Porphyrin Assemblies and Their Scaffolds - Langmuir (ACS

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Porphyrin Assemblies and Their Scaffolds J.-H. Fuhrhop Institut für Organische Chemie der Freien Universität Berlin, Takustr. 3, 14195 Berlin, Germany ABSTRACT: The chlorophyll and heme molecules of chloroplasts and mitochondria are brought to life by “the global fold of the protein scaffolds”. Proteins in hydrophobic cell regions touch the dye platelets from both sides, pushing and orienting them according to their life-spending activities in light and electron transfers. The conjugated π-electron systems or planarity of the porphyrin macrocycles are never disturbed. Most artificial porphyrin assemblies contain meso-tetraphenylporphyrins (TPPs), because the four phenyl groups rotate freely and carry their substituents above or below the macrocycle. A single porphyrin molecule can, for example, be attached to an anionic surface with ammonium groups on its 2,3carbons, be located within a hydrophobic membrane with its alkyl chains on the 4-position, and then fixate a cationic polymer with 4,5-sulphonates. Charged TPPs also show unique spectroscopic changes at different pH values and a reversible loss of the macrocycle’s planarity. On smooth silicate, graphite, or gold scaffolds TPPs have been attached irreversibly as single molecules, as extended non-covalent H or J aggregates as well as acetylene or thiophene-linked polymers. Soft, mobile porphyrin ladders conduct excited electrons (“excitons”) better than rigid porphyrin wires (“polarons”). he expression “global fold of the protein” and the sentence “cyt c does not fold without the bound heme” appear in the heme protein review of Reedy and Gibney (pp 621−622).1 They furthered my imagination, when I was looking back, following the kind invitation of Langmuir to write a historical review on porphyrin assemblies. A title was found. The history of porphyrins-in-scaffold chemistry starts in 1958 with the X-ray structure of myoglobin.2 The 1 nm2 porphyrin platelets become visible here, and one begins to understand why and how they function. The Fe(II) hemes can add only oxygen reversibly, Fe(II)O2, without being oxidized to hemin, Fe(III)OH, in the hydrophobic protein capsules providing two histidines. The properties of the heme in cytochrome P450 of mitochondria, which burns food in reductive environments, and of the large number of chlorophyll molecules, which oxidize water and reduce carbon dioxide in microorganisms and plants, are also determined by protein scaffolds. It all starts in the complicated π systems of protoporphyrin,3 porphyrin dimers,4 and chlorophylls,5 but without the protein scaffolds no repetitive process occurs. A major task of chemists is the transfer of porphyrins from biology to technology and medicine. Let us produce electricity, clean water, and kill deadly bacterial and cancer cells with sunlight and oxygen. We shall look here first at the porphyrin macrocycles and their substituents at some attractive planar surfaces, mainly silicate, and then graphite and gold. Section 2 then characterizes porphyrins in biological scaffolds because they are seen by X-rays in crystal structures. The remaining sections address synthetic assemblies fixed on smooth solid surfaces, the fundamental, most simple scaffolds. They immobilize and arrange porphyrin monomers, noncovalent dimers, and polymeric H and J aggregates as well as ionic and ladder wires and covalent polymers of porphyrins and thiophenes.

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1. PORPHYRINS AND SMOOTH SOLID SURFACE SCAFFOLDS The essential property of porphyrin dyes on solid surfaces is the separation of an upper, freely accessible surface from an inner, scaffold-dominated surface. The upper side of the combined aromatic as well as polyene-like π-electron system of the porphyrins is accessible to microscopes and chemicals. The lower porphyrin side, the one that is attached to the scaffold, becomes an invisible part of the silicate, graphite, or gold surface. Depending on the size and flexibility of its binding substituents, the fixed side of the porphyrin π system may or may not react with dissolved chemicals. Porphin (Scheme 1a) is the parent compound of all porphyrins. It has no substituents, is difficult to synthesize and handle, and is altogether useless. It is introduced here because it demonstrates the fundamental importance of the substituents in porphyrin chemistry. Without them, the porphyrin macrocycle packs into endless molecular stacks, which are affected neither by solvents nor scaffolds. Porphin and octamethyl porphyrin are nearly insoluble in all solvents.6a,b The prototype of natural porphyrins, which interact with all types of scaffolds, is protoporphyrin IX. Two reactive vinyl and propionic acid side chains may connect it covalently with proteins (Scheme 1b,c).3 It is accessible on the 100 g scale from the blood of cows, which may be collected by young students in a slaughter houseone typical experience of chemists that they never forget. The vinyl and propionate side chains rotate, bringing their reactive CC and CO double bonds above and below the porphyrin plane (Scheme 1c). This rotation prevents polymeric stacking. Only face-to-face dimers are formed.4 Received: June 12, 2013 Revised: October 11, 2013

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dx.doi.org/10.1021/la402228g | Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Structures of Porphin, Protoporphyrin IX, Protoporphyrin3, Copper(II) Chlorophyllin, Zinc-mesotetraphenylporphyrin, and 52,56,102106,152,156,202,206-Octaoxyalkyl-meso-tetraphenyl-porphyrina

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(a) Porphin, the unsubstituted porphyrin. (b) Planar rotamer of protoporphyrin IX (or 2,7,12,18-tetramethyl-3,8-divinyl-13,17-dipropionic acidporphyrin). (c) Crystal structure of the nonplanar rotamer of protoporphyrin3 (Adapted from ref 3. Copyright 1977, American Chemical Society). (d) Copper(II) chlorophyllin, a commercial food additive containing 65% of the indicated chlorine4-e6-mixture. (e) Planar rotamer of zinc-mesotetraphenylporphyrin (5,10,15,20-tetraphenylporphyrin). (f) Nonplanar rotamer of 52,56,102106,152,156,202,206-octaoxyalkyl-meso-tetraphenylporphyrin.

The rigid 1 × 1 nm2 platelets possess a heterocyclic πelectron system around the diprotonated or metalated nitrogen. The inner conjugation path containing the nitrogen atoms is aromatic. All bonds have a length of 0.136−0.137 nm. To the 16 carbon π electrons, one has to add 1 π electron each from the protonated nitrogen atoms in order to obtain 18, which corresponds to an aromatic 4n + 2π electron system. Without this formality, the inner ring does not look conjugated, but in reality, it is as shown by the uniform bond lengths. Alternating bond lengths of 0.145 and 0.135 nm, typical of conjugated polyenes,3 are observed in the periphery. The NH protons are, both in natural and in synthetic scaffolds, usually replaced by metal ions. Magnesium and iron dominate in nature, but almost all metals of the periodic system are of interest in synthetic assemblies because they change the magnetism, the visible spectra, and the redox potentials of porphyrins.6a,b Pheophorbides, the chlorophylls without magnesium, surprisingly show only weak alternations of the bond lengths everywhere. The strongest deviation occurs at methine carbon 5 (0.137 and 0.141 nm), and the weakest, at methine carbon 15 (0.1388 and 0.1391).5 A mixture of 30% each of copper chlorins e4 and e6 (copper chlorophyllin) is, however, the only chlorophyll derivative that is accessible on the desirable 100 g scale for scaffold-assembly studies. It is commercially applied as a food additive (Scheme 1d).7 Polyene character, low oxidation

potential, and long-wavelength absorption are interesting properties for solar and photovoltaic cells, and only the copper has to be removed, to be replaced by zinc(II) or tin(IV), for example. By far the most favored porphyrins for the assembly on scaffolds are the meso-tetraphenylporphyrins (TPPs). Their one-step syntheses on the 10 g scale from substituted pyrrole and benzaldehyde derivatives are often simple and fast. The unique and most fascinating aspects of TPP chemistry are, however, the large number of charged, hydrogen-bonding, or hydrophobic substituents that may be bound to the phenyl groups, their location in, above, or below the porphyrin plane, and the free rotation of all four phenyl groups. A multisubstituted TPP molecule may be first attached to a silicate surface by four ammonium groups on C2 or C3. Four octadecyl groups on C4 would then render the silicate surface partially hydrophobic, and carboxylate groups on C5 or C6 would appear above the porphyrin; the scaffold-bound porphyrin can also bind a cationic polymer. The phenyl group rotation in substituted TPPs provides the most powerful synkinetic tool for artificial porphyrin assemblies. The diffusion coefficient of TPP in chloroform and benzene was measured to be relatively high (3.80 × 10−6 cm2/s) in the pores of muscovite mica. The diffusion rate was directly determined by continuously monitoring the low-concentration side of the membrane by absorption spectroscopy at 424 nm.8 B


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Figure 1. (a) Energy-minimized model of a double-helical myoglobin.20 The histidine side chains and the disulfide bottom of the hydrophobic box are indicated; its Fe(II)-O2 complex is stable for only a few seconds. Adapted from ref 20. Copyright 1994, American Chemical Society. (b) Binding site of human cytochrome P450, which is known to oxidize drugs in humans. Hydrogen bonds to glutamic acid 216 and serine 304 fix the side chain of (S)-bufuralol, and the aromatic cycle finds the active iron(IV) center in the hole between four protein helices.23 The dotted Fe−CH bond now waits for the oxygen molecule and NADPH. The beauty of the protein helices always surrounding the protoporphyrin plane in nature becomes apparent, and porphyrin and bufuralol are accessible on both sides of the π-electron cycles. This is a picture of life on the nanometer scale. Adapted from ref 23. Copyright 2011, American Chemical Society.

Langmuir−Blodgett films made of TPPs can also be transferred onto mica substrates but rearrange there to yield wormlike aggregates. The area per molecule is 0.76 nm2, which is even smaller than the area occupied by the vertically oriented tetraarylporphyrin ring of 0.9 nm2. The observed height of 5−8 nm is in agreement with a highly packed multilayer worm.9 Floating layers of TPP derivatives on the water surface10 are, however, typically monomolecular without or with short substituents. The tilt angle of the monolayer is mostly 35°. If substituted tetraphenylporphyrins TPPs are to be immobilized as single molecular islands on solid surfaces, they need several cooperating bonds as well as extremely smooth surfaces as described in section 3. The absorption spectrum of porphyrins has its strongest band at the UV−vis edge of 400 nm (Soret or B band, ε > 100 000), followed by four Q bands in the visible range from 480 to 630 nm (ε < 10 000) for free base porphyrins or two bands between 530 and 600 nm for metal complexes. Chlorophylls show a weaker Soret band but additional intense absorptions in the near-infrared (680−900 nm).6a,b Irradiation of porphyrins and diamagnetic metalloporphyrins at 400 nm produces a red fluorescence band (λmax ≈ 620 nm) in free base porphyrins or diamagnetic metal complexes. The red fluorescence is visible and quenched by the addition of paramagnetic metalloporphyrins such as Mn(III)- and Cu(II)porphyrinates. This effect depends on the distance between fluorescing and quenching monomers6a,b and provides the most simple, sensitive, and quantitative tool in the characterization of porphyrin interactions in scaffolds. Metalloporphyrins can in general also be oxidized (preferably stable and diamagnetic zinc porphyrins) or reduced (usually tin(IV) porphyrins) to π radicals and/or phlorins with oxidized or reduced methine bridges. All of these disturbances of the inner conjugation pathway lead to very broad absorption bands in the visible region, and the original metalloporphyrin can always be recovered in high yields by the addition of appropriate oxidants or reductants. In zinc porphyrin radical dimers, the π-electron systems of both molecules merge together, and an 800 nm band appears.11 An equally intense dimer absorption at 800 nm was obtained when two iron(II) porphyrins formed a cation−anion pair upon the addition of pyrazine, a pyridine ligand with two

opposite nitrogen atoms in the 1,4 position. The porphyrin−porphyrin, FeN(CHCH)2NFe, distance increased to about 1 nm, but the photoinduced electron transfer was as strong as with the direct π,π dimer.12 The most important scaffolds today for the stabilization and fixation of porphyrin assemblies are a few materials that can be produced with extremely planar surfaces. Potash mica or muscovite, KAl2(AlSi3O10)(F,OH)2, provides a large, negatively charged surface area with a roughness below 0.3 nm and is commercially available.8 Silicate nanoparticles with a diameter of about 100 nm are equally smooth and are quickly prepared in the laboratory from silyl chlorides. Treatment with α,ωdiamines changes their surface charge to positive.13,14 Gold(0) surfaces are soft because the metal atoms are large and airstable. They bind SH-substituted porphyrins and polymers covalently, are commercially available on quartz, or require very careful preparation techniques in the laboratory in order to be reproducible.15−18 Graphite sheets provide the most common hydrophobic, nonmetallic scaffold (section 5). The dynamics of smooth interface formation during crystal growth must also be mentioned. A review of crystal interfaces from 2001 describes the “ledge-directed epitaxy” (LDE),19 but it has not been applied yet as a porphyrin “terrace”. Selective molecular recognition events should be observable here by atomic force microscopy (AFM).

2. PORPHYRINS IN PROTEIN SCAFFOLDS The Protein Data Bank (PDB) contains about 2000 protein crystal structures with hemes and/or chlorophylls. Both sides of the porphyrin platelets are always equally well visible here because the porphyrin planes are not lying flat on the protein rods. Proteins bind to porphyrins by single side chains (Figure 1). Myoglobin is the heme protein that adsorbs, stores, and releases molecular oxygen in muscles. The protein holds only one porphyrin molecule but already applies eight α-helical and several short random-coil sections. They entrap the heme molecule between different hydrophobic, helical sections, bind it loosely with two histidines, stabilize the capsule with cysteine, and leave no room for a single water molecule. An oxygen molecule, however, quickly enters because it is also hydrophobic and because its paramagnetic triplet ground state likes C


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the iron(II) ion. The more loosely bound histidine ligand of the Fe(II) ion is replaced by O2, but no oxidation to hemin “rust”, Fe(III)OH, takes place. The molecular oxygen may then remain in resting muscles for a long time, but it is also immediately released if another protein or membrane surface touches it. Finally, the reductive potential of blood (∼30 mV above the hydrogen electrode) stabilizes the chemically menaced Fe(II)-O2 species. The globin alone is not sufficient; the whole organism is needed to keep the oxygen molecule inactive as long as necessary.2 A synthesis model, simply a cysteine-stabilized hydrophobic box providing space for a heme molecule as well as two axial histidine side chains, looks great20 (Figure 1a), but its molecular oxygen adduct survives only a few seconds. Then it becomes rust, an Fe(III)OH hemin. Hemes are not only of interest to blood and muscle cells but also also an essential nutrient for bacterial pathogens visiting humanity. Such bacteria quickly remove the heme from erythrocytes by a “hand clasp” between human hemoglobin and a similar globin provided by bacterium Staphylococcus aureus. During infections, the erythrocytes and the bacteria meet each other in blood and let their globins touch shortly at the cell walls. The heme then leaves the erythrocytes 70 000 times faster than upon treatment with the solvents, which are used for heme isolation from the blood of cows. About 310 protein helices and loops perform the perfect hand clasp, after which the bacterium disappears and multiplies with the many stolen heme molecules.21 The mechanisms of hand clasp and heme release are not known. Chemical research preferably deals with cytochrome P450 enzymes. Cytochrome means a dye for electron conduction in cells, and P450 signifies a porphyrin or pigment with a specific Soret band shift from 400 to 450 nm upon addition of carbon monoxide. P450 reduces oxygen molecules to water and keeps one oxygen atom as an Fe(IV)O heme in mitochondria. Then it oxidizes CH bonds of hydrocarbons to alcohols, −COH, stereoselectively.22 Figure 1b shows one of several known cytochrome P450 Xray structures. It is combined with the drug molecule (S)bufuralol (2-(2-tert-butylamino-1-hydroxyethyl)-7-ethylbenzofurane), a heart tranquillizer. The picture is created by a computer program specializing in sites of metabolism (SOM) models. Efficient drugs dock only on SOMs. The model exactly predicts (or reproduces) the experimental results concerning the stereospecific oxygenation of the ethyl group (Figure 1b) and the enzyme’s amino acid bonds to the S-bufarol side chain.23 The highly flexible reaction center of human cytochrome P450 is again occupied by hydrophobic protein helices. Two hydrophilic side chains, namely, serine and glutamic acid, reach into a polyfunctional hole above the iron reaction center and bind to the enol and alcohol oxygen atoms of bufaralol. The Fe(IV)O bond then catches and oxidizes the benzylic proton, and the next incoming oxygen molecule pushes the oxidized bufaralol away. The binding site in the hole is then weak enough to release the product and catches the next educt molecule. Fe(IV)O and the protein hole are separated by 10 bond lengths, but they cooperate perfectly. Substrate binding and metabolism in cytochromes P450 also produce free electrons and currents. The currents are measurable with the tip of an atomic force microscope if the P450 proteins are placed on the heads of gold nanopillars standing on a graphite layer. Even immobilized on the gold pillars, the cytochrome electron-transfer rates are indeed always faster when drug oxidation takes place.24

Photosynthesis in the chloroplasts of plants and bacteria is the only source of life energy and materials on earth. Photosystem II crystal structures (resolution of 0.19 nm) show 35 chlorophyll molecules, mostly effective antenna for light absorption, a redox-active center with 2 chlorophyll dimers, an oxygen-delivering manganese complex, Mn4CaO5, and about 40 protein helices, which organize and steer all components. The electron and electron hole producing and separating system with charge transfer times of between 3 ps and 500 μs (10−12 and 0.5 ms) totally depends on the “global fold of a protein scaffold”.25 Synthetic photoinduced electron-transfer models hardly apply any chlorophyll derivative or protein. A covalent βcarotene-Zn tetraphenylporphyrin-tetraphenylporphyrin-naphthoquinone-benzoquinone polyene, however, yields ZnP+quinone− ion pair visible light flashes with a quantum yield of 83% and a lifetime of 55 ps. This is much longer than for quinone anion radicals in photosynthesis, where the back reaction with a close carotene cation radical is immeasurably fast.26,27 The development of functional artificial photosynthetic systems by self-assembly is, however, out of reach. Strategies that ensure self-repair after damage by a series of demanding excited-state and radical reactions are also unknown, even for covalent chlorophyll oligomers.28 Hydrophilic nanoparticles made of a lactic acid and glycolic acid copolymer loaded with a water-insoluble meso-tetraphenylchlorin are easy to prepare, air-stable, and nonphototoxic upon systemic administration. After cellular internalization, however, the chlorophyll photosensitizer is released from the nanoparticle and becomes highly phototoxic. Irradiation with visible light results in cancer-cell-specific killing. In vivo experiments with mice successfully eradicated cancers with visible light.29

3. PORPHYRIN MONOMERS, DIMERS, AND TRIMERS ON SOLID SURFACE SCAFFOLDS One square nanometer porphyrin macrocyles fixed on a smooth surface make an irresistible platform for molecular architecture. However, the molecule must lie parallel to the planar ground and be very well cemented onto it; otherwise, all tower constructs will fail. Negatively charged, spherical 100 nm silica particles are accessible by a one-pot reaction sequence from tetraethoxysilane.13 Their anionic surface may be used directly to bind phenols or form salt or amide layers with organic amines. The most useful starting material, however, is (3-aminopropyl)triethoxysilane. It directly produces the only known positively charged, absolutely smooth surface. Both the anionic and the cationic silicate nanoparticles adsorb water without swelling, remaining dissolved in water up to pH 11. Above this pH and below pH 3, they coagulate quickly and redissolve completely at pH 9. Surprisingly, the smooth polyammonium surface does not fix a triacetate foot of porphyrin 2 in water at pH 7. The positive charges on silica are presumably too far apart from each other to bind all three neighboring acetic acid groups, and one or two carboxylate−ammonium bonds are obviously too weak to pull a porphyrin out of the bulk water volume. Centrifugation of the silica particles leaves the porphyrin fluorescence in water.14 The sine qua non for the fixation on anionic surfaces is always the guanidinium group, ((NH2)2CNH2+), the most basic substituent of nature. It appears in the excrement of seabirds instead of urea and tightly binds to the silicate rocks of Peru, so it should also attach the three feet of propionate D


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Figure 2. (a) Model of the calyx[4]arene (blue) and the substituted meso-tetraphenyl-tricarboxylateporphyrin (red). (b) AFM picture of the silicate surface, showing smoothness on the left side, two 1-nm-high flat-lying calyx[4]arene 1 molecules on right side, and 1 + 2 in the middle.14 (c) Flat calixarene or porphyrin 1 nm2 platelets never arrange side by side but with empty space in between. The flexible and/or rotating substituents together with the rotation of the platelets separate them. Adapted from ref 14. Copyright 2005, American Chemical Society.

Figure 3. (a) Fixation of a manganese(III) porphyrin at a distance of 1.0 nm above the free base does not influence its time-dependent decrease in the fluorescence of a free base porphyrin at a distance of 0.5 nm. It causes a 60% decay within 0.16 ns, indicating the removal of the high-energy electron from the bottom free base.31 Adapted from ref 31. Copyright 2006, American Chemical Society. (b) Trimer in a yoctowell.32 (Red) silicatefixed bottom as in panel a. (Blue) Mn(III)Cl-tetra-2-N-methylpyridinium-porphyrin. (Green) Mn(III)Cl-tetraphenyl-4-sulphonato-porphyrin. Adapted from ref 32. Copyright 2004, American Chemical Society.

tripods. Guanidine-coated silica particles yielded, however, only ill-defined clusters; no single tripod stood upright. Tetraphenoltetraguanidine calixarene 1 (Figure 2a) was obtained from a European network colleague and solved the problem of rapid, uncontrollable precipitation.30 The highly-soluble calixarene locks on smooth polyammonium-coated silica nanoparticle surfaces and is cemented there. Four guanidinium groups on the upper side of the plane are not involved in phenolic hydrogen bonds. Dense coverage with 1-nm-high guanidine islands of calixarene 1 is achieved, and they are large enough to cover the whole ammonium plane, which the porphyrin 2 tripod cannot reach now. The three propionate groups attach 2 on top of 1, and a 3-nm-high dimer is now detectable by AFM. For the first time, a single porphyrin molecule stands upright and remains so for months in water and air (Figure 2b). Both partners, 1 and 2, are, as monomers, very soluble in water, but

at pH 7, nothing now splits the phenol−ammonium and guanidinium−tricarboxylate bonds. Both the monomolecular and the bimolecular rocks now shape the particle landscape.14 Calixarene 1 covers about 40% of the surface, often in optimal Monte Carlo packing (Figure 2c). The 3-nm-high dimer is stable but appears only in a small quantity. Noncoated silicate particles with −O−Si(O−,OH)−O− surfaces produce four covalent −Si−O−Si bonds with mesotetraphenylporphyrins carrying a silyl chloride substituent, meta-CO−NH(CH2−CH2)−SiO2Cl, on each phenyl group. Immobile hydrophobic spot patterns as shown in Figure 2c are detectable by AFM on the now fluorescing silicate surface. The remaining Si(OH)2 sites around the porphyrin islands, between 60 and 80% of the particle surface, can again be esterified quantitatively. Long-chain silyl chlorides with a Michael reactive double bond in the center and an E


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Figure 4. (a) Absorption spectra of meso-tetraphenylsulphonato-porphyrin (TPPS) at the pH values given. At pH 0.5 (red), there is almost no Soret band. Spectra of mixed PDDA/TPPS LBL multilayers with either (b) TPPS or (c) PDDA as the outermost layer. J aggregates with the 490 nm band appear only with the loosely ordered TPPS layer on top. The planar porphyrin chromophore with a strong Soret band always disappears at low pH values. (d) Models corresponding to spectra b with the TPPS H aggregate on the bottom covered by its J aggregate and (c) only an H aggregate of TPPS and the polymer PDDA (or a PDDA/TPPS mixture) on top! Adapted from ref 34. Copyright 2009, American Chemical Society.

Early theoretical modeling of porphyrin dimers in solution suggests that the rigid porphyrin macrocycles would always lie together, on top of each other, strictly parallel and laterally shifted, with a pyrrole ring above the central hole of the partner, nonshifted if substituent interactions become dominant.4 Spectra hardly change. Unpaired electrons change the situation. The oxidized zinc-octaethylporphyrin-π-cation radical dimer gives very broad proton NMR signals and visible absorption bands at room temperature. They become narrow in the diamagnetic dimers formed in acetonitrile at −30 °C. The methine proton signal now appear at 5.65 ppm instead of at −0.05 ppm, and the porphyrin ring current is lost.11 Only homodimers, no pure heterodimers of porphyrins, are known in solution. They are accessible only by time-separated step-by-step reactions on solid scaffolds and remain there because of their insolubility. The scaffold-bound part is then accessible only from the solid surface, which could be heated, cooled, electrified, reduced, or oxidized, and the upper porphyrin can be reached by other molecules or by an electrode tip in a solvent. Pure noncovalent porphyrin trimers also need a scaffold and step-by-step addition. The coating porphyrins then isolate the one in the center.

oligoethleneglycol at the other end produce a molecular monolayer around all of the porphyrin islands. Yoctowells (1 yL = 10−24 L) with a uniform diameter of 2 nm and depths of between 2 and 6 nm (the early name was therefore nanowells) now cover the nanoparticles.31,32 The porphyrin bottoms’ fluorescence is quenched by watersoluble porphyrins, which fit into the tubule. The 1D diffusion of an exactly fitting Cu(II) porphyrin molecule along the 4 nm pathway takes several minutes, but derivatives with a diameter of 3 nm have no quenching effect whatsoever. A ring of methylammonium groups, fixed at the walls of the wells by Michael addition at a distance of either 0.5 or 1.0 nm with respect to the bottom porphyrin, adsorbs tetra-anionic manganese(III) meso-(tetraphenyl-4-sulfonato)porphyrinate (Mn(III) TPPS) molecules here. The transient fluorescence of the bottom porphyrin now decays within 0.2 ns as assisted by the Mn(III) porphyrin 0.5 nm away. The same porphyrin at a distance of 1.0 nm has no quenching effect whatsoever (Figure 3). The time constant for the diffusion of a fitting porphyrin into the well is on the order of 1000 s and corresponds to formal diffusion constants close to D = 10−23 m2 s−1 in both water and chloroform. Porphyrins in hydrophobic pores that are twice as large diffuse 1014 times faster!31 These observations prove that the whole surface of a nanoparticle is smooth. There are no edges, and no quickly quenched, unreachable porphyrins exist on any nanoparticle surface. If fitting, paramagnetic, cationic manganese(III)-porphyrin B is deposited at the bottom above fluorescing, fixed porphyrin A, then the fluorescence disappears quantitatively. A third anionic addition, namely, manganese(III) tetrasulfonate porphyrin C, is also irreversibly fixed by the pyridinium porphyrin in position B (Figure 3b). However, the electrode of cyclic voltammetry recognizes no trimer but exclusively top component C. It gives a strong signal, and B and A are not detectable at all.32 Similar experiments on gold electrodes show that only porphyrin A could be oxidized and reduced electrochemically. The number and total volume of yoctowells in a given volume of a particle solution can be calculated from the weight and diameter of the dry particles and the intensity of the Soret band in their aqueous suspensions. A total of 1500 yoctowells per particle or 20% surface coverage is a typical value.

4. POLYMERIC FACE-TO-FACE AGGREGATES The aromatic and polyene character of the two connected conjugation pathways of porphyrins, the stiffness of their electroneutral, and the mobility of their charged macrocycles cause intense polarization in their molecular assemblies. Meso-p-tetraphenylsulfonate-porphyrin (TPPS) in water is the most drastic case known. Its spectrum varies much more strongly at different pH values than that of any other porphyrin. At pH 10, the four sulfonates are charged, the pyrrole nitrogen atoms are neutral, and monomeric porphyrins prevail. The standard visible porphyrin spectrum with a Soret band at 413 nm (Figure 4a, see section 1) appears. At pH 0.5, the Soret band intensity drops by a factor of more than 10, and a new, intense band at 490 nm appears, accompanied by a weaker band at 700 nm. It is the diacid of the porphyrin, which forms a side-by-side stacking J aggregate connected by a network of hydrogen bonds. The repulsion between NH+-pyrroles also destroys the planarity of the macrocycle. Two positive charges F


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in the center combined with four negative charges in the periphery thus reversibly destroy the planar π system, its aromaticity, and the near-UV absorption. This extraordinary spectacle is caused by the strong charge repulsion in the center coupled with the long-distance interaction between the center and the periphery. At pH 3, only one nitrogen atom is protonated and the planarity and the strong Soret band reappear, but the band is shifted from 413 to 434 nm. No J aggregate appears, and the conjugation pathway is hardly disturbed.33 Thin films containing porphyrins have always been manufactured by using Langmuir−Blodgett (LB) techniques on liquid water or by their self-assembly of monolayers (SAM) on solid surfaces. Both procedures require insoluble amphiphiles and a strong interaction of the whole intact layer with the substrate (e.g., hydrosulfides with gold). Today, layer-by-layer, LBL, technique is the most popular because the total transfer of an intact LB monolayer is often difficult to reproduce.34 LBL is the sequential layering of oppositely charged polyelectrolytes directly on a solid substrate, and it works with almost any polycation and polyanion (e.g., with poly(diallyldimethylammonium chloride) (PDDA, a misleading name with respect to its structure; it is in fact a polycyclic Ndimethyl-pyrrolidinium polymer)) and a charged dye (e.g., tetraphenylsulfonate porphyrin (TPPS)). In multilayers made of PDDA and TPPS, the porphyrin J aggregate is observed only if TPPS is the outermost layer. A cationic PDDA layer or a mixed PDDA/TPPS layer on top immediately destroys the porphyrin J aggregate and rearranges it to the stable H aggregate with fully overlapping, upright-standing porphyrin macrocycles (Figure 4d) and a Soret band at 423 nm.34 The specific Soret bands of TPPS are thus due to the free base (413 nm), H aggregate (423 nm), diacid (434 nm), and J aggregate (491 nm). In the H-type aggregates, the porphyrins stack directly over each other, and the slipped packing of J aggregates leads to loose packing and mobility (Figure 4d). H aggregates are usually poor emitters, whereas J aggregates typically show strong luminescence.34−37 Electron or ion transfer via light- or redox-initiated membrane potentials is the major business of natural porphyrin assemblies and their scaffolds. Thousands of bacteria and plant generations optimized it with protein scaffolds by evolution (section 3), and artificial systems approach effectivity by a nematic self-organization or liquid crystallinity. Slimy porphyrin threads made of hydrophobic meso-tetra-4n-butyl-phenyl-porphyrin, for example, are 30 nm thick and simply packed on a titanium dioxide surface. They produce currents of the excited electrons (excitons) with diffusion lengths above 40 nm at 90 K and 22 nm at 300 K when irradiated with intense 430 nm light pulses (2 × 1011 photons/ cm2). The motion of the exciton here is not forced on a stiff and sluggish atomic lattice of crystalline materials (section 6), but it moves freely through a broad conductive band.35 No wire or scaffold is needed, only structural order and rigidity of the nematic, liquid-crystalline material.

A zinc tetraphenylporphyrin with an imidazoyl group on a methine bridge produces 100−300-nm-long molecular tapes on graphite. Linear zinc-imidazole coordinations and hydrophobic interactions between upright-standing octyl or octadecyl chains work together. Heating in pyridine for 8 days enhances the dissociation to monomers, and the tapes grow, surprisingly, to lengths of 600−900 nm. The related observation that the shorter octyl chains always generate the longer tapes also fits: it is the immobility of four long alkyl chains that shortens the wires on HOPG.36 4′-Phosphonate side chains, 4-O(CH2)3-PO3Na2, on tetraphenylporphyrin first turn away from a HOPG surface as far as they can and become isolated. Monolayers of formless, 0.5-nmthick patches of flat-lying porphyrins are formed. Within minutes, fibers with heights of between 1.4 and 2.8 nm, a typical mixture of J and H aggregates, and lengths of up to 500 nm appear. They are oriented parallel to each other at an angle of 60° corresponding to the HOPG crystal axes and are still surrounded by the disordered and mobile patches of flat-lying porphyrins. The fibers then melt to form uniform 2.9-nm-high fibers, which are H aggregates of upright-standing sodium phosphate porphyrins. The HOPG surface does not fix the porphyrins but rather allows fast adsorption and slows down crystallization.37 Anionic silicate surfaces create with the same tetraphosphonate at acidic pH values long fibers of uprightstanding porphyrins and/or monolayers, and at high pH, charge repulsion causes shorter fibers, which tend to a rearrange to disordered monolayers. Zirconium(IV), first envisaged as an optimal cement between the phosphonate groups, precipitates the porphyrins as ill-defined clusters. It is, however, the most soluble sodium phosphonate that produces the longest rigid wires on planar mica with a uniform width and height of 2.8 nm (Figure 5a). meso-Octopusporphyrin containing eight positive tetraalkylammonium charges instead of the eight negative phosphonate charges causes very different soft fibers. The −CH2OCO(CH2)18−phosphocholine chains at the 2′,5′ positions of the phenyl groups of TPP and its zinc(II) complex again appear as very stable colloidal solutions in water, and electron microscopy

Figure 5. (a) Fiber made from tetraphenylporphyrins with four ionic side chains at the 4′ positions The polar salt (−O(CH2)3PO3Na2) produces rigid wires on graphite as indicated by the sharp tapping signals of the AFM tip. Adapted from ref 37. Copyright 2004, American Chemical Society. (b) Fluid membrane made of octopusporphyrin with eight octadecyl-phosphocholine −(CH2OCO(CH2)18−PO4−−(CH2)2N(CH3)3+) chains. These long alkyl chains in the pyrrolic β positions produced flexible, micellar fibers that look like long strings of fat droplets. Such long-chain and short-chain tetraphenylporphyrin phosphonates and amines look like very promising candidates for evolutionary tests! See section 7. Adapted from ref 38. Copyright 1996, American Chemical Society.

5. NONCOVALENT NANOWIRES We now turn to noncovalent porphyrin fibers or “wires” of mono- or bimolecular thickness. They appear on hydrophobic (or lipophilic) highly ordered pyrolytic graphite (HOPG) substrates,36,37 gold nanoparticles,38 and hydrophilic silicate spheres.13,39 G


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Figure 6. Zirconium(IV)-porphyrinphosphonate 4d with four phosphonates on each side forms multilayered towers around upright standing Alizarin-sulfonate-Zr(IV) crystallites and small rocks on a silicate surface. One tower (circled) was moved 200 nm with the AFM tip in tapping mode. Their width is about 40 nm at the base.39 Adapted from the Ph.D. thesis of A. Klyszcz, FU Berlin, 2003. Copyright by the author.

Figure 7. AFM of (a) the TMPyP/PSS brush on mica (b) after the addition of a 0.01 M iodide solution. The gel structure fades away, but the bound porphyrins remain photochemically active and produce singlet oxygen.40,41 Adapted from ref 41. Copyright 2012, American Chemical Society.

on graphite shows 7-nm-thick micellar fibers (Figure 5b). Each single porphyrin molecule can be seen on the electron micrograph, the fibers fluoresce strongly, and laser flash photolysis is followed by electron transfers from the porphyrin to the hydrophobic benzoquinone as well as to the more hydrophilic 1,2-naphtoquinone. A molecular fat droplet is aligned as a string of pearls in an extremely open fiber structure of the dimethylviologen acceptors. The fibers also transfer electrons from amines to dimethylviologen.38 Well-defined and reproducible zirconium(IV)-phosphonate porphyrin assemblies can be realized only on separated islands on silica surfaces. The addition of a tetraphenylporphyrin (4d) with eight phosphonate groups (Figure 6) to a zirconium phosphonate layer on silica yields only ill-defined precipitates. However, islands of a few nanocrystals of the zirconium(IV)sulfate of Alizarin S (1,2-dihydroxy-3-sulphonato-anthraquinone) on a mica surface coated with phosphonate, −O− (CH3)2Si−(CH2)3−OPO3H2, produce a perfect foundation for tower building. Porphyrinphosphonate 4d now leaves the mica

phosphonate alone but lets the Alizarine islands grow. About 1 tower per 1000 nm2 is formed and remains stable in water or air for several months. The small number of islands also leaves enough free space for the pushing of towers across the silica plate by several micrometers with an AFM tip. No Zr(IV)-rock changes its shape (Figure 6).39 In a 40 000-meric poly(styrenesulfonate), the large phenylsulfonate groups turn the polymer chain into a cylindrical “bottle brush” with a 12 nm diameter and a 100 nm length. It is not flexible and does not form networks.40,41 If, however, tetracationic anilinium or pyridinium porphyrins are added, then each single brush adsorbs up to 4000 porphyrin molecules and AFM now detects only large cross-linked assemblies (Figure 7a). Visible light produces singlet oxygen as usual with porphyrins, but surprisingly, added iodide is also oxidized much more rapidly than in the absence of the polymer, although the anionic polymer network is almost completely disassembled with iodide (Figure 7b). The porphyrin molecules always remain on the oppositely charged brushes and catalyze the H


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Figure 8. AFM of a poly-TAPP film on FTO glass. The formula of the least-conductive polymer with a phenazine link is given here; two mobile anilines in a row conduct better. The photovoltaic properties are given, and it is the PCBM fullerene that carries the electrons away.47 Adapted from ref 47. Copyright 2010, American Chemical Society.

Figure 9. AFM picture of a covalent carotene−porphyrin vesicle membrane on hydrophobic graphite and hydrophilic mica surfaces.49 Adapted from ref 49. Copyright 1997, American Chemical Society.

photoxidation of I− to I3−. Porphyrins alone, however, are totally inactivated by the precipitation with iodide ions, and the addition of monomeric benzosulfonate has no activating effect. The polymer thus functions as a solubilizing scaffold, and its negative surface charge keeps the porphyrins separated, even the water-insoluble iodides. An irradiated dye polycation− polyanion−iodide system has, of course, unpredictable, strongly time-dependent properties, but the components are so easily accessible and the polymer scaffold has such a dramatic proteinlike effect that it is also very promising for a manyexperiment evolution (section 7). Another cationic tetraphenylporphyrin with four tetraalkylammonium groups produces singlet oxygen under white light when attached to a water-soluble poly(dicarboxylatethiophene). A dilute aqueous solution kills Gram-negative Escherichia coli and Gram-positive Bacillus subtilis within minutes under visible light.42

porphyrin polymers should, however, be most helpful for the utilization of incoming visible light. Related polymer dimers (ladders) replacing the thiophene in the copolymer by noncovalent bipyridine bridges in water conduct excitons much better than does a single porphyrin chain.44 Here it turns out, finally, that the electronic band and tunnel effects are in general massively overestimated.45 The stereochemical polaron model considering only geometric effects, namely, wire planarization and ordering upon ladder formation, yields much more realistic and quantitative predictions. Such exciton transfers are also most critical for the efficiency of dye-sensitized solar photovoltaic cells invented by Grätzel. Phthalocyanines are generally preferred to porphyrins because they absorb at longer wavelengths, their excitons diffuse longer, and their holes are more mobile. Measured exciton diffusion lengths ranged from about 10 nm in zinc porphyrin layers to 70 nm in copper phthalocyanine layers. However, nitrogen bridges instead of the methine hydrocarbons do not allow any substituents. Four positive charges would kill the π-electron pathway. Because phthalocyanines cannot provide the dynamic stereochemistry of tetraphenylporphyrins, they are of limited interest to chemists. Tetra(4-aminophenyl)porphyrin (TAPP), for example, polymerizes upon oxidation in dichloromethane, and its structure looks like that of polyaniline. The addition of pyridine slows the polymerization and favors the formation of thin and lightyellow films. Without pyridine, thick black films are formed. Both types are, however, equally good at conducting electrons. The morphology of poly-TAPP shows a highly interconnected nanofibrous network with fiber diameters in the range of 40− 100 nm (Figures 8 and 9). High-surface-area polymer fibers (Figure 8) with bulky and mobile bis-aniline polyamide linkages create a photovoltaic cell with commercial electron acceptor

6. COVALENT WIRES AND LAYERS The four octyloxy side chains, −O(CH2)7CH3, above and below a stiff meso-acetylene-connected porphyrin 40-mer, produce highly concentrated organic solutions. They deposit directly on Au(111) surfaces as highly ordered, parallel molecular rows of about 40 monomers when applied as an ultrahigh vacuum electrospray. The rigid acetylene connection and the gold planarity work together.15 The octyl chains should also be useful as a matrix for polythiophene wires parallel to the porphyrin chain. Micrometer-long covalent porphyrin polymers with acetylene−thiophene bridges hardly conduct electricity (∼10−3 S cm−1) when compared to polythiophene wires (6−8 S cm−1).43 A conducting and ordering polythiophene scaffold dissolved in the hydrocarbon layer consisting of the alkyl substituents of the I


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they cannot be copied. Chlorophyll and heme need cells and organisms; chemists need stable, versatile, symmetric, and quickly accessible porphyrins. The artificial world of tetraphenylporphyrins (TPPs) and smooth surfaces remembers sunlight, oxygen, and even carbon dioxide but forgets about proteins and biology. The one-step synthesis, the free rotation of the phenyl groups that transports four charged, hydrogen-bonding, or hydrophobic substituents to selected surfaces, and the stability and versatility of the metal complexes are a unique combination. A proposal for the next experiment is the following: the study of tetraalkylate, the central nitrogen atoms of tetraphenylsulphonato-porphyrin, and the folded, polyene-type porphyrins in neutral water. In other words, we should keep going with TPPs. Smooth silicate particles will probably remain the common scaffolds of TPPs, whereas mica is good for well-defined pores. The endless cellulose tubules of thoroughly dried wood tissue, however, should also be investigated as scaffolds for noncovalent porphyrin assemblies as well as for covalent polymers. Polished glassy carbon electrodes (GCEs)48 as well as conductivity measurements on platinum electrodes43 should always be applied for conducting assemblies. Any improvement of a porphyrin assembly property must be quantified. Electrospinning machines for highly concentrated polymer solutions are waiting to establish dynamic networks. In other words, choose TPP substitution patterns and scaffold combinations and then change assembly conditions in order to create the desired functionality. Concrete, commercial applications are important but apparently are difficult to achieve with porphyrin assemblies. Dye-sensitized voltaic cells are, after 30 years of experimentation, always the least efficient ones in Wikimedia comparisons; inorganic semiconductors always win. A porphyrin-coated cathode, which reduces oxygen to water, is unlikely to be stable in an electrical machine, which burns hydrogen or hydrocarbons on the anode. Whether porphyrin dyes and light can really replace poisons, which also kill bacteria and cancer cells, must be proven in hospitals. Chemists may attempt to purify dirty drinking water from carefully selected, endangered areas in Africa. Floating, smooth, or porous silicate particles coated with cationic TPPs should be applied under light and then filtered off with simple, natural devices. Interested African students of chemistry paid by drinking water fellowships could take a look at Europe and, if successful, practice these principles in their homelands. In other words, be realistic and modest with applications.

PCBM, a fullerene with a phenyl group, and a butyric acid methyl ester as side chains.46,47 Rigid, tricyclic phenazine links, however, cause a loss of both chain flexibility and electron conductivity. The TAPP polymers are then overoxidized, a phenomenon called pernigraniline in polyaniline. Another application of stable porphyrin polymers is possible in the production of electricity by fuel cells, namely, in the reduction of oxygen to water on the cathode, which accompanies the oxidation of the fuel on the anode. Metalloporphyrin polymers (e.g., conducting poly{[mesotetrakis(2-thienyl) porphyrin]-Co(II)} networks containing Co−Co bifacial binding clefts) perform the desired fourelectron reduction part in acidic, neutral, and basic solutions on conductive glass electrodes and show a stable peak current density for 100 cycles.48 Solid membranes made of porphyrin polymers with carotene side chains are also known. Zinc-tetra(4-aminophenyl)porphyrin with four bixinyl groups produces at first stable vesicles with a monomolecular 4.7 nm membrane and an absorption band stretching from 300 to 600 nm. Irradiation with visible light cross-links the membrane efficiently without disturbing the membrane’s shape; the tiny vesicle diameters between 30 and 120 nm do not change after polymerization, upon addition of sodium chloride or 95% ethanol or upon drying on graphite or mica (Figure 9). Only on strongly hydrated mica is a slow collapse observed. The tetrabixinyl porphyrin also produces stable Langmuir−Blodgett monolayers on water and silica, which again polymerize upon irradiation. The combination of the porphyrin−polyene vesicle with an electron-accepting guanidinium porphyrin leads to relatively long lifetimes of light-induced charge separations.49 The successful syntheses of so many covalent porphyrin polymers indicate the tertraphenylporphyrin polymers with the most impressive monomer molecular weights above 600, a tetrasulfonate of about 1000. 40mers are usually accessible and isolable. TPPs also yield the most reproducible and well-defined microscopic assemblies in water and on solid surfaces. It is probably the rotation of the substituted phenyl groups that leads to extremely complicated structural mixtures and stops precipitation. The final note on covalent dye polymers, possibly containing porphyrins, concerns a hydrothermal reaction of unsubstituted pyrrole with any of the following three phenylene dialdehydes:

The conditions are tough: glacial acetic acid at high pressure in a Teflon-lined autoclave at 453 K for 3 days and small amounts of FeCl3 are all crucial. A dark-brown, ill-defined polymer network is obtained in isolated yields of between 20 and 44%. The material is porous, and it reversibly adsorbs 19 wt % carbon dioxide at 273 K and 1 bar.50 Such dirty sponges can probably be made quickly on the 100 g scale. In the presence of a reducing agent such as thiophene, they may also fix molecular oxygen.

7. PERSPECTIVES Both natural life and the luxury of civilization use sunlight and molecular oxygen as energy sources. The chlorophylls and their protein scaffolds bring us plants, hydrocarbons, and coal, and hemes perform respiration. In the laboratories of chemists, however, biological systems can only be isolated and analyzed;

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. J


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Biography

(6) (a) Smith, K. M. (ed.), Porphyrins and metalloporphyrins; Elsevier:1975, Amsterdam; (b) Kadish, K. M.; Smith, K. M.; Guilard, R. (eds), The Porphyrin Handbook, Elsevier: 1995, 2003 and 2011, Amsterdam (7) Egner, P. A.; Stansbury, K. H.; Snyder, E. P.; Rogers, M. E.; Hintz, P. A.; Kensler, T. W. Identification and Characterization of Chlorin e4 Ethyl Ester in Sera of Individuals Participating in the Chlorophyllin Chemoprevention Trial. Chem. Res. Toxicol. 2000, 13, 900−906. (8) Kathawalla, I. A.; Anderson, J. L.; Lindsey, J. S. Hindered Diffusion of Porphyrins and Short-Chain Polystyrene in Small Pores. Macromolecules 1989, 22, 1215−1219. (9) Sun, P.; Jose, D. A.; Shukla, A. D.; Shukla, J. J.; Das, A.; Rathman, J. F.; Ghosh, P. Covalently Linked, Porphyrin-Based Amphiphiles: A Detailed Atomic Force Microscopic Study. Langmuir 2005, 21, 3413− 3423. (10) Kazak, A. V.; Usol’tseva, N. V.; Yudin, S. G.; Sotsky, V. V.; Semeikin, A. S. Influence of meso-Substituted Tetraphenylporphyrin Derivatives Structure on Their Supramolecular Organization in Floating Layers and Langmuir−Blodgett Films. Langmuir 2012, 28, 16951−16957. (11) Fuhrhop, J. H.; Wasser, P.; Riesner, D.; Mauzerall, D. Dimerization and π−Bonding of a Zinc Porphyrin Cation Radical. J. Am. Chem. Soc. 1972, 94, 7996−8001. (12) Kuroda, Y.; Kawashima, A.; Hayashi, Y.; Ogoshi, H. SelfOrganized Porphyrin Dimer as a Highly Specific Receptor for Pyrazine Derivatives. J. Am. Chem. Soc. 1997, 119, 4929−4933. (13) van Blaaderen, A.; Vrij, J. Synthesis and Characterization of Monodisperse Colloidal Organo-silicon Spheres. J. Colloid Interface Sci. 1993, 156, 1−12. (14) Kopaczynska, M.; Wang, T.; Schulz, A.; Dudic, M.; Casnati, A.; Sansone, F.; Ungaro, R.; Fuhrhop, J. H. Scanning Force Microscopy of Upright-Standing, Isolated Calixarene-Porphyrin Heterodimers. Langmuir 2005, 21, 8460−8465. (15) Saywell, A.; Sprafke, J. K.; Esdaile, L. J.; Britton, A. J.; Rienzo, A.; Anderson, H. L.; Shea, J.N. O; Beton, P. H. Conformation and Packing of Porphyrin Polymer Chains Deposited Using Electrospray on a Gold Surface. Angew. Chem., Int. Ed. 2010, 49, 9136−9139. (16) Barlow, D. E.; Scudiero, L.; Hipps, K. W. Phthalocyanine and Cobalt(II) Tetraphenylporphyrin on Au(111): Mixed Composition Films. Langmuir 2004, 20, 4413−4421. (17) Yoshimoto, S.; Tada, A.; Suto, K.; Yau, S. L.; Itaya, K. Microscopy of Molecular Assemblies of Cobalt(II)- and Copper(II)Coordinated Tetraphenylporphine and Phthalocyanine on Au(100). Langmuir 2004, 20, 3159−3165. (18) Fuhrhop, J. H.; Bedurke, T.; Gnade, M.; Schneider, J.; Doblhofer, K. Hydrophobic Gaps of Steroid Size in a Surface Monolayer Collect 1,2-trans-Cyclohexanediol and Glucose from Bulk Water. Langmuir 1997, 13, 455−459. (19) Ward, M. D. Bulk Crystals to Surfaces: Combining X-ray Diffraction and Atomic Force Microscopy to Probe the Structure and Formation of Crystal Interfaces. Chem. Rev. 2001, 101, 1697−1725. (20) Choma, C. T.; Lear, J. D.; Nelson, M. J.; Dutton, P. L.; Robertson, D. E.; DeGrado, W. F. Design of a Heme-Binding FourHelix Bundle. J. Am. Chem. Soc. 1994, 116, 856−865. (21) Villareal, V. A.; Spirig, T.; Robson, S. A.; Liu, M.; Lei, B.; Clubb, R. T. Transient Weak Protein Protein Complexes Transfer Heme Across the Cell Wall of Staphylococcus aureus. J. Am. Chem. Soc. 2011, 133, 14176−14179. (22) Brown, C. M.; Reisfeld, B.; Mayeno, A. N. Cytochromes P450: a structure-based summary of biotransformations using representative substrates. Drug Metab. Rev. 2008, 40, 1−100. (23) Moors, S. L.C; Vos, A. M.; Cummings, M. D.; Van Vlijmen, V.; Ceulemans, A. Structure-Based Site of Metabolism Prediction for Cytochrome P450 2D6. J. Med. Chem. 2011, 54, 6098−6105. (24) Jett, J. E.; Lederman, D.; Wollenberg, L. A.; Li, D.; Flora, D. R.; Bostick, C. D.; Tracy, T. S.; Gannett, P. M. Measurement of Electron Transfer through Cytochrome P450 Protein on Nanopillars and the Effect of Bound Substrates. J. Am. Chem. Soc. 2013, 135, 3834−3840.

Jürgen Fuhrhop was born on February 4, 1940, in war-torn Berlin. He grew up with positive impressions of American culture and life from his family’s movie theater “Casa Candida”, which they obtained from an american general in 1946. At the age of 16, he independently decided to become a chemist. Three years later, after learning Holleman-Wiberg’s Inorganic Chemistry by heart, he quickly passed all of the laboratory courses and examinations at the Free University of Berlin but decided to continue his studies at the University of Braunschweig because his professor of organic chemistry at the FU Berlin became ill. His Ph.D. thesis on the formylation and lead dioxide oxidation of porphyrins prepared him well for a 2-year postdoctoral research stay with Professors Sam Granick and David Mauzerall at Rockefeller University, New York. Jürgen Fuhrhop did his habilitation at U Braunschweig, Professor Hans Herloff Inhoffen, and returned in 1979 to the FU Berlin as a professor of bioorganic chemistry. There he switched to bola amphiphiles and monolayer vesicle membranes in water and headed the Collaborative Research Center (Sonderforschungsbereich) “Vectorial Membrane Processes” from 1985 to 2000. He also combined membranes with porphyrins and solid surfaces and wrote 10 books on various aspects of biological and synthetic organic chemistry. In 2006, he completed his research work with the invention of yoctowells on planar silicate surfaces but is still reading and writing books at the Institute of Chemistry and Biochemistry, FU Berlin. Also, he wrote one of the very first papers to be published in Langmuir in the year of its founding, 1985.

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ACKNOWLEDGMENTS Professor Weiss invited me to write a historical porphyrin assembly review, and Dr. Pamela Winchester and eight reviewers helped enormously to render my first attempt readable. I am very grateful for all of the interest and enormous help and thank them all.

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REFERENCES

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Historical Review

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Porphyrin Assemblies and Their Scaffolds - Langmuir (ACS

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