Jennifer A. Wytko

Surface-Tuned Assembly of Porphyrin Coordination Oligomers

Two self-complementary phenanthroline-strapped porphyrins bearing imidazole arms and C 12 or C 18 alkyl chains were synthesized, and their surface self-assembly was investigated by atomic force microscopy (AFM) on mica and highly ordered pyrrolitic graphite (HOPG). Upon zinc(II) complexation, stable porphyrin dimers formed, as confirmed by DOSY (1)H NMR and UV-visible spectroscopy. In solution, the dimers formed J-aggregates. AFM studies of the solutions dip-coated onto mica or drop-casted onto HOPG revealed that the morphologies of the assemblies formed were surface-tuned. On mica, fiber-like assemblies of short stacks of J-aggregates were observed. The strong influence of the micas epitaxy on the orientation of the fibers suggested a surface-assisted assembly process. On HOPG, interactions between the alkyl chains and the graphite surface resulted in the stabilization and trapping of monomer species followed by their subsequent association into coordination polymers on the surface. Interdigitation of the alkyl chains of separate polymer strands induced lateral association of wires to form islands that grew preferentially upon drop-casting and slow evaporation. Clusters of laterally assembled wires were observed for the more mobile functionalized porphyrins bearing C 12 chains.

Synthesis of extended porphyrins by connection of meso-aryl groups with β-pyrrolic positions

This review focuses on synthetic strategies leading to extended porphyrins, but is limited to examples in which at least one β-pyrrolic carbon is linked to a meso-aryl substituent. Friedel–Crafts reactions are covered first because historically they were the first reactions employed to link β-pyrrolic carbons with meso-aryl groups, thus creating new six-membered fused rings. Other reactions inserting heteroatoms follow. Direct cyclizations between an ortho-carbon and a β-pyrrolic position are presented next. Finally, new highly extended chromophores are described. These compounds are generally obtained in two consecutive reactions. First a Suzuki coupling is used to introduce an aryl group in one or two meso positions. Then the new fused rings are generated by oxidative cyclizations.

Porphyrin‐[(E)‐1,2‐Diethynylethene] Scaffolding: Synthesis and Optical and Electrochemical Properties of Multinanometer‐Sized Porphyrin Arrays

Two series of linearly conjugated hybrid materials, consisting of (E)-1,2-diethynylethene (DEE; hex-3-ene-1,5-diyne) and ZnII porphyrin components, were prepared by Pd0-catalyzed cross-coupling reactions. In one series, one or two DEE substituents were introduced into the meso-positions of the ZnII porphyrins, leading from 5⋅Zn, to 9 and 1 (Scheme 1). The second series contains the linearly π-conjugated molecular rods 1 – 3 that span a length range from 23 A (1) to 53 A (3) (Fig. 1). The larger rods 2 and 3 consist of two or three porphyrin moieties, respectively, that are bridged at the meso-positions by trans-enediynediyl (hex-3-ene-1,5-diyne-1,6-diyl) linkers (Scheme 2). The UV/VIS spectra in the series 5⋅Zn, 9, and 1 (Fig. 2) showed a strong bathochromic shift of both Soret and Q bands of the ZnII porphyrin as a result of the addition of DEE substituents. Upon changing from 1 to 2 (Fig. 3), the Q band was further bathochromically shifted, whereas the Soret band remained nearly at the same position but became broadened and displayed a shoulder on the lower-wavelength edge as a result of excitonic coupling. The close resemblance between the UV/VIS spectra of 2 and 3 suggests that saturation of the optical properties in the oligomeric series already occurs at the stage of dimeric 2. Stationary voltammetric investigations showed that the DEE substituents act as strong electron acceptors which induce large anodic shifts in the first reduction potential upon changing from 5⋅Zn to 9 (ΔE=190 mV) and to 1 (ΔE=340 mV). Increasing the number of porphyrin moieties upon changing from 1 to 2 had no effect on the first reduction potential yet the first oxidation potential was substantially lowered (ΔE=110 mV). Large differences in the potentials for one-electron oxidation of the two porphyrin moieties in 2 (ΔE=200 mV) confirmed the existence of substantial electronic communication between the two macrocycles across the trans-enediynediyl bridge.

Highly linear self-assembled porphyrin wires.

An efficient noncovalent assembly process involving high geometrical control was applied to a linear bis(imidazolyl zinc porphyrin) 7Zn, bearing C(18) substitutents, to generate linear multiporphyrin wires. The association process is based on imidazole recognition within the cavity of the phenanthroline-strapped zinc porphyrin. In chlorinated solvents, discrete soluble oligomers were obtained after (7Zn)(n) was end-capped with a terminal single imidazolyl zinc porphyrin derivative 4Zn. These soluble species, as well as their destabilization in the presence of protic solvents, were studied by UV-visible and time-resolved luminescence. In the solid state, assemblies as long as 480 nm, which corresponds to 190 iterative units or a total of 380 porphyrins, were observed by atomic force microscopy measurements on mica. The length and linearity of the porphyrin wires obtained illustrate the potential of phenanthroline-strapped porphyrins for the directional control of self-assembly processes.

Modulation ofπ-Electron Conjugation in Oligo(triacetylene) Chromophores by Incorporation of a Central Spacer

A comprehensive series of trimeric hybrid oligomers 4 – 14 (Fig. 2) was prepared by insertion of different hetero-spacers between two (E)-hex-3-ene-1,5-diyne (=(E)-1,2-diethynylethene, DEE) moieties, and the optical and electrochemical properties of the resulting π-conjugated materials compared to those of the DEE dimer 2 and trimer 3, which formally contains a DEE moiety as homo-spacer. The hetero-spacers varied from benzenoid (phenylene, naphthalene, biphenylene, anthracene), to π-electron-deficient (pyrazine, pyridine) and π-electron-rich (thiophene, furan) aromatic rings, and to an organometallic trans-Pt(PEt3)2 fragment. The hybrid oligomers were synthesized following a general strategy which relied on the Sonogashira cross-coupling between mono-deprotected DEE 15 and the appropriately bis-functionalized spacer (Scheme and Table 1). UV/VIS Studies revealed that the majority of the hetero-spacers were less effective than the homo-spacer DEE in facilitating π-electron delocalization along the linearly conjugated oligomeric backbone (Table 2 and Fig. 3). With increasing degree of benzenoid aromaticity in the hetero-spacer, the electronic communication between the terminal DEE moieties in the hybrid oligomers was reduced. As a remarkable exception, a large bathochromic shift of the longest-wavelength absorption maximum, which is indicative of enhanced π-electron delocalization, was obtained upon introducing an anthracene-9,10-diyl moiety as hetero-spacer into oligomer 7 (Figs. 3 and 6). Electrochemical investigations by cyclic and steady-state voltammetry confirmed the limited extent of π-electron delocalization in the majority of the hybrid oligomers (Table 3). The fluorescence properties of many of the DEE hybrid materials were dramatically enhanced upon incorporation of the hetero-spacers (Table 2). The heterocyclic derivatives 10 – 12, containing pyridine, pyrazine, or thiophene spacers, respectively, displayed a strong fluorescence emission, which is present to a significant extent neither in DEE homo-oligomers nor in the individual heteroaromatic spacer components, demonstrating the value of combining different repeat units to modulate oligomeric and polymeric properties. The pyridine derivative 10 provided an interesting example of a molecular system, in which both the electronic absorption (Fig. 4) and emission characteristics (Table 2) can be reversibly switched as a function of pH (Fig. 5).

Cyclotriveratrylene as preorganizing matrix : a new tripole ligand.

Abstract The synthesis of a tripode ligand combining three pyridine rings arranged around a rigid derivative of cyclotriveratrylene is described.

Photodynamics of excitation energy transfer in self-assembled dyads. Evidence for back transfer

Three self-assembled photonic dyads comprising a zinc porphyrin donor and a free base acceptor have been studied by time-resolved fluorescence spectroscopy. The driving force of the assembly is the site selective binding of an imidazole connected to a free base porphyrin. Three spacers have been incorporated between the imidazole connector and the free base porphyrin, providing three different distances separating the donor and the acceptor. The high efficiencies and the rates of energy transfer in the set of dyads is consistent with the Forster energy transfer mechanism. Evidence for Forster back transfer has been obtained, and its efficiency and rate have been quantitatively evaluated for the first time.

Molecular Tools for the Self-Assembly of Bisporphyrin Photodyads: A Comprehensive Physicochemical and Photophysical Study

Accessible and hindered phenanthroline-strapped Zn(II) porphyrin receptors exhibited suitable topography tailored to strongly and selectively bind N(1)-unsubstituted imidazoles and imidazoles appended to free-base porphyrins. Distal binding was clearly driven by the formation of strong bifurcated hydrogen bonds with the phenanthroline unit of the receptors. An extensive physicochemical study emphasized the influence of bulkiness of the substrate and of the porphyrin receptor on the binding and self-assembly mechanism. Knowledge of the corresponding spectroscopic, thermodynamic, and kinetic data were of fundamental importance to elucidate and to model the photoinduced properties which occur within the self-assembled porphyrin dyads.

Toward generic models of hemoproteins.

The versatility of a resorcinol-substituted phenanthroline-strapped porphyrin was enhanced by appending allyl chains via a double Claisen rearrangement. Two pyridine arms were then appended to the resorcinol oxygens. This multifunctional porphyrin is a suitable generic building block for the preparation of liposoluble heme protein models. X-ray characterization of the pentacoordinated zinc complex and the binding of a sixth imidazole ligand to the ferric complex both suggest a facilitated access to an empty distal site.

Comparative studies in series of cytochrome c oxidase models.

This study compares the behavior as cytochrome c oxidase (CcO) functional and structural models of a series of reported and unreported ligands that provide either a binding site for copper without a built-in proximal base, or both a flexible binding site for copper and a built-in proximal base, or a fixed binding site for copper with a built-in proximal base. The comparisons of the models show that the relative position of the two metal sites is not only a crucial parameter in the control of the catalytic behavior but also essential in mimicking other features of the enzyme such as CO exchange between the ferrous heme a(3) and the cuprous Cu(B) center.