Johannes K. Sprafke

Vernier templating and synthesis of a 12-porphyrin nano-ring

Templates are widely used to arrange molecular components so they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods. Nature uses sophisticated templates such as the ribosome, whereas chemists use simple ions or small molecules. But as we tackle the synthesis of larger targets, we require larger templates—which themselves become synthetically challenging. Here we show that Vernier complexes can solve this problem: if the number of binding sites on the template, nT, is not a multiple of the number of binding sites on the molecular building blocks, nB, then small templates can direct the assembly of relatively large Vernier complexes where the number of binding sites in the product, nP, is the lowest common multiple of nB and nT (refs 8, 9). We illustrate the value of this concept for the covalent synthesis of challenging targets by using a simple six-site template to direct the synthesis of a 12-porphyrin nano-ring with a diameter of 4.7 nm, thus establishing Vernier templating as a powerful new strategy for the synthesis of large monodisperse macromolecules.

Modular Functionalization of Carbon Nanotubes and Fullerenes

A series of highly efficient, modular zwitterion-mediated transformations have been developed which enable diverse functionalization of carbon nanotubes (CNTs, both single-walled and multi-walled) and fullerenes. Three functionalization strategies are demonstrated. (1) Trapping the charged zwitterion intermediate with added nucleophiles allows a variety of functional groups to be installed on the fullerenes and carbon nanotubes in a one-pot reaction. (2) Varying the electrophile from dimethyl acetylenedicarboxylate to other disubstituted esters provides CNTs functionalized with chloroethyl, allyl, and propargyl groups, which can further undergo S(N)2 substitution, thiol addition, or 1,3-dipolar cycloaddition reactions. (3) Postfunctionalization transformations on the cyclopentenones (e.g., demethylation and saponification) of the CNTs lead to demethylated or hydrolyzed products, with high solubility in water (1.2 mg/mL for MWCNTs). CNT aqueous dispersions of the latter derivatives are stable for months and have been successfully utilized in preparation of CNT-poly(ethylene oxide) nanocomposite via electrospinning. Large-scale MWCNT (10 g) functionalization has also been demonstrated to show the scalability of the zwitterion reaction. In total we present a detailed account of diverse CNT functionalization under mild conditions (60 degrees C, no strong acids/bases, or high pressure) and with high efficiency (1 functional group per 10 carbon atoms for SWCNTs), which expand the utility of these materials.

Stepwise Effective Molarities in Porphyrin Oligomer Complexes: Preorganization Results in Exceptionally Strong Chelate Cooperativity

Complexes of zinc porphyrin oligomers with multivalent ligands can be denatured by adding a large excess of a monodentate ligand, such as quinuclidine. We have used denaturation titrations to determine the stabilities of the complexes of a cyclic zinc-porphyrin hexamer with multidentate ligands with two to six pyridyl coordination sites. The corresponding complexes of linear porphyrin oligomers were also investigated. The results reveal that the stepwise effective molarities (EMs) for the third through sixth intramolecular coordination events with the cyclic hexamer are extremely high (EM = 10(2)-10(3) M), whereas the values for the linear porphyrin oligomers are modest (EM ≈ 0.05 M). The speciation profiles for the denaturation reactions demonstrate that intermediate species are not significantly populated and that these equilibria are well described by a highly cooperative two-state model.

All‐or‐Nothing Cooperative Self‐Assembly of an Annulene Sandwich

Large p-conjugated macrocycles and annulenes are intriguing compounds for testing theories of electronic structure because of their high symmetry, lack of end-group effects, strain, and distorted p overlap. Some of these features lead to extraordinary linear and nonlinear optical properties as well as unusual host–guest chemistry based on interactions of curved p surfaces. With increasing ring size, these unique properties disappear because the rings become so flexible that they adopt polymer-like random-coil conformations. We recently introduced Vernier templating as a strategy for the synthesis of large monodisperse p-conjugated macrocycles, such as the [12]porphyrin nanoring c-P12 (Figure 1 a). Structural rigidification of such macrocycles by coordination to radial templates becomes challenging because of the inaccessibility of large rigid templates. Linear conjugated porphyrin oligomers bind bidentate ligands, such as 1,4diazabicyclo[2.2.2]octane (DABCO) and 4,4’-bipyridine, to form rigid, self-supported double strands. Planarization of the p systems in these ladder complexes enhances the twophoton absorption and the charge-carrier mobilities. Here we show that double-strand formation can also be used to lock the conformations of large p-conjugated macrocycles, as illustrated for c-P12 in Figure 1. The two simplest geometries providing complete p conjugation around an annulene are those in which the p orbitals are radial (as in carbon nanotubes) or axial (as in classical aromatic molecules). In principle, the formation of a double-strand complex offers a unique opportunity to switch between these two geometries, without altering the covalent structure. Recent STM imaging experiments indicated that porphyrin nanoring c-P12 lies flat on a gold(111) surface, with the porphyrin macrocycles parallel to the substrate, in the correct conformation to assemble a double-strand sandwich. Encouraged by this result, we tested the formation of (cP12)2·(DABCO)12 by adding DABCO to c-P12. When following this process by H NMR spectroscopy, we observed the appearance of a new species of lower symmetry which is in

Electronic and Mechanical Modification of Single-Walled Carbon Nanotubes by Binding to Porphyrin Oligomers

We report on the noncovalent binding of conjugated porphyrin oligomers to small diameter single-walled carbon nanotubes (SWNTs) and highlight two remarkable observations. First, the binding of the oligomers to SWNTs is so strong that it induces mechanical strain on the nanotubes in solution. The magnitudes of the strains are comparable to those found in solid-state studies. Comparable strains are not observed in any other SWNT-supramolecular complexes. Second, large decreases in polymer band gap with increasing length of the oligomer lead to the formation of a type-II heterojunction between long chain oligomers and small-diameter nanotubes. This is demonstrated by the observation of enhanced red-shifts for the nanotube interband transitions. These complexes offer considerable promise for photovoltaic devices.

Conformation and Packing of Porphyrin Polymer Chains Deposited Using Electrospray on a Gold Surface

Conjugated porphyrin polymers have stimulated great interest due to their potential applications in nonlinear optics, light harvesting and nanoscale charge transport. As with many other organic materials, interfacial properties are likely to play an important role in their applications in molecular electronics. However, it has not so far been possible to study these effects due to the difficulty in preparing suitable monolayers, since the relevant polymers and oligomers cannot be sublimed. A question of particular interest relates to the influence of the flexibility of such a large molecule on the ordering within interfacial regions. We have investigated the adsorption of two oligomers, a porphyrin tetramer (P4, N= 4; see Figure 1 for structural diagrams), a hexamer (P6, N= 6), and a polymer Pn (N = 30–50) on the Au(111) surface using scanning tunneling microscopy (STM). The porphyrin units have long octyloxy side chains to promote solubility in organic solvents. Our experiments are performed at room temperature under ultrahigh vacuum (UHV) conditions (base pressure 5 10 11 Torr) and we use UHV electrospray deposition (UHV-ESD) to transfer the oligomers and polymers directly from solution onto a surface. In our approach to UHV-ESD, a volatilized mixture of solvent and solute molecules is produced in atmosphere by electrospray. This mixture enters the UHV system through a small aperture and is passed through a series of differentially pumped chambers, to the Au(111) substrate (for further details see Supporting Information). UHV-ESD and related approaches have been used to introduce nanotubes, fullerenes, dye molecules, and polymers into a UHV environment. Images acquired after deposition of a sub-monolayer coverage of P6 (Figure 2) show that, despite their large size, the porphyrin oligomers diffuse on the surface and form Figure 1. a) Structure of porphyrin oligomers and polymers. b) P6 molecule with the trihexylsilyl end groups truncated to trimethylsilyl groups for clarity.

Vernier-templated synthesis, crystal structure, and supramolecular chemistry of a 12-Porphyrin nanoring

Vernier templating exploits a mismatch between the number of binding sites in a template and a reactant to direct the formation of a product that is large enough to bind several template units. Here, we present a detailed study of the Vernier-templated synthesis of a 12-porphyrin nanoring. NMR and small-angle X-ray scattering (SAXS) analyses show that Vernier complexes are formed as intermediates in the cyclo-oligomerization reaction. UV/Vis/NIR titrations show that the three-component assembly of the 12-porphyrin nanoring figure-of-eight template complex displays high allosteric cooperativity and chelate cooperativity. This nanoring–template 1:2 complex is among the largest synthetic molecules to have been characterized by single-crystal analysis. It crystallizes as a racemate, with an angle of 27° between the planes of the two template units. The crystal structure reveals many unexpected intramolecular C–H⋅⋅⋅N contacts involving the tert-butyl side chains. Scanning tunneling microscopy (STM) experiments show that molecules of the 12-porphyrin template complex can remain intact on the gold surface, although the majority of the material unfolds into the free nanoring during electrospray deposition.

A Discrete Three-Layer Stack Aggregate of a Linear Porphyrin Tetramer: Solution-Phase Structure Elucidation by NMR and X-ray Scattering

Formation of stacked aggregates can dramatically alter the properties of aromatic π-systems, yet the solution-phase structure elucidation of these aggregates is often impossible because broad distributions of species are formed, giving uninformative spectroscopic data. Here, we show that a butadiyne-linked zinc porphyrin tetramer forms a remarkably well-defined aggregate, consisting of exactly three molecules, in a parallel stacked arrangement (in chloroform at room temperature; concentration 1 mM-0.1 μM). The aggregate has a mass of 14.7 kDa. Unlike most previously reported aggregates, it gives sharp NMR resonances and aggregation is in slow exchange on the NMR time scale. The structure was elucidated using a range of NMR techniques, including diffusion-editing, (1)H-(29)Si HMBC, (1)H-(1)H COSY, TOCSY and NOESY, and (1)H-(13)C edited HSQC spectroscopy. Surprisingly, the (1)H-(1)H COSY spectrum revealed many long-range residual dipolar couplings (RDCs), and detailed analysis of magnetic field-induced (1)H-(13)C RDCs provided further evidence for the structural model. The size and shape of the aggregate is supported by small-angle X-ray scattering (SAXS) data. It adopts a geometry that maximizes van der Waals contact between the porphyrins, while avoiding clashes between side chains. The need for interdigitation of the side chains prevents formation of stacks consisting of more than three layers. Although a detailed analysis has only been carried out for one compound (the tetramer), comparison with the NMR spectra of other oligomers indicates that they form similar three-layer stacks. In all cases, aggregation can be prevented by addition of pyridine, although at low pyridine concentrations, disaggregation takes many hours to reach equilibrium.