Porphyrin Template Research Articles

Large Enhancement of the Single-Molecular Conductance of a Molecular Wire through a Radical Substituent.

The single-molecular conductance between two π-conjugated wires with and without a radical substituent has been compared. Specifically, methyl- and iminonitroxide-substituted 4-(biphenyl-4-yl)pyridine wires bound onto a porphyrin template were subjected to scanning tunneling microscopy (STM) apparent-height measurement at the interface between highly oriented pyrolytic graphite (HOPG) and octan-1-oic acid. Statistical analysis of the STM images revealed that the radical-substituted wire has 3.2±1.7-fold higher conductance than the methyl-substituted reference. Although density functional theory (DFT) calculation suggests that only 17 % of the SOMO is distributed on the wire moiety, the effect was significant. This study presents the potential of radical substituents to achieve high conductivity in molecular wires.

Inside Cover: Assembly of Multi‐Phthalocyanines on a Porphyrin Template by Fourfold Rotaxane Formation (Chem. Eur. J. 35/2016)

Inside Cover: Assembly of Multi‐Phthalocyanines on a Porphyrin Template by Fourfold Rotaxane Formation (Chem. Eur. J. 35/2016)

Assembly of Multi-Phthalocyanines on a Porphyrin Template by Fourfold Rotaxane Formation.

A stacked assembly composed of a porphyrin and two phthalocyanines was prepared through fourfold rotaxane formation. Two phthalocyanine molecules, bearing four 24-crown-8 units, were assembled onto a porphyrin template incorporating four sidechains with two dialkylammonium ions each through pseudorotaxane formation between crown ether units and ammonium ions. The Staudinger phosphite reaction, as the stoppering reaction, resulted in the formation of the stacked heterotrimer composed of a porphyrin and two phthalocyanines connected through a fourfold rotaxane structure. UV/Vis spectroscopic and electrochemical studies of the heterotrimer indicated that there is a significant electronic interaction between the two phthalocyanine units due to the close stacking. The electrochemical oxidation process of the stacked heterotrimer was studied by cyclic voltammetry and spectroelectrochemistry. Electron paramagnetic resonance (EPR) spectroscopy of a dinuclear Cu(II) complex, in which two Cu(II) phthalocyanines were assembled on a metal-free porphyrin template, revealed that two Cu(II) phthalocyanines were located within the stacking distance, which resulted in an antiferromagnetic interaction between the two S=1/2 spins in the ground state of the Cu(2+) ions in the heterotrimer.

Charge-induced distortion and stabilization of surface transfer doped porphyrin films

The interaction between zinc-tetraphenylporphyrin (ZnTPP) and fullerenes (C60 and C60F48) are studied using ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy (STM). Low temperature STM reveals highly ordered ZnTPP monolayers on Au(111). In contrast to C60, a submonolayer coverage of C60F48 results in long-range disorder of the underlying single ZnTPP layer and distortion of individual ZnTPP molecules. This is induced by substantial charge transfer at the organic-organic interface, revealed by the interface energetics from UPS. However, a second layer of ZnTPP prevents C60F48 guests from breaking the self-assembled porphyrin template. This finding is important for understanding the growth behaviour of "bottom-up" functional nanostructures involving strong donor-acceptor heterojunctions in molecular electronics.

Porphyrin-templated synthetic G-quartet (PorphySQ): a second prototype of G-quartet-based G-quadruplex ligand

Template-assembled synthetic G-quartet (TASQ) has been reported recently as a G-quadruplex ligand interacting with DNA according to an unprecedented, nature-inspired 'like likes like' approach, based on the association between two G-quartets, one being native (quadruplex) and the other one artificial (ligand). Herein, a novel TASQ-based ligand is designed, synthesized and its quadruplex-recognition properties are evaluated in vitro: PorphySQ (for porphyrin-templated synthetic G-quartet) displays enhanced quadruplex recognition properties as compared to the very first reported prototype (DOTASQ, for DOTA-templated synthetic G-quartet), since the porphyrin template insures a more stable intramolecular G-quartet fold due to self-stabilizing interactions that may take place intramolecularly between the porphyrin ring and the formed G-quartet.

Fine‐Tuning Ligands for Catalysis Using Supramolecular Strategies

AbstractCoordinative bonds have been used to prepare supramolecular ligands leading to well‐defined catalysts formed by assembly. The construction of these ligands is based on selective metal–ligand interactions between nitrogen donor atoms of phosphorus‐nitrogen building blocks and various zinc(II) porphyrins. The major advantage of this supramolecular approach of catalyst preparation is the simplification of ligand variation enabling straightforward modification of steric, electronic and chiral properties of the supramolecular ligand. A large number of new ligands becomes accessible by this modular variation of the building blocks. The ligand assembly based on pyridyl phosphites and zinc(II) porphyrin with electron‐withdrawing substituents led to a twelve‐fold increase in activity and an increase in enantioselectivity from 17 to 50 % in the rhodium‐catalyzed hydrogenation of dimethyl itaconate. The first examples of assemblies based on non‐chiral ligands and chiral zinc(II) porphyrin template molecules show, as proof of principle, an enantiomeric excess up to 18 % in the asymmetric palladium‐catalyzed allylic alkylation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Water-soluble porphyrins as a dual-function molecular imaging platform for MRI and fluorescence zinc sensing

We report a molecular platform for dual-function fluorescence/MRI sensing of mobile zinc. Zinc-selective binding units were strategically attached to a water-soluble porphyrin template. The synthetic strategy for achieving the designed target ligand is flexible and convenient, and the key intermediates can be applied as general building blocks for the construction of other metal sensors based on a similar mechanism. The metal-free form, (DPA-C(2))(2)-TPPS(3) (1), where DPA is dipicolylamine and TPPS(3) is 5-phenyl-10,15,20-tris(4-sulfonatophenyl)porphine, is an excellent fluorescent sensor for zinc. It has certain superior physical properties compared with earlier-generation zinc sensors including emission in the red and near-IR regions [lambda(em) = 645 nm (s) and 715 nm (m)], with a large Stokes shift of >230 nm. The fluorescence intensity of 1 increases by >10-fold upon zinc binding. The fluorescence "turn-on" is highly selective for zinc versus other divalent metal ions and is relatively pH-insensitive within the biologically relevant pH window. The manganese derivative, [(DPA-C(2))(2)-TPPS(3)Mn(III)] (2), switches the function of the molecule to generate an MRI contrast agent. In the presence of zinc, the relaxivity of 2 in aqueous solution is significantly altered, which makes it a promising zinc MRI sensor. Both metal-free and Mn(III)-inserted forms are efficiently taken up by live cells, and the intracellular zinc can be imaged by either fluorescence or MR, respectively. We anticipate that in vivo applications of the probes will facilitate a deeper understanding of the physiological roles of zinc and allow detection of abnormal zinc homeostasis for pathological diagnoses.

Rigid bis-zinc(ii) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.

Cover Picture: A Conjugated Polycarbazole Ring around a Porphyrin (Angew. Chem. Int. Ed. 28/2006)

Like the rings of Saturn a fully conjugated cyclododeca‐2,7‐carbazole surrounds a porphyrin template. The macrocycle can be released from the template. The cover shows a single‐molecule STM measurement with the structure of cyclododeca‐2,7‐carbazole in the foreground. The tunneling current is dominated by the conjugated cyclic structure and shows an “electronic hole” in the center. For further details on these molecules and their self‐assembly into columnar stacks see the Communication by K. Müllen et al. on page 4685 ff.

Bidentate ligands formed by self-assembly.

A novel supramolecular strategy to prepare bidentate ligands via the assembly of functionalised monomeric ligands on a dimeric zinc(II) porphyrin template is presented; the assembled bidentate ligands show chelating behaviour and their rhodium complexes display enhanced selectivity in the hydroformylation compared to the non-template analogue.

Synthetic hosts by monomolecular imprinting inside dendrimers.

Synthetic host systems capable of selectively binding guest molecules are of interest for applications ranging from separations and chemical or biological sensing to the development of biomedical materials. Such host systems can be efficiently prepared by 'imprinting' polymers or inorganic materials with template molecules, which, upon removal, leave behind spatially arranged functional groups that act as recognition sites. However, molecularly imprinted polymers have limitations, including incomplete template removal, broad guest affinities and selectivities, and slow mass transfer. An alternative strategy for moulding desired recognition sites uses combinatorial libraries of assemblies that are made of a relatively small number of molecules, interconverting in dynamic equilibrium; upon addition of a target molecule, the library equilibrium shifts towards the best hosts. Here we describe the dynamic imprinting of dendritic macromolecules with porphyrin templates to yield synthetic host molecules containing one binding site each. The process is based on our general strategy to prepare cored dendrimers, and involves covalent attachment of dendrons to a porphyrin core, cross-linking of the end-groups of the dendrons, and removal of the porphyrin template by hydrolysis. In contrast to more traditional polymer imprinting, our approach ensures nearly homogeneous binding sites and quantitative template removal. Moreover, the hosts are soluble in common organic solvents and amenable to the incorporation of other functional groups, which should facilitate further development of this system for novel applications.

A Membrane Protein Model: Polypeptides with Four α-Helix Bundle Structure on 5,10,15,20-Tetrakis[2-(carboxymethoxy)phenyl]porphyrin

Abstract A novel template for the polypeptide–porphyrin hybrid, 5,10,15,20-tetrakis[2-(ethoxycarbonylmethoxy)phenyl]porphyrin (2) was synthesized in 17% yield. Four protected carboxyl groups are installed in 2 at the ortho-phenyl groups as the anchoring points, to which the N-termini of the polypeptides can be linked after the hydrolysis. Thus, 2 is a new analog of the porphyrin template tetrakis[2-(methoxycarbonyl)phenyl]porphyrin. The atropisomerization of 2 was much more rapid than that of tetrakis[2-(methoxycarbonyl)phenyl]porphyrin: At 353 K, 2 reached their equilibrium within 15 min. Such atropisomeric flexibility of 2 allows the self-organization of the α-helical peptide segments which are connected to 2. After the hydrolyses of the ethyl ester groups of 2, four α-helical 21-peptides, H–(–Gln–Leu–Leu–Gln–Ala–Leu–Ala–)3–NHCH2CH2OH were combined through the amide bonds to yield a new polypeptide–porphyrin hybrid (12). The polypeptide moieties of 12 showed typical α-helix profiles on circular dichroism measurements in methanol–water (3/1, v/v) and in Tris·HCl buffer solution. Thus, the four α-helix polypeptides are considered to fold into a bundle structure on the porphyrin template through the hydrophobic interaction. The circular dichroism spectrum of 12 in methanol showed an induced Cotton effect at the porphyrin region, and the CD band was symmetrically split. The hybrid (12) was also successfully incorporated into the egg yolk lecithin membrane almost quantitatively. When the molar ratio of egg yolk lecithin/12 was 500/1, the α-helix structure of the peptide segments in 12 was completely retained in the vesicles.

The roles of templates in the syntheses of porphyrin oligomers

4,4′-Bipyridyl is an efficient template for the formation of cyclic porphyrin dimer from a diethynyl porphyrin monomer; s-tri(4-pyridyl)triazine promotes the formation of cyclic trimer from the same starting material to a lesser degree whereas meso-tetra(4-pyridyl)porphyrin is not a practical template for the preparation of cyclic porphyrin tetramer from monomer. The factors responsible for these differences in template efficiency were elucidated by analysing the product distributions from coupling linear porphyrin dimer and tetramer in the presence of these and other ligands. The 4,4′-bipyridyl template increases the kinetic effective molarity for the cyclisation of linear dimer by a factor of 20. meso-Tetra(4-pyridyl)porphyrin is a good template for the cyclisation of linear porphyrin tetramer; it is also a good template for the preparation of cyclic tetramer from linear dimer and this is the best preparative route to cyclic tetramer. There are three roles for the meso-tetra(4-pyridyl)porphyrin template in the latter case: (i) as a negative template it prevents linear dimer cyclising to cyclic dimer, (ii) as a linear template it promotes the formation of linear tetramer from two linear dimers, and (iii) as a cyclisation template it accelerates the closure of linear tetramer to cyclic tetramer. All three roles were detected under optimum conditions but the second role is not significant when TMEDA is present during coupling.