- and carrier-free nanodrugs for synergetic phototherapies of cancers both in vitro and in vivo, which are highly desirable by combining the advantages and avoiding the disadvantages of the single use of PDT or PTT.

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.

Simulating Assembly Landscapes for Comprehensive Understanding of Supramolecular Polymer-Solvent Systems

Complexity in supramolecular polymer systems arises from interactions between different components, including solvent molecules. By varying their concentration or temperature in such multicomponent systems, complex phenomena can occur such as thermally bisignate and dilution-induced assembly of supramolecular polymers. Herein, we demonstrate that both these phenomena emerge from the same underlying interaction mechanism between the components. As a model system, amide-decorated supramolecular polymers of porphyrins were investigated in combination with aliphatic alcohols as hydrogen-bond scavengers, and thermodynamic mass-balance models were applied to map the three-dimensional assembly landscapes. These studies unveiled that the interaction between hydrogen-bond scavengers and monomers is temperature-dependent and becomes dominant at high monomer concentrations. With these insights, we could exploit competitive monomer-alcohol interactions to prompt the dilution-induced assembly of various common monomers as well as bisignate assembly events. Moreover, kinetic insights were obtained by navigating through the assembly landscape. Similar to phase diagrams of covalent polymers, these assembly landscapes provide a comprehensive picture of supramolecular polymerizations, which helps to precisely regulate the system properties. The generality of this approach using assembly landscapes makes it relevant for any supramolecular system, and this enhanced control will open the door to build complex and functional supramolecular polymer systems.

A sensing platform based on zinc-porphyrin derinative in hexadecyl trimethyl ammonium bromide (CTAB) microemulsion for highly sensitive detection of theophylline

A new porphyrin-based sensing platform in hexadecyl trimethyl ammonium bromide (CTAB) microemulsion is developed for highly sensitive detection of theophylline. In this sensing system, the zinc-porphyrin-cinnamic acid conjugate (Zn-TPPCA) works as fluorescence probe while theophylline can decrease fluorescence intensity of the probe. Further studies indicate the linear relationship between the fluorescence quenching value and the concentration of theophylline within a given range. And the introduction of CTAB microemulsion can greatly enhance sensibility and stability of this detecting system and facilitate the detection of theophylline. On the basis above, a highly sensitive sensing platform for theophylline is created with a low limit of detection (LOD) of 0.0083 μg mL^-1 under the optimal detection conditions. And further application of this method in determination of commercially available theophylline preparation shows excellent results. Subsequent studies on quenching mechanism indicate that static quenching appears between Zn-TPPCA and theophylline. Therefore, this work provides not only a highly sensitive method for determination of theophylline but also further evidence for creation of biosensors for drugs with porphyrin derivatives.

Ionic supramolecular polymerization of water-soluble porphyrins: balancing ionic attraction and steric repulsion to govern stacking

We have synthesized novel water-soluble anionic porphyrin monomers that undergo pH-regulated ionic supramolecular polymerization in aqueous media. By tuning the total charge of the monomer, we selectively produced two different supramolecular polymers: J- and H-stacked. The main driving force toward the J-aggregated supramolecular polymers was the ionic interactions between the sulfonate and protonated pyrrole groups, ultimately affording neutral supramolecular polymers. In these J-aggregated supramolecular polymers, amide groups were aligned regularly along polymer wedges, which further assembled in an edge-to-edge manner to afford nanosheets. In contrast, the H-aggregated supramolecular polymers remained anionic, with their amide NH moieties acting as anion receptors along the polymer chains, thereby minimizing repulsion. For both polymers, varying the steric bulk of the peripheral ethylene glycol (EG) units controlled the rates of self-assembly as well as the degrees of polymerization. This steric effect was further tunable, depending on the solvation state of the EG chains. Accordingly, this new family of supramolecular polymers was created by taking advantage of unique driving forces that depended on both the pH and solvent.

Optimal acceleration voltage for near-atomic resolution imaging of layer-stacked 2D polymer thin films

Despite superb instrumental resolution in modern transmission electron microscopes (TEM), high-resolution imaging of organic two-dimensional (2D) materials is a formidable task. Here, we present that the appropriate selection of the incident electron energy plays a crucial role in reducing the gap between achievable resolution in the image and the instrumental limit. Among a broad range of electron acceleration voltages (300 kV, 200 kV, 120 kV, and 80 kV) tested, we found that the highest resolution in the HRTEM image is achieved at 120 kV, which is 1.9 Å. In two imine-based 2D polymer thin films, unexpected molecular interstitial defects were unraveled. Their structural nature is identified with the aid of quantum mechanical calculations. Furthermore, the increased image resolution and enhanced image contrast at 120 kV enabled the detection of functional groups at the pore interfaces. The experimental setup has also been employed for an amorphous organic 2D material.

High-resolution imaging of organic 2D materials using transmission electron microscopes is challenging. Here, the authors find the optimal electron acceleration voltage, and demonstrate 1.9 Å resolution, enabling detection of interstitial defects and functional groups in 2D polymer thin films.

Using transient equilibria (TREQ) to measure the thermodynamics of slowly assembling supramolecular systems

Supramolecular chemistry involves the noncovalent assembly of monomers into materials with unique properties and wide-ranging applications. Thermal analysis is a key analytical tool in this field, as it provides quantitative thermodynamic information on both the structural stability and nature of the underlying molecular interactions. However, there exist many supramolecular systems whose kinetics are so slow that the thermodynamic methods currently applied are unreliable or fail completely. We have developed a simple and rapid spectroscopic method for extracting accurate thermodynamic parameters from these systems. It is based on repeatedly raising and lowering the temperature during assembly and identifying the points of transient equilibrium as they are passed on the up- and down-scans. In a proof-of-principle application to the coassembly of polydeoxyadenosine (polyA) containing 15 adenosines and cyanuric acid (CA), we found that roughly 30% of the CA binding sites on the polyA chains were unoccupied, with implications for high-valence systems.

Supramolecular Chirality in Porphyrin Self-assembly Systems in Aqueous Solution

Abstract:

The self-assembly process appears as a powerful and attractive strategy for constructing complex supramolecules by the spontaneous organization of appropriate building blocks. In this scenario, water-soluble porphyrinoids lend themselves as ideal paradigms to disclose the self-assembly phenomenon by exploiting their well-known tendency to build aggregates in aqueous media via weak non-covalent forces. Nevertheless, the spontaneous organization of achiral porphyrins can result in a final chiral superstructure moving away from single-molecule behaviour to supramolecular chirality. Therefore, over the years numerous attempts have been implemented to investigate how a porphyrin aggregate, made up of achiral monomers, becomes not-symmetric and which processes govern the bias for a certain enantiomeric assembly rather than another. Thus, in this mini-review, we exclusively discuss the main strategies for designing and building chiral aggregates in water from achiral porphyrin monomers, with particular regard to their chiroptical features.

Guanine-Based G-Quadruplexes Templated by Various Cations toward Potential Use as Single-Ion Conductors

Inspired by the atomic-sized, shape-regulated features of G-quadruplexes comprising guanine motifs with a monovalent metal cation, the G-quadruplex-forming ability, and properties of a guanine-based π-conjugated Y2 molecule containing bithiophene and peripheral dodecyl chain units in the presence of various cation salts (Li⁺ , Na⁺ , K⁺ , and Mg²⁺ ) were exploited. A series of structural characterization revealed that Y2 yielded desirable G-quadruplexes with all the tested cations as a consequence of the combination of a hydrogen-bonded cyclic G-quartet, π-stacking, and cation-dipole interactions. The radius and nature of the coordinating cations crucially affected the structural characteristics of G-quadruplexes, leading to variations in the ion migration ability inside the cavity of the G-quadruplex (Li⁺ >Na⁺ >K⁺ >Mg²⁺ ), as characterized through theoretical and experimental investigations. These results not only improve the understanding of G-quadruplex self-assemblies based on guanine but also provide an impetus for their diverse potential applications, especially in the field of Li batteries.

Novel Aggregation-Enhanced PEC Photosensitizer Based on Electrostatic Linkage of Ionic Liquid with Protoporphyrin IX for Ultrasensitive Detection of Molt-4 Cells

Nowadays, aggregation quenching of most organic photosensitizers in aqueous media seriously restricts analytical and biomedical applications of photoelectrochemical (PEC) sensors. In this work, an aggregation-enhanced PEC photosensitizer was prepared by electrostatically bonding protoporphyrin IX (PPIX) with an ionic liquid of 1-butyl-3-methylimidazole tetrafluoroborate ([BMIm][BF₄]), termed as PPIX-[BMIm] for clarity. The resultant PPIX-[BMIm] showed weak photocurrent in pure dimethyl sulfoxide (DMSO, good solvent), while the PEC signals displayed a 44.1-fold enhancement in a water (poor solvent)/DMSO binary solvent with a water fraction (f_w) of 90%. Such PEC-enhanced mechanism was critically studied by electrochemistry and density functional theory (DFT) calculation in some detail. Afterward, a label-free PEC cytosensor was built for ultrasensitive bioassay of acute lymphoblastic leukemia (molt-4) cells by electrodepositing Au nanoparticles (Au NPs) on the PPIX-[BMIm] aggregates and sequential assembly of protein tyrosine kinase (PTK) aptamer DNA (aptDNA). The resultant cytosensor showed a wide linear range (300 to 3 × 10⁵ cells mL^-1) with a limit of detection (LOD) as low as 63 cells mL^-1. The aggregation-enhanced PEC performance offers a valuable and practical pathway for synthesis of advanced organic photosensitizer to explore its PEC applications in early diagnosis of tumors.

Controlling the length of porphyrin supramolecular polymers via coupled equilibria and dilution-induced supramolecular polymerization

Multi-component systems often display convoluted behavior, pathway complexity and coupled equilibria. In recent years, several ways to control complex systems by manipulating the subtle balances of interaction energies between the individual components have been explored and thereby shifting the equilibrium between different aggregate states. Here we show the enantioselective chain-capping and dilution-induced supramolecular polymerization with a Zn²⁺-porphyrin-based supramolecular system when going from long, highly cooperative supramolecular polymers to short, disordered aggregates by adding a monotopic Mn³⁺-porphyrin monomer. When mixing the zinc and manganese centered monomers, the Mn³⁺-porphyrins act as chain-cappers for Zn²⁺-porphyrin supramolecular polymers, effectively hindering growth of the copolymer and reducing the length. Upon dilution, the interaction between chain-capper and monomers weakens as the equilibria shift and long supramolecular polymers form again. This dynamic modulation of aggregate morphology and length is achieved through enantioselectivity in the aggregation pathways and concentration-sensitive equilibria. All-atom and coarse-grained molecular simulations provide further insights into the mixing of the species and their exchange dynamics. Our combined experimental and theoretical approach allows for precise control of molecular self-assembly and chiral discrimination in complex systems.

Sub-5 nm homeotropically aligned columnar structures of hybrids constructed by porphyrin and oligo(dimethylsiloxane)

A series of tetraphenylporphyrin-based thermotropic liquid crystals containing oligo(dimethylsiloxane) were synthesized. These disc-coil hybrids form ordered nanostructures with periodic sizes on the sub-5 nm scale, including oblique columnar, lamellar, and hexagonal columnar phases. Films with sub-5 nm line patterns and homeotropically aligned columnar structures can be obtained by substrate-induced self-assembly.

Rational Design of Chiral Nanohelices from Self-Assembly of Meso-tetrakis (4-Carboxyphenyl) Porphyrin-Amino Acid Conjugates

In this article, meso-tetrakis (4-carboxyphenyl) porphyrins modified with different amino acids were designed, synthesized, and researched. The chiral self-assembly behavior of these porphyrin-amino acid molecules can be precisely controlled by adjusting the pH, constituent amino acids, and temperature, thereby giving rise to chiral nanostructures with precisely tailored helical pitch and handedness. This research provides a certain reference for the design and preparation of chiral nanomaterials and has potential application prospects in chiral resolution and chiral catalysis.

Consequences of Amide Connectivity in the Supramolecular Polymerization of Porphyrins: Spectroscopic Observations Rationalized by Theoretical Modelling

The correlation between molecular structure and mechanism of supramolecular polymerizations is a topic of great interest, with a special focus on the pathway complexity of porphyrin assemblies. Their cooperative polymerization typically yields highly ordered, long 1D polymers and is driven by a combination of π-stacking due to solvophobic effects and hydrogen bonding interactions. Subtle changes in molecular structure, however, have significant influence on the cooperativity factor and yield different aggregate types (J- versus H-aggregates) of different lengths. In this study, the influence of amide connectivity on the self-assembly behavior of porphyrin-based supramolecular monomers was investigated. While in nonpolar solvents, C=O centered monomers readily assemble into helical supramolecular polymers via a cooperative mechanism, their NH centered counterparts form short, non-helical J-type aggregates via an isodesmic pathway. A combination of spectroscopy and density functional theory modelling sheds light on the molecular origins causing this stunning difference in assembly properties and demonstrates the importance of molecular connectivity in the design of supramolecular systems. Finally, their mutual interference in copolymerization experiments is presented.

A soft on/off switch based on the electrochemically reversible H-J interconversion of a floating porphyrin membrane

Soft molecular assemblies that respond reversibly to external stimuli are attractive materials as on/off switches, in optoelectronic, memory and sensor technologies. In this Edge Article, we present the reversible structural rearrangement of a soft porphyrin membrane under an electrical potential stimulus in the absence of solid-state architectures. The free-floating porphyrin membrane lies at the interface between immiscible aqueous and organic electrolyte solutions and is formed through interfacial self-assembly of zinc(ii) meso-tetrakis(4-carboxyphenyl)porphyrins (ZnPor). A potential difference between the two immiscible electrolyte solutions induces the intercalation of bis(triphenylphosphoranylidene)ammonium cations from the organic electrolyte that exchange with protons in the porphyrin membrane. In situ UV/vis absorbance spectroscopy shows that this ionic intercalation and exchange induces a structural interconversion of the individual porphyrin molecules in the membrane from an H- to a J-type molecular configuration. These structural rearrangements are reversible over 30 potential cycles. In situ polarisation-modulation fluorescence spectroscopy further provides clear evidence of structural interconversion of the porphyrin membrane, as intercalation of the organic electrolyte cations significantly affects the latter's emissive properties. By adjusting the pH of the aqueous phase, additional control of the electrochemically reversible structural interconversion can be achieved, with total suppression at pH 3.

Bioinspired Applications of Porphyrin Derivatives

Porphyrin derivatives are ubiquitous in nature and have important biological roles, such as in light harvesting, oxygen transport, and catalysis. Owing to their intrinsic π-conjugated structure, porphyrin derivatives exhibit characteristic photophysical and electrochemical properties. In biological systems, porphyrin derivatives are associated with various protein molecules through noncovalent interactions. For example, hemoglobin, which is responsible for oxygen transport in most vertebrates, consists of four subunits of a globular protein with an iron porphyrin derivative prosthetic group. Furthermore, noncovalently arranged porphyrin derivatives are the fundamental chromophores in light-harvesting systems for photosynthesis in plants and algae. These biologically important roles originate from the functional versatility of porphyrin derivatives. Specifically, porphyrins are excellent host compounds, forming coordination complexes with various metal ions that adds functionality to the porphyrin unit, such as redox activity and additional ligand binding at the central metal ion. In addition, porphyrins are useful building blocks for functional supramolecular assemblies because of their flat and symmetrical molecular architectures, and their excellent photophysical properties are typically utilized for the fabrication of bioactive functional materials. In this Account, we summarize our endeavors over the past decade to develop functional materials based on porphyrin derivatives using bioinspired approaches. In the first section, we discuss several synthetic receptors that act as artificial allosteric host systems and can be used for the selective detection of various chemicals, such as cyanide, chloride, and amino acids. In the second section, we introduce multiporphyrin arrays as mimics of natural light-harvesting complexes. The active control of energy transfer processes by additional guest binding and the fabrication of organic photovoltaic devices using porphyrin derivatives are also introduced. In the third section, we introduce several types of porphyrin-based supramolecular assemblies. Through noncovalent interactions such as metal-ligand interaction, hydrogen bonding, and π-π interaction, porphyrin derivatives were constructed as supramolecular polymers with formation of fiber or toroidal assembly. In the last section, the application of porphyrin derivatives for biomedical nanodevice fabrication is introduced. Even though porphyrins were good candidates as photosensitizers for photodynamic therapy, they have limitations for biomedical application owing to aggregation in aqueous media. We suggested ionic dendrimer porphyrins and they showed excellent photodynamic therapy (PDT) efficacy.

Porphyrin-based supramolecular assemblies and their applications in NLO and PDT

Tetrapyrrolic systems largely inspired by nature have attracted much attention in organic electronics and biomedical applications owing to their planar structure and extended [Formula: see text]-conjugated double bonds. As a result, delocalization of [Formula: see text]-electron cloud leads the excellent optical absorption and fluorescent properties. Nonetheless, the utilization of non-covalent interactions result in the self-assembled nanostructures providing applications in bioimaging and electronics. In this review, it is demonstrated that the recent reports on the self-assembly in tetrapyrrolic systems via supramolecular interactions lead to well-defined nanoarchitectures. Moreover, the importance of porphyrin based derivatives in nanoelectronics and chemotherapeutic applications is reported. Therefore, the inclination of tetrapyrroles towards the design and development of novel supramolecular nanostructures are considered the hallmark for nanorobotics, shape memory polymers and bionic arms.

Porphyrins Through the Looking Glass: Spectroscopic and Mechanistic Insights in Supramolecular Chirogenesis of New Self-Assembled Porphyrin Derivatives

The solvent driven aggregation of porphyrin derivatives, covalently linked to a L- or D-prolinate enantiomer, results in the stereospecific formation of species featuring remarkable supramolecular chirality, as a consequence of reading and amplification of the stereochemical information stored in the proline-appended group. Spectroscopic, kinetic, and topographic SEM studies gave important information on the aggregation processes, and on the structures of the final chiral architectures. The results obtained may be the seeds for the construction of stereoselective sensors aiming at the detection, for example, of novel emergent pollutants from agrochemical, food, and pharmaceutical industry.

The Self-Aggregation of Porphyrins with Multiple Chiral Centers in Organic/Aqueous Media: The Case of Sugar- and Steroid-Porphyrin Conjugates

An overview of the solvent-driven aggregation of a series of chiral porphyrin derivatives studied by optical methods (UV/Vis, fluorescence, CD and RLS spectroscopies) is herein reported. The investigated porphyrins are characterized by the presence in the meso-positions of glycol-, steroidal- and glucosteroidal moieties, conferring amphiphilicity and solubility in aqueous media to the primarily hydrophobic porphyrin platform. Aggregation of the macrocycles is driven by a change in bulk solvent composition, forming architectures with supramolecular chirality, steered by the stereogenic centers on the porphyrin peripheral positions. The aggregation behavior and chiroptical properties of the final aggregated species strongly depend on the number and stereogenicity of the ancillary groups that dictate the mutual spatial arrangement of the porphyrin chromophores and their further organization in larger structures, usually detectable by different microscopies, such as AFM and SEM. Kinetic studies are fundamental to understand the aggregation mechanism, which is frequently found to be dependent on the substrate concentration. Additionally, Molecular Mechanics calculations can give insights into the intimate nature of the driving forces governing the self-assembly process. The critical use of these combined methods can shed light on the overall self-assembly process of chirally-functionalized macrocycles, with important implications on the development of chiral porphyrin-based materials.

Bioactivity studies of porphyrinoids against microsporidia isolated from honeybees

Microsporidian infections are dangerous to honeybees due to the absence of an efficient treatment for nosemosis. In the present work, the abilities of several porphyrins to directly inactivate microsporidia derived from Nosema-infected honeybees were studied in vitro. Amide derivatives of protoporphyrin IX (PPIX) conjugated with one and two amino acid moieties were synthesized, and their activities were compared with those of two cationic porphyrins, TMePyP and TTMePP. The most active porphyrins, PP[Lys-Asp]₂, PP[Lys-TFA]₂, PP[Asp(ONa)₂]₂ and PP[Lys-Lys]₂ at concentrations as low as 10–50 µM exerted significant effects on microsporidia, reducing the number of spores by 67–80% compared to the control. Live-cell imaging of the spores treated with porphyrins showed that only 1.6% and 3.0% of spores remained alive after 24 h-incubation with 50 µM PP[Asp(ONa)₂]₂ and PP[Lys-Asp]₂, respectively. The length of the amino acid side chains and their identity in the PPIX molecules affected the bioactivity of the porphyrin. Importantly, the irradiation of the porphyrins did not enhance their potency in destroying Nosema spores. We showed that the porphyrins accumulated inside the living spores but not inside dead spores, thus the destruction of the microsporidia by non-metallated porphyrins is not dependent on photosensitization, but is associated with their active transport into the spore cell. When administered to honeybees in vivo, PPIX[Lys-TFA]₂ and PPIX[Lys-Lys]₂ reduced spore loads by 69–76% in infected individuals. They both had no toxic effect on honeybees, in contrast to zinc-coordinated porphyrin.

Highly Efficient and Tunable Filtering of Electrons' Spin by Supramolecular Chirality of Nanofiber-Based Materials

Organic semiconductors and organic-inorganic hybrids are promising materials for spintronic-based memory devices. Recently, an alternative route to organic spintronic based on chiral-induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical π-conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin-filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both "up" and "down" orientations of filtered spins can be obtained in a single enantiopure system via the temperature-dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure-property relationship for spintronic applications.

Covalently Linked Bis(Amido-Corroles): Inter- and Intramolecular Hydrogen-Bond-Driven Supramolecular Assembly

Four bis-corroles linked by diamide bridges were synthesized through peptide-type coupling of a trans-A₂ B-corrole acid with aliphatic and aromatic diamines. In the solid state, the hydrogen-bond pattern in these bis-corroles is strongly affected by the type of solvent used in the crystallization process. Although intramolecular hydrogen bonds play a decisive role, they are supported by intermolecular hydrogen bonds and weak N-H⋅⋅⋅π interactions between molecules of toluene and the corrole cores. In an analogy to mono(amido-corroles), both in crystalline state and in solutions, the aliphatic or aromatic bridge is located directly above the corrole ring. When either ethylenediamine or 2,3-diaminonaphthalene are used as linkers, incorporation of polar solvents into the crystalline lattice causes a roughly parallel orientation of the corrole rings. At the same time, both NHCO⋅⋅⋅NH corrole hydrogen bonds are intramolecular. In contrast, solvation in toluene causes a distortion with one of the hydrogen bonds being intermolecular. Interestingly, intramolecular hydrogen bonds are always formed between the -NHCO- functionality located further from the benzene ring present at the position 10-meso. In solution, the hydrogen-bonds pattern of the bis(amido-corroles) is strongly affected by the type of the solvent. Compared with toluene (strongly high-field shifted signals), DMSO and pyridine disrupt self-assembly, whereas hexafluoroisopropanol strengthens intramolecular hydrogen bonds.

Supramolecular polymerization: challenges and advantages of various methods in assessing the aggregation mechanism

Oligothiophenes with branched alkyl end groups show distinct aggregation in organic solvents. The process of supramolecular polymerization is assessed by three different methods (UV-vis absorption and fluorescence emission spectroscopy and dynamic light scattering) to exclude artifacts. An apparent dependence of the degree of aggregation on the concentration of the oligomers is observed. Above the upper limit of concentration (a lower micromolar range for the present class of compounds), experimental data delivered conflicting results and the concentration should not therefore be exceeded. Scanning force microscopy and molecular dynamics simulations confirm the formation of one-dimensional aggregates with presumably helical arrangement of the achiral monomers.

Reconfiguration of π-conjugated superstructures enabled by redox-assisted assembly

We show that n-doping supramolecular assemblies built from perylene diimide units provides a means to modulate the structure-function properties of these materials. In addition to highlighting the design principles, a combination of spectroscopic and microscopic characterization correlates an increase in the free-exciton bandwidth with the formation of mesoscale hierarchical superstructures.

Kinetic and spectroscopic studies on the chiral self-aggregation of amphiphilic zinc and copper (l)-prolinate-tetraarylporphyrin derivatives in different aqueous media

The chiral self-aggregation of (l)-proline porphyrin derivatives depends on both the nature of the media and the coordinated metal ion.

Energy-Landscape-Independent Kinetic Trap of an Incomplete Cage in the Self-Assembly of a Pd2 L4 Cage

A kinetic trap is the metastable species that is transiently or constantly produced during the reaction by trapping in a deep energy well. In most cases, the reactivity of kinetically trapped species is relatively low under the reaction conditions. Herein, we report another type of kinetically trapped species that is an incomplete cage (IC) intermediate produced during the self-assembly of a Pd₂ L₄ cage from ditopic ligand (L) and Pd^II ions with a certain lifetime, although IC has a high enough reactivity to be converted into the cage with the reaction of free L, which was confirmed by the reaction of the isolated IC and L under the self-assembly conditions. IC was kinetically trapped not because IC lies on the bottom of a deep energy well but because the conversion of the intermediates essential for the conversion of IC to the cage preferentially takes place; IC was kinetically trapped independently of the shape of the energy landscape of the self-assembly.

Competing Interactions in Hierarchical Porphyrin Self-Assembly Introduce Robustness in Pathway Complexity

Pathway complexity in supramolecular polymerization has recently sparked interest as a method to generate complex material behavior. The response of these systems relies on the existence of a metastable, kinetically trapped state. In this work, we show that strong switch-like behavior in supramolecular polymers can also be achieved through the introduction of competing aggregation pathways. This behavior is illustrated with the supramolecular polymerization of a porphyrin-based monomer at various concentrations, solvent compositions, and temperatures. It is found that the monomers aggregate via an isodesmic mechanism in weakly coupled J-type aggregates at intermediate solvent quality and temperature, followed by nucleated H-aggregates at lower solvent qualities and temperatures. At further increased thermodynamic driving forces, such as high concentration and low temperature, the H-aggregates can form hierarchical superhelices. Our mathematical models show that, contrary to a single-pathway polymerization, the existence of the isodesmic aggregation pathway buffers the free monomer pool and renders the nucleation of the H-aggregates insensitive to concentration changes in the limit of high concentrations. We also show that, at a given temperature or solvent quality, the thermodynamically stable aggregate morphology can be selected by controlling the remaining free external parameter. As a result, the judicious application of pathway complexity allows us to synthesize a diverse set of materials from only a single monomer. We envision that the engineering of competing pathways can increase the robustness in a wide variety of supramolecular polymer materials and lead to increasingly versatile applications.

Transforming a C 3-Symmetrical Liquid Crystal to a π-Gelator by Alkoxy Chain Variation

Rational understanding of the structural features involving different noncovalent interactions is necessary to design a liquid crystal (LC) or an organogelator. Herein, we report the effect of the number and positions of alkoxy chains on the self-assembly induced physical properties of a few π-conjugated molecules. For this purpose, we designed and synthesized three C ₃-symmetrical molecules based on oligo(p-phenylenevinylene), C ₃ OPV1-3. The self-assembly properties of these molecules are studied in the solid and solution states. All of the three molecules follow the isodesmic self-assembly pathway. Upon cooling from isotropic melt, C ₃ OPV1 having nine alkoxy chains (-OC₁₂H₂₅) formed a columnar phase with two-dimensional rectangular lattice and retained the LC phase even at room temperature. Interestingly, when one of the -OC₁₂H₂₅ groups from each of the end benzene rings is knocked out, the resultant molecule, C ₃ OPV2 lost the LC property, however, transformed as a gelator in toluene and n-decane. Surprisingly, when the -OC₁₂H₂₅ group from the middle position is removed, the resultant molecule C ₃ OPV3 failed to form either the LC or the gel phases.

Multiple Pathways in the Self-Assembly Process of a Pd4L8 Coordination Tetrahedron

The self-assembly of a Pd₄1₈ coordination tetrahedron (Tet) from a ditopic ligand, 1, and palladium(II) ions, [PdPy*₄]²⁺ (Py* = 3-chloropyridine), was investigated by a ¹H NMR-based quantitative approach (quantitative analysis of self-assembly process, QASAP), which allows one to monitor the average composition of the intermediates not observed by NMR spectroscopy. The self-assembly of Tet takes place mainly through three pathways and about half of the Tet structures were produced through the reaction of a kinetically produced Pd₃L₆ double-walled triangle (DWT) and 200-nm-sized large intermediates (Int^L). In two of the three pathways, the leaving ligand (Py*), which is not a component of Tet, catalytically assisted the self-assembly. Such a multiplicity of the self-assembly process of Tet suggests that molecular self-assembly takes place on an energy landscape like a protein-folding funnel.

Tunable Energy Landscapes to Control Pathway Complexity in Self-Assembled N-Heterotriangulenes: Living and Seeded Supramolecular Polymerization

Herein, the synthesis and self-assembling features of N-heterotriangulenes 1-3 decorated in their periphery with 3,4,5-trialkoxy-N-(alkoxy)benzamide moieties that enable kinetic control of the supramolecular polymerization process are described. The selection of an appropriate solvent results in a tunable energy landscape in which the relative energy of the different monomeric or aggregated species can be regulated. Thus, in a methylcyclohexane/toluene (MCH/Tol) mixture, intramolecular hydrogen-bonding interactions in the peripheral side units favor the formation of metastable inactivated monomers that evolve with time at precise conditions of concentration and temperature. A pathway complexity in the supramolecular polymerization of 1-3 cannot be determined in MCH/Tol mixtures but, importantly, this situation changes by using CCl₄ . In this solvent, the off-pathway product is a face-to-face H-type aggregate and the on-pathway product is the slipped face-to-face J-type aggregate. The autocatalytic transformation of the metastable monomeric units, as well as the two competing off- and on-pathway aggregates allow the realization of seeded and living supramolecular polymerizations. Interestingly, the presence of chiral, branched side chains in chiral (S)-2 noticeably retards the kinetics of the investigated transformations. This work brings to light the relevance of controlling the pathway complexity in self-assembling units and opens new avenues for the investigation of complex and functional supramolecular structures.