Two‐Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion

Self‐assembly of porphyrin molecules can be controlled kinetically to form structures with lengths extending from the nano‐ to the micrometer scale, through a programmed solvent‐diffusion process in designed microflow spaces. Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.     

Exploring Supramolecular Self‐Assembly of Tetraarylporphyrins by Halogen Bonding: Crystal Engineering with Diversely Functionalized Six‐Coordinate Tin(L)2–Porphyrin Tectons

This study targets the construction of porphyrin assemblies directed by halogen bonds, by utilizing a series of purposely synthesized Sn(axial ligand)₂–(5,10,15,20‐tetraarylporphyrin) [Sn(L)₂‐TArP] complexes as building units. The porphyrin moiety and the axial ligands in these compounds contain different combinations of complimentary molecular recognition functions. The former bears p‐iodophenyl, p‐bromophenyl, 4′‐pyridyl, or 3′‐pyridyl substituents at the meso positions of the porphyrin ring. The latter comprises either a carboxylate or hydroxy anchor for attachment to the porphyrin‐inserted tin ion and a pyridyl‐, benzotriazole‐, or halophenyl‐type aromatic residue as the potential binding site. The various complexes were structurally analyzed by single‐crystal X‐ray diffraction, accompanied by computational modeling evaluations. Halogen‐bonding interactions between the lateral aryl substituents of one unit of the porphyrin complex and the axial ligands of neighboring moieties was successfully expressed in several of the resulting samples. Their occurrence is affected by structural (for example, specific geometry of the six‐coordinate complexes) and electronic effects (for example, charge densities and electrostatic potentials). The shortest intermolecular I???N halogen‐bonding distance of 2.991 Å was observed between iodophenyl (porphyrin) and benzotriazole (axial ligand) moieties. Manifestation of halogen bonds in these relatively bulky compounds without further activation of the halophenyl donor groups by electron‐withdrawing substituents is particularly remarkable.     

Controlling supramolecular polymerization through multicomponent self‐assembly

The self‐assembly into supramolecular polymers is a process driven by reversible non‐covalent interactions between monomers, and gives access to materials applications incorporating mechanical, biological, optical or electronic functionalities. Compared to the achievements in precision polymer synthesis via living and controlled covalent polymerization processes, supramolecular chemists have only just learned how to developed strategies that allow similar control over polymer length, (co)monomer sequence and morphology (random, alternating or blocked ordering). This highlight article discusses the unique opportunities that arise when coassembling multicomponent supramolecular polymers, and focusses on four strategies in order to control the polymer architecture, size, stability and its stimuli‐responsive properties: (1) end‐capping of supramolecular polymers, (2) biomimetic templated polymerization, (3) controlled selectivity and reactivity in supramolecular copolymerization, and (4) living supramolecular polymerization. In contrast to the traditional focus on equilibrium systems, our emphasis is also on the manipulation of self‐assembly kinetics of synthetic supramolecular systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 34–78     

Fabrication of Microspheres from Phthalimide‐Substituted Porphyrin Derivatives

Microspheres were fabricated from phthalimide‐substituted porphyrin derivatives. Microscopic analysis showed that the structures of the supramolecular assemblies synthesized in the present study were spherical, with diameters in the sub‐micrometer to micrometer range. The size of the microspheres could be controlled by changing the concentration of the casting solution. The spectroscopic properties of the microspheres were measured to determine the influence of their structural components. Thermal studies indicated that the temperature at which these structures became unstable was lower than the bulk melting point. During I–V measurements on devices composed of these microspheres, it was found that the current increased upon light irradiation, and the characteristic photoresponse properties of these devices were reproducible.     

Self‐Assembly of Anionic Porphyrins and Alkaline or Alkaline Earth Metal Ions Mediated by Cucurbit[7,8]uril

Nanoscaled coordination polymers based on biologically prevalent ions have potential applications in drug delivery and biomedical imaging. Herein, coordination polymer nanoparticles of anionic porphyrins, including meso‐tetra(4‐carboxyphenyl)‐porphyrin (H₂TCPP^4?) and meso‐tetra(4‐sulfonatophenyl)‐porphyrin (H₂TPPS^4?), and alkaline or alkaline earth metal cations, such as K⁺ and Ca²⁺, were constructed in aqueous solution in the presence of cucurbit[7]uril (CB7) or cucurbit[8]uril (CB8). UV/Vis absorption and fluorescence spectroscopy, dynamic light scattering (DLS), scanning electron spectroscopy (SEM), and atomic force microscopy (AFM) were applied to explore the assembly and particle formation of porphyrin anions and metal cations mediated by CBn. The particle size depends on the kinds of CBn and metal cations and their concentrations. The uptake of H₂TPPS^4? particles by tumor cells (A549 cells) was found to be more efficient than H₂TPPS^4? at 37 °C, showing the application potential of such assembled particles in biology and medicine.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

Unusual Stepwise Protonation and J‐Aggregation of meso‐Tetrakis(N‐methylpyridinium‐4‐yl)porphine on Binding Poly(sodium vinylsulfonate)

The binding of a tetracationic porphyrin to a highly charged polymer like poly(sodium vinylsulfonate) has been investigated over a wide pH range and under various experimental conditions. We present evidence that, depending on the pH, the high electrostatic field exerted by the polymer stabilizes the diprotonated form of the free base porphyrin at unusual pH values or otherwise causes the formation of H‐type aggregates. In particular, at a low polymer concentration, lowering the pH at first allows the formation of the diacid species then it determines its reorganization in close‐packed J‐type aggregates. The employment of various metallo‐derivatives of the title porphyrin enables a better insight into the nature of all the detected species.     

Coadsorption-induced reconstruction of supramolecular assembly characteristics.

A host supramolecular structure consisting of bis-(2,2':6',2"-terpyridine)-4'-oxyhexadecane (BT-O-C16) is shown to respond to coadsorbed molecules in dramatic ways, as observed by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface under ambient conditions. Interestingly, the lattice parameter of the triphenylene-filled complex differs significantly from that of the coronene-filled one, although the triphenylene and coronene molecules are nearly the same size. The STM study and density functional theory calculations reveal that intermolecular hydrogen-bond interactions play an essential role in forming the assembly structures. The different electronic properties of coronene and triphenylene molecules are responsible for the difference in lattice parameters and consequently for the difference in filling behaviors in the coronene/BT-O-C16 and triphenylene/BT-O-C16 binary systems.     

Supramolecular ssDNA Templated Porphyrin and Metalloporphyrin Nanoassemblies with Tunable Helicity

Free‐base and nickel porphyrin–diaminopurine conjugates were formed by hydrogen‐bond directed assembly on single‐stranded oligothymidine templates of different lengths into helical multiporphyrin nanoassemblies with highly modular structural and chiroptical properties. Large red‐shifts of the Soret band in the UV/Vis spectroscopy confirmed strong electronic coupling among assembled porphyrin–diaminopurine units. Slow annealing rates yielded preferentially right‐handed nanostructures, whereas fast annealing yielded left‐handed nanostructures. Time‐dependent DFT simulations of UV/Vis and CD spectra for model porphyrin clusters templated on the canonical B‐DNA and its enantiomeric form, were employed to confirm the origin of observed chiroptical properties and to assign the helicity of porphyrin nanoassemblies. Molar CD and CD anisotropy g factors of dialyzed templated porphyrin nanoassemblies showed very high chiroptical anisotropy. The DNA‐templated porphyrin nanoassemblies displayed high thermal and pH stability. The structure and handedness of all assemblies was preserved at temperatures up to +85 °C and pH between 3 and 12. High‐resolution transition electron microscopy confirmed formation of DNA‐templated nickel(II) porphyrin nanoassemblies and their self‐assembly into helical fibrils with micrometer lengths.     

Syntheses and energy transfer in multiporphyrinic arrays self-assembled with hydrogen-bonding recognition groups and comparison with covalent steroidal models

A number of new porphyrins equipped with complementary triple hydrogen-bonding groups were synthesized in good yields. Self-assembly was investigated by NMR spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM). These artificial antenna systems were further characterized by stationary and time-resolved fluorescence techniques to investigate several yet unsolved questions on the mechanism of excitation energy transfer (EET) in supramolecular systems. For example, the photophysics of a simple D--U[triple chemical bond]P--A dyad was studied, in which donor D and acceptor A are ZnII- metalated and free-base porphyrins, respectively, and U (uracyl) and P (2,6-diacetamidopyridyl) are complementary hydrogen-bonding groups linked by flexible spacers. In this dyad, the EET occurs with about 20 % efficiency with a lifetime of 14 ps. Reversal of the nonsymmetric triple hydrogen-bonding groups to give a A--U[triple chemical bond]P--D construct results in an EET efficiency of about 25 % and a lifetime of 19 ps. Thus, there is a slight directionality of EET mediated by these asymmetric triple hydrogen-bonding units tethered to flexible spacers. In polymeric systems of the type P-D-P[triple chemical bond]U-A-U[triple chemical bond]P-D-P, or U-D-U[triple chemical bond]P-A-P[triple chemical bond]U-D-U, the EET efficiency doubles as each donor is flanked by two acceptors. Because doubling the probability of photon capture doubles the EET efficiency, there is no energy amplification, which is consistent with the "antenna effect". For these polymeric systems, AFM images and DLS data indicate large rodlike assemblies of a few hundred nanometers, whereas the components form much smaller aggregates under the same conditions. To understand the importance of the flexible hydrogen-bonding zipper, three different covalently bridged D-B-A molecules were synthesized in which the bridge B is a rigid steroidal system and the same ester chemistry was used to link the porphyrins to each end of the steroid. The geometry inferred from molecular modeling of D-B-A indicates geometric similarities between B and some conformations of the --P[triple chemical bond]U-- supramolecular bridge. Although the EET efficiency is a factor of two greater for the steroidal systems relative to the supramolecular dyads, the rate is 50-80 times slower, but still slightly faster than that predicted by F?rster-type mechanisms. Circular dichrosim (CD) spectra provide a conformational sampling of the porphyrin groups appended on the steroidal skeleton, thus allowing an estimation of the orientation factor kappa for the transition dipole moments, which significantly affects the EET rate. We conclude that the flexible hydrogen-bonded linked systems are adaptive and have variable geometries with foldamers in which the D and A groups can approach well under 1 nm. In these folded conformations, a rapid EET process occurs, probably also involving a Dexter-type exchange mechanism, thus explaining the fast EET relative to the rigid steroidal compounds. This study predicts that it is indeed possible to build large supramolecular antennas and the component design and supramolecular dynamics are essential features that dictate EET rates and efficiencies.     

Copper-metallomesogen structures obtained by ionic self-assembly (ISA): molecular electromechanical switching driven by cooperativity

In a stepwise noncovalent multiple-interaction strategy, copper(II) salts were complexed with the sodium salts of bathophenanthrolinedisulfonic acid (BPS) and bathocuproinedisulfonic acid (BCS), and organized into nanostructured materials by the addition of ammonium surfactants by means of the ionic self-assembly (ISA) route. In the case of the methyl-substituted BCS complexes, a slow color change from green to brick red was observed. UV and EPR investigations showed that the color change was due to a change in oxidation state, the resulting brick red color is typical for Cu(I) species. It is concluded that steric interactions and mechanical packing into a supramolecular structure drive this electronic transition at the metal center. When complexation is performed with double-tail ammonium surfactants, these metallomesogenic materials exhibit thermotropic liquid-crystalline phase behavior, as investigated by polarized light microscopy, differential scanning calorimetry (DSC), and temperature-dependent wide-angle and small-angle X-ray analyses. The complexity of the observed phases increased with increasing tail length of the surfactants. Complexation with double-tail C(18) surfactants yielded highly organized materials for both the BPS and BCS ligands.