Peripheral Ruthenium(II) Complexes and Orthogonal, Ferrocyanide

D-14195 Berlin, Germany, and Max-Volmer-Institut fu¨r Physikalische Chemie,. Technische Universita¨t Berlin, Strasse des 17, Juni 135, D-10623 Berli...
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Langmuir 1999, 15, 5029-5039

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Peripheral Ruthenium(II) Complexes and Orthogonal, Ferrocyanide-Linked Bilayers of (β-Tetraethyl-β-p- and m-Tetrapyridyl)porphyrins in Bulk Water Dan Donner,† Christoph Bo¨ttcher,† Christian Messerschmidt,† Ulrich Siggel,‡ and Ju¨rgen-Hinrich Fuhrhop*,† Institut fu¨ r Organische Chemie der Freien Universita¨ t Berlin, Takustrasse 3, D-14195 Berlin, Germany, and Max-Volmer-Institut fu¨ r Physikalische Chemie, Technische Universita¨ t Berlin, Strasse des 17, Juni 135, D-10623 Berlin, Germany Received December 31, 1998. In Final Form: March 30, 1999 2,8,12,18-Tetraethyl-3,9,13,17-tetra-m- and p-pyridylporphyrins have been synthesized via dipyrromethanes. They form strongly fluorescing planar monolayer leaflets in water. Upon titration with ferrocyanide the m-pyridylporphyrin adsorbs these anions and the fluorescence decreases drastically. One ferrocyanide ion quenches the fluorescence of about 50 porphyrin units. In the case of the p-isomer the quenching effect is much less pronounced. The zinc complex gives no defined monolayer in water, but assembles in the presence of equimolar amounts of ferrocyanide to a triple layer. Here, the central porphyrins are rotated into two different orthogonal positions: one of these porphyrin molecules provides its β-pyridyl groups as axial ligands of the central zinc ions for the outer layers, the other takes part in a closed hydrophobic region, in which the β-ethyl groups of two orthogonal porphyrin ligands interact. Transmission electron and atomic force microscopy were used to characterize this new type of molecular assembly.

Introduction β-Tetraethyl-β-tetra-4-pyridinium porphyrins have, so far, only been synthesized as a mixture of the four regioisomers. This mixture is very soluble in water (10-1 M) and does not precipitate upon charge neutralization in heterodimers with meso-tetraphenylsulfonatoporphyrins.1 The binding constant is between 107 and 108 M and the water solubility of the electroneutral dimer is around 10-3 M. The pure isomer II, which has hydrophobic east and west edges and hydrophilic north and south edges, is the least-soluble of the four regioisomers. It assembles in water to form fluorescing monolayer sheets.2 These sheets provide positively charged surfaces, which may be used as a basis for the construction of complex and redoxactive molecular assemblies by their interaction with appropriate anions. We report here on a rational total synthesis of isomer II of tetra-4- and tetra-3-pyridinium porphyrins 1 (p-pyridyl) and 2 (m-pyridyl) (Scheme 1). Both porphyrins form stable monolayer leaflets, which do not roll up under salt-free conditions. The planar monolayer, on one hand, which is partially protonated at pH 2.5 and therefore provides two positively charged surfaces reacts with appropriate anions. Two m-pyridinium substituents, on the other hand, also complex metal cations. This paper gives a first structural account of the monoand bilayers, which are formed upon reaction with ferrocyanide and ruthenium(II) ions. Results 2,8,12,18-Tetraethyl-3,7,13,17-tetra-4-pyridylporphyrin 1 and the corresponding 3-pyridyl regioisomer 2 were synthesized by regioselective condensation reactions via * To whom correspondence should be addressed. † Freie Universita ¨ t Berlin. ‡ Technische Universita ¨ t Berlin. (1) Endisch, C.; Fuhrhop, J.-H.; Buschmann, J.; Luger, P.; Siggel, U. J. Am. Chem. Soc. 1996, 118, 6671. (2) Endisch, C.; Bo¨ttcher, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1995, 117, 8273.

dipyrromethanes. The synthesis of 1 started with a Claisen condensation of ethyl isonicotinoate.3 Nitrosation, catalytic hydrogenation, and condensation with acetylacetone yielded the m-pyridylpyrrole 3 in 78% yield.4 Attempts to reduce the oxime with zinc in acetic acid failed. The acetyl group of 3 was reduced to ethyl with borohydrideborotrifluoride,5 the ethyl ester on C2 transesterified with benzyl alcohol, and the 5-methyl group was acetoxylated with lead(IV) acetate and converted to the 2-acetoxymethyl group of 4.6,7 Selective hydrolysis of the acetate and deformylation8 of 4 to give dipyrromethane 6 was achieved by heating in 33% trifluoroacetic acid under nitrogen. Debenzylation by hydrogenation yielded the dicarboxylate 7, which had to be thoroughly purified by reverse phase chromatography for the final steps to occur in acceptable yields. Thermal decarboxylation9 and diformylation10 led to dipyrromethanes 8 and 9, which were then cyclized in HBr/HOAc to the a,c-biladiene 1011 and dehydrogenated to give the desired porphyrin 2. The overall yield of the 12-step synthesis starting with nicotinic acid ethyl ester was 17%. One gram could be prepared within about 3 weeks. Porphyrin 1 with 4-pyridyl substituents was synthesized by an analogous procedure, but the experimental conditions were often quite different because of drastic differences in solubilities (see the Experimental Section; Scheme 2). The 1H NMR spectra of porphyrins 1 and 2 in DMSO/ TFA (dimethyl sulfoxide/trifluoroacetic acid) 9:1 clearly (3) Gilman, H.; Broadbent, H. S. J. Am. Chem. Soc. 1948, 70, 2757. (4) Ochiai, E.; Tsuda, K.; Ikuma, S. Chem. Ber. 1935, 68, 1710. (5) Whitlock, H. W.; Hanauer, R. J. Org. Chem. 1968, 33, 2169. (6) Dolphin, D. The Porphyrins; Academic Press: New York, 1978; Vol. 1, Chapter 4. (7) Trost, B. H.; Fleming, I. Comprehensive Organic Synthesis; Pergamon Press: Oxford, 1991; Vol. 7, p 92. (8) Johnson, A. W.; Kay, I. T.; Markham, E.; Price, R.; Shaw, K. B. J. Chem. Soc. 1959, 3416. (9) Young, M.; Chang, C. K. J. Am. Chem. Soc. 1985, 107, 898. (10) Silverstein, R. M.; Ryskiewicz, E. E.; Willard, C.; Koehler, R. C. J. Org. Chem. 1955, 20, 670. (11) Arsenault, G. P.; Bullock, E.; MacDonald, S. F. Can. J. Chem. 1960, 38, 4384.

10.1021/la981791+ CCC: $18.00 © 1999 American Chemical Society Published on Web 06/05/1999

5030 Langmuir, Vol. 15, No. 15, 1999 Scheme 1

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Transmission electron micrographs of negatively stained probes (not shown) revealed leaflets of 1 and 2 which were not rolled up as observed in preparations starting with 1 in 10-1 M NaCl solution.1 The platelets of 2 nm thickness made of 2 (Figure 3) were then titrated in water at pH 3 with ferro- or ferricyanide. Stern-Volmer plots indicate efficient fluorescence quenching. The initial slope was 5 × 107 L/mol corresponding to fluorescence quenching of 50 porphyrin molecules by 1 ferrocyanide anion. Ten molar percent of ferrocyanide quenched about 90% of the fluorescence of a 10-6 M porphyrin solution. The quenching of the p-tetrapyridylporphyrin 1 was less effective by a factor of 10 and for the rolled-up sheets of 1 in the presence of NaCl (10-1 M) by a factor of 100. 1,2-Naphthoquinone and iron(II) or iron(III) salts were much less efficient as quenchers when compared to ferrocyanide (Figure 4). The visible spectrum of 2 (Figure 1) did hardly change upon the addition of ferrocyanide (not shown). The zinc porphyrinates, Zn-1 and Zn-2, did not form stable leaflets of single-layer thickness when the injection method was applied. Electronic spectra and electron microscopy indicated monomers and small, ill-defined aggregates. Titration of these leaflets with ferrocyanide in water at pH 2.7, however, caused a color change to green. In UV/vis spectra the Soret band at 438 nm (λ1/2 ) 40 nm) changed to two broad bands at 445 and 515 nm, which points to excitonic effects in lateral assemblies.. The Q-bands were red-shifted to 596 and 654 nm (Figure 5a). A similar spectrum has been observed earlier for Zn-1 leaflets which were formed in the presence of poly(vinyl sulfate) coatings.12 The corresponding m-tetrapyridyl regioisomer 1a showed no 654 nm band upon ferrocyanide titration and the line broadening was much less important (Figure 5b). The Zn-1 ferrocyanide leaflets show regular striations under the electron microscope stretching over the whole length of the monolayered crystallite. The interlayer distance has been determined by calculating Fourier transforms of the digitized images (“power spectra”) and is 20 Å; another regular striation, perpendicular to the first one, occurs and corresponds to a distance of 8.8 Å (Figure 6, inset; for AFM see Experimental Section). It was then also attempted to prepare peripheral ruthenium(II) complexes of 1 and 2. For this purpose we applied cis-dichloro-bis(2-bipyridyl)ruthenium in acetic acid/methanol.3 Both porphyrins showed, in variance to the 1H NMR spectra, similar reactivities toward Ru(II) ions. The major effects of the peripheral Ru(II) ions was water solubilization. UV/vis spectra showed only a broadening of the Soret band at 401 nm and the four visible bands at pH 6. The kinetically inert ruthenium ion remained tightly fixed even in 10-6 M aqueous solutions. The fluorescence of both porphyrins 1 and 2 was totally quenched. Both complexes also survived reversed phase chromatography. The ruthenium complexes were quickly eluted at pH 6, whereas the noncomplexed starting materials only migrated at pH values below 3.

showed the absence of regioisomers, which have been characterized earlier in the case of 1 (Figure 1). The anticipated complexation of the m-pyridyl groups at C3, C7 and C13, C17 with ruthenium ions was indicated by extra splittings of the pyridyl proton signals and small upfield shifts after the addition of ruthenium dichloride to a DMSO solution of 2. The p-regioisomer did not show these effects under identical conditions. Both porphyrins 1 and 2 were soluble in water below pH 1 and formed monolayered molecular leaflets upon titration with sodium hydroxide at pH 2.5 and 1.9. The resulting colloidal solutions were, however, unstable and the monolayers needed protection by wrapping with soft polymer coatings to prevent precipitation.12 Nitrogen bubbling, for example, led immediately to precipitation of noncoated leaflets. It was now found out that this instability and the tendency of the leaflets to roll up to scrolls were induced by the salts formed in the neutralization process. If this was avoided by using an injection technique, then perfectly stable colloidal solutions were formed. In the optimized procedure we used 10-2 M solutions of 1 or 2 in TFA/DMSO (2:3) and injected it into 0.04% TFA in water (pH 1.9; Soret band: 344 and 473 nm). Total conversion of the monomers to the monolayer leaflets took about 2 h and was indicated by the shift of the Soret band from 409 to 473 nm (Figure 2). Atomic force microscopy (AFM) experiments in the tapping mode on mica surfaces showed that the first molecular assembly of the p-tetrapyridinium compound 1 appeared in the form of fibers with a height of 7 Å (Figure 3a,b). This corresponds to monomers lying flat on the negatively charged mica surface with orthogonal pyridinium rings. The width of the fibers, however, is about 30-50 nm and therefore far away from a molecular dimension. At least 10 molecules of porphyrin 1 lie parallel to each other in a flat ribbon. The typical length of these ribbons is in the order of 300 nm corresponding to about 200-300 molecules (Figure 3a). After a few hours, however, these bulky fibers disintegrate. Aged assemblies of the same porphyrin obtained under the same conditions appear as monomolecular leaflets with a thickness of 17 Å, corresponding to a monolayer of upright standing porphyrins (Figure 3c). In this arrangement, the mica subphase can only be in direct contact with two pyridinium rings. The phase shift in the tapping mode is 3° ( 1° in the fiber case and 5° ( 1° for the monolayer. The fibers are thus more of a fluid arrangement than the twodimensional (2D) crystal.13-15 Porphyrin 2 shows similar structures on mica.

The regioselective synthesis of 1 and 2 ran astonishingly well considering the possibilities of acid-base-catalyzed rearrangements of the β-pyridyl substituents. Fulvenyltype intermediates, which are quite common in pyrrole chemistry,5,6 did obviously not occur. All preparative

(12) Bindig, U.; Endisch, C.; Fuhrhop, J.-H.; Komatsu, T.; Tsuchida, E.; Siggel, U. J. Colloid Interface Sci. 1998, 199, 123. (13) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, 385.

(14) Noy, A.; Sanders, C. H.; Vezenov, D. V.; Wong, S. S.; Lieber, C. M. Langmuir 1998, 14, 1508-1511. (15) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, J.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807.

Discussion

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Langmuir, Vol. 15, No. 15, 1999 5031 Scheme 2

difficulties, which are described in detail in the experimental part,7 stem from the amphiphilic character and the nucleophilicity of the pyridylpyrroles. It was anticipated that the m-pyridyl substituents in 3,8- and 13,18-positions would function as 2,2′-dipyridylanalogous ligands for metal ions, in particular for redoxactive ruthenium(II) and ferrocyanide ions. In the corresponding p-pyridylporphyrins we expected lesser effects of redox-active metal ions because the binding angle and distance are not optimal. The experimental results showed, however, practically no difference for the ruthenium complexes, which may be due to the fact that single pyridine units already bind to ruthenium(II) bis(bipyridyl).8 Up to four ruthenium ions may therefore associate the periphery of 1. In the case of ferrocyanide, however,

the expected selectivity was observed: fluorescence quenching efficiency of the m-bipyridyl units was higher by a factor of about 100 as compared to the p-bipyridyl units. This is presumably caused by better fitting binding angles rather than fitting distances, since the ferrocyanide diameter is large enough for both types of ligands. The most astonishing supramolecular effect, however, was observed with the zinc complex of the p-pyridyl isomer 1 and ferrocyanide. Enormous red-shifts and line broadening in UV/vis spectra and well-defined monolayered crystal plates under the electron microscope were observed (Figure 6). Neither the m-tetrapyridyl zinc porphyrinate Zn-2 nor the free bases 1 and 2 produced similar spectra or TEM patterns. The 2.0 nm distance observed in electron micrographs is in good agreement with the width of a

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Figure 1.

1H

Donner et al.

NMR spectrum of 1 in CDCl3.

Figure 2. Time-dependent UV/vis spectra after injection of 2a in 10 µL of TFA/DMSO 2:3 into 10 mL of 0.04% TFA.

Zn-1 molecule with two peripheral ferrocyanide ions bound to it. The porphyrins must lie parallel to the solid subphase. The 0.88 nm distance occurs orthogonal to the 2 nm striations corresponding to porphyrin-ferrocyanide layers. The only reasonable explanation, which occurred to us, is the coordination of a second porphyrin Zn-1 layer orthogonal to the first one. The connection occurs by zincpyridine ligation similar to the one found in a crystal structure of 5-pyridyl-10,15,20-tetraphenyl zinc porphyrinate.8 Upon introduction of a central zinc ion, some porphyrins obviously rotate by 90° around the y-axis within the original monolayer leaflet of the free base porphyrin. These porphyrins now act as pyridyl donors for the zinc ions of the first porphyrin layer. The intermittent space between the orthogonal-lying porphyrinates is filled by another layer of zinc porphyrinates, which is again rotated by 90°, this time around the x-axis. This brings the peripheral ethyl groups in contact. The 8.8 Å distance then corresponds to the distance between the porphyrin

units of the orthogonal layer (Figure 7a,b). This proposal is in agreement with all findings including the similarity of the UV/vis spectrum with the one of polymer-coated Zn-1, including the 16 Å height of the platelets in AFM. The netlike structure thus contains three types of porphyrins: (i) two Zn-1-[Fe(CN)6]24- planes parallel to the subphase which are connected by (ii) orthogonal Zn-1 molecules connected by Zn-pyridine coordination and (iii) Zn-1 porphyrins, which fill up the space between the type (ii) porphyrins. Their ethyl groups form hydrophobic envelopes at the non-pyridyl edges and provide slippery regions in the net. Edges formed by the ethyl groups do not lead to reflexes in the electron micrographs. The 5.3 nm height found in AFM pictures finally corresponds to the ferrocyanide-linked bilayer depicted in Figure 7. The m-pyridyl group cannot bind to zinc ions of neighboring porphyrinates. Its most useful supramolecular property is the tight binding of ferrocyanide combined with cooperative quenching. If one assumes a stacking

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Figure 3. Atomic force micrographs (AFM) of (a,b) fibers and (c,d) monolayers of porphyrin 1.

distance of ∼9 Å and a lateral distance of 16 Å between porphyrin centers, a typical leaflet would, according to the TEM pictures, contain about 80 molecules. The finding that a ferrocyanide ion quenches the fluorescence of about just that number of porphyrins suggests, that efficient energy transfer occurs along the stacks. The hydrophobic, fluid regions could also be used to anchor hydrophobic molecules or amphiphiles. The phenomenon is similar to the fluorescence quenching observed in Scheibe-type fibers made of pseudoisocyanine dyes, where up to a few thousand molecules loose their fluorescence upon addition of one quinone molecule.16 The major attractions of the noncovalent β-pyridylporphyrins with two opposite hydrophobic and hydrophilic edges are the following: (i) The pyridyl surfaces allow the integration of redox-active anions as well as of metal ions and (ii) the flexible side-on connection by a hydrophobic ethyl group allows various reorientations of the porphyrins within monolayer and bilayer assemblies. In this respect, they provide unique material properties and relative dye orientations. Experimental Section Methods. 1H and 13C NMR were recorded on a Bruker AM 270 SY spectrometer. Melting points were measured on a Bu¨chi capillary melting-point apparatus. Data were not corrected. Solvents and reagents were of reagent grade quality, purchased from Acros. Preparative chromatography was performed on ICN (16) Katheder, F. Kolloid-Z. 1940, 92, 299.

silica gel 60 µm. Reversed phase chromatography on Aldrich octadecyl-functionalized silica gel, and analytical thin layer chromatography (TLC) on precoated silica gel 60 F-254 plates. Atomic Force Microscopy. A 25 µl solution of porphyrin 1 at pH ) 2.5 was spread on freshly cleaved mica. After 30 s the solution was carefully blotted off. Scanning force microscopy (SFM) was performed using a Digital Instruments Nanoscope III (Santa Barbara, CA) in the tapping mode. Silicon cantilevers (Digital Instruments) with a spring constant of 17-64 N/m and a resonance frequency in the range of 240-400 kHz were used. The scanning rate was usually 1 Hz. AFM measurements of freshly prepared leaflet solutions showed short fibers, having a length of up to a few micrometers and a width of 30-50 nm (Figure 3a). A height histogram (bearing analysis) gave fibers with a height of 0.7 nm. A few leaflets with a height of 1.7 nm were also detected as small bright spots having a length of 200-300 nm and a width of 50-100 nm (Figure 3a). AFM measurements of a leaflet solution after 3 h showed only 1.7 nm leaflets. No fibers were left (Figure 3b). These aggregates correspond well to the leaflets found in transmission electron micrographs. The porphyrins lie perpendicular to the mica surface in a crystalline arrangement. The fibers correspond to stacks as indicated. The blue-shift of the Soret band developed very quickly. The red-shift, on the other hand, needed many minutes to hours to evolve (Figure 2). The height difference points to a high tilt angle of 65° of the porphyrins on the mica surface. Finally, crystalline leaflets continue to grow and eventually precipitate. AFM measurements have revealed three-dimensional crystals of different heights. The smallest unity, however, was always the proposed triple layer consisting of 2 layers of Zn porphyrin and a single layer of hexacyanoferrat(II) in between. Its height

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Figure 4. (a) UV/vis (s) and fluorescence excitation (- - -) spectra of 2 leaflets in water; (b) Stern-Volmer plots of the 644 nm fluorescence quenching of 10-5 M solutions of porphyrin 1 leaflets in water at pH 2.4, excitation wavelength 527 nm; (c) a qualitative model of energy transfer. amounts to 5.2 nm. The next higher structure has a height of 10.3 nm, corresponding to 2 three-layer units. Transmission Electron Microscopy (TEM). A droplet (5 µl) of the freshly prepared solution was placed on hydrophilized (treated for 60 s at 10 mA (8 W) and 5 × 10-1 bar in argon plasma using a BALTEC MED 020 (BALTEC, Liechtenstein)), carboncoated copper grids and blotted after 45 s with the help of a filter paper. The air-dried grids were transferred into a Philips CM12 microscope (TEM). Imaging was carried out at a primary magnification of 58300× and high-resolution imaging was at 160000×. Fourier transform calculations of the micrographs were performed in the context of the IMAGIC 5 software (Image Science GmbH, Heilbronner Str. 10, 10711 Berlin, Germany). All new compounds were characterized by TLC, 1H NMR, and IR spectra and MS. Only a few typical pieces of data are given in the following. Syntheses. Ethyl 3-Oxo-3-pyrid-3-yl Propionate. Sodium hydride (60% dispersion in mineral oil) (60 g, 1.5 mol) was washed oil free with ether and dispersed in 600 mL of dry ether. Then, 92.5 mL (1.65 mol) of absolute ethanol was added dropwise over 30 min. Absolute ethyl acetate (210 mL, 2.145 mol) and 123 mL of freshly distilled ethyl-3-pyridyl acetate (0.9 mol) were added, and the inhomogeneous reaction mixture was distilled to remove the ether. The resulting slurry was stirred under nitrogen at 100 °C for 18 h. After cooling, 172 mL of acetic acid was added and the mixture was neutralized with saturated sodium bicarbonate solution. The mixture was extracted with CH2Cl2 (400 mL × 3). The extract was washed with water and dried over Na2SO4. The solvent was evaporated to give a brownish paste which was ready to proceed to the next step. Crude yield: 139 g, (80%). Ethyl 2-Hydroxyimino-3-oxo-3-pyrid-3-yl Propionate. The above pyridyl acetoacetate (139 g, 0.72 mol) in 360 mL of acetic acid was cooled to 15 °C and 72 g of sodium nitrite (1.044 mol) in 150 mL of water was added dropwise to the bottom of the flask. A precipitate formed. The mixture was allowed to stand overnight at 4 °C and then was filtered and washed with water

until a pH of approximately 7 was reached. Crystallization from ethanol gave the oxime as a white solid. Yield 143.9 g (90%), mp 150 °C. 4-Acetyl-2-ethoxycarbonyl-5-methyl-3-pyrid-3-ylpyrrole (3). A 250-mL autoclave was charged with 40 g (180 mmol) of the above oxime, 33 mL (320 mmol) of acetyl acetone, and 10% palladium on carbon (500 mg, 4.7 mmol) in 150 mL of acetic acid. The inhomogeneous mixture was stirred under 150 atm of H2 at 30 °C for 18 h. The catalyst was removed by filtration through Celite and the solvent removed by rotary evaporation. To the residue was added sodium bicarbonate solution, and the product was extracted with chloroform (200 mL × 3). The combined organic layers were washed with 100 mL of water, dried with sodium sulfate, and evaporated. Ether (100 mL) was added to cause precipitation of colorless crystals. Yield 38.2 g (78%), mp 129 °C. 2-Ethoxycarbonyl-4-ethyl-5-methyl-3-pyrid-3-ylpyrrole (4). Compound 3 (30.4 g, 112 mmol; dried over phosphorus pentoxide under vacuum) was dissolved in 300 mL of absolute tetrahydrofurane. Sodium borohydride (10.5 g, 275 mmol) was added with stirring at 5 °C under nitrogen. Borontrifluoride etherate (47 mL, 372 mmol) was added dropwise at 5-10 °C. The mixture was stirred for 1 h at room temperature, and 2 N hydrochloric acid (200 mL) was added slowly to quench the reaction. After being stirred overnight, the biphasic solution became almost clear. The solvent was evaporated, and the residue was neutralized with sodium carbonate solution. The pyrrole was extracted three times with 400 mL of chloroform. The chloroform was dried over sodium sulfate and evaporated. The remaining paste was washed with 50 mL of boiling ether, leaving 27.7 g of a white powder (96%) (mp 122 °C). 5-Acetoxymethyl-2-benzoxycarbonyl-4-ethyl-3-pyrid-3ylpyrrole (5). Compound 4 (25.8 g, 100 mmol) and 250 mg of sodium were refluxed in 120 mL of absolute benzyl alcohol at 25 mbar under nitrogen. Reversed phase TLC in methanol/water 8:2 revealed the product at an Rf of 0.31. After evaporation of the

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Figure 5. (a) UV/vis titration of p-tetrapyridylporphyrin Zn-1 in water (pH 2.7) with ferrocyanide; (b) same with Zn-2. benzyl alcohol, 1 mL of acetic acid, 100 mL of methanol, and 5 mL of water were added to cause precipitation. The light orange precipitate of the 2-benzylester was collected by filtration and crystallized from methanol, which gave fine colorless crystals. Yield 31 g (97%), mp 113 °C. The 2-benzylester analogue of 4 (32 g, 100 mmol) in 500 mL of acetic acid and 25 mL of acetic anhydride was stirred under nitrogen for 1 h. Lead tetraacetate (48.8 g, 110 mmol) was added and the suspension was stirred at 50 °C for 16 h. During this time, the solution became clear. The solvent was removed by rotary evaporation, and the residue was basified with saturated sodium carbonate solution to a pH of 8. After dispersion with 500 mL of chloroform, a white insoluble solid (lead carbonate) was removed by filtration. The organic layer was separated, washed with brine, dried over sodium sulfate, and evaporated, leaving behind a pale yellow paste. Purification was done on a octadecyl-functionalized silica gel column with 8:2 methanol/water as the eluent (Rf ) 0.38). Yield 36.7 g (97%), mp 102-103 °C. Bis(5-benzoxycarbonyl-3-ethyl-4-pyrid-3-ylpyrrole-2-yl)methane (6). Compound 5 (18.9 g, 50 mmol) in 200 mL of trifluoroacetic acid was diluted with 400 mL of water. The solution was refluxed for 4 h under a positive pressure of argon, during which time the solution changed from light orange to dark brown

and a tar formed. The solvent was evaporated under vacuum, and the resulting black pasty mass was lyophilized. The residue was redissolved in 200 mL of chloroform, and the solution was basified with triethylamine. The mixture was filtered and evaporated. The resulting black oil was passed through a silica gel column (12 × 20 cm) with 96:4 chloroform/methanol eluent. The first fraction was collected and evaporated. After the addition of methanol (200 mL) and concentration to about 50 mL, the product crystallized spontaneously. Further purification was achieved by crystallization from methanol. Yield 13.1 g (84%), mp 112 °C. Bis(3-ethyl-4-pyrid-3-ylpyrrole-2-yl)methane (8). A mixture of 6 (9.37 g, 15 mmol) and 10% palladium on carbon (400 mg, 3.8 mmol) in absolute dimethylformamide (DMF) (150 mL) was stirred under 150 atm of H2 at 30 °C for 18 h in an autoclave vessel. The resulting mixture of the air-sensitive diacid 7 and carbon in DMF was taken directly to the next step. After the addition of triethylamine (5 mL), the suspension was heated in the above autoclave vessel to 160 °C for 18 h and was filtered through Celite. The filtrate was concentrated, and the resulting brownish paste was redissolved in 20 mL of methanol. The desired product 8 began to crystallize by the dropwise addition of water to a final concentration of 10%. The light yellow crystals were

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Figure 6. (top) Transmission electron micrograph of zinc p-tetrapyridyl porphyrinate Zn-1 leaflets in the presence of ferrocyanide; (inset) Fourier transform [Inner reflexes: 2 nm. Outer reflexes: 0.88 nm]; (bottom) AFM of the same preparation on mica with height section analysis. quickly filtered off and washed with methanol/water 8:2. Yield 4.38 g (82%), mp 175 °C. 1H NMR (DMSO-d , δ in ppm): 0.94 (t, 6H, CH ), 2.56 (q, 4H, 6 3 CH2), 3.86 (s, 2H, dipyrrylmethane), 6.88 (d, 2H, pyrrole-CH), 7.35 (dd, 2H, 5-pyridyl) 7.74 (dt, 2H, 4-pyridyl), 8.36 (dd, 2H, 6-pyridyl), 8.59 (d, 2H, 2-pyridyl), 10.55 (s, 2H, pyrrole-NH). IR (KBr, cm-1): 3440 (NH), 1598 (CdC), 841 (pyridyl CH). MS (El, 80 eV, 170 °C) m/z: 356 (M+, 52%), 184 (C12H12N2+, 100%), 173 (C11H13N2+, 67%). Bis(3-ethyl-5-formyl-4-pyrid-3-ylpyrrole-2-yl)methane (9). Freshly distilled POCl3 (3.67 mL, 40 mmol) was added dropwise

to 3.85 mL of DMF (50 mmol) in absolute chloroform (830 mL) at 15 °C. The solution was stirred at room temperature for 1 h before a solution of 8 (3.56 g, 10 mmol) in absolute chloroform (60 mL) was added dropwise. The resulting brown mixture was refluxed for 30 min, at which time TLC showed no remaining 8. The mixture was cooled, and saturated aqueous sodium acetate (10 mL) was added slowly to quench the reaction. The mixture was basified with saturated aqueous NaHCO3. The aqueous layer was extracted several times with chloroform (total 300 mL). The organic layer was dried over sodium sulfate, filtered, and concentrated. The residue was redissolved in methanol (100 mL)

Ru(II) Complexes and Bilayers of Porphyrins

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Figure 7. Structural model of the bilayer leaflet of Zn-1-(ferrocyanide)2 derived from the electron micrograph in Figure 6. The orthogonal porphyrin arrangements come about by the interaction of the β-pyridyl units with the central zinc ions and hydrophobic interactions between ethyl groups. The indicated distances in nanometers are taken from the TEM and AFM images in Figure 6. and concentrated to remove chloroform. After the dark brown residue was redissolved in 20 mL of methanol, the desired product 9 began to crystallize by the dropwise addition of water to a final concentration of 15%. The white crystals were quickly filtered off and washed with methanol/water 7:3. Yield 2.80 g (68%), mp >250 °C. Rf value (reverse phase C18, MeOH/H2O 4:1): 0.34. Anal. Calcd for C25H24N4O2 (412.49 g/mol): C, 72.80; H, 5.86; N, 13.58. Found: C, 72.56; H, 5.82; N, 13.22. 1H NMR (270 MHz, DMSO-d , δ in ppm): 0.75 (t, 6H, CH ), 6 3 2.39 (q, 4H, CH2), 4.08 (s, 2H, dipyrrylmethane), 7.46 (dd, ZH, 5-pyridyl), 7.80 (dt, 2h, 4-pyridyl), 8.55 (d, 2H, 6-pyridyl), 8.56 (s, 2h, 2-pyridyl), 9.15 (d, 2H, CHO), 12.16 (s, 2H, pyrrole-NH). MS (El, 80 eV, 220 °C) m/z: 412 (M+, 100%), 397 (M-CH3+, 15%), 383 (M-C2H3+, 30%). 2,8,12,18-Tetraethyl-3,9,13,17-tetra-3-pyridylporphyrin (2). To a stirred -5 °C solution of 9 (413 mg, 1 mmol) in absolute acetic acid (40 mL) under argon was added acetic anhydride (300 mL) followed by HBr (2 mL of a 33% solution in acetic acid, 11.2 mmol). The solution was stirred for 10 min before a solution of 8 (178 mg, 0.5 mmol) in absolute acetic acid (5 mL) and acetic anhydride (45 mL) was added over a period of 30 min. To the resulting dark red solution was added additional HBr (1 mL of a 33% solution in acetic acid, 5.6 mmol) followed by 8 (178 mg, 0.5 mmol) in absolute acetic acid (5 mL) and acetic anhydride (45 mL). The mixture was allowed to warm to room temperature and stirred for 2 h before 2,3-dichloro-5,6-dicyano-benzoquinone (DDQ) (227 mg, 1 mmol) in 20 mL of acetic anhydride was added. The resulting dark brown mixture was stirred for an additional 4 h, at which time UV/vis showed no increase of the Soret band (410 nm). The mixture was cooled on ice, and ethanol (200 mL) was added slowly to quench the reaction. During this time the color of the solution changed from dark brown to yellow and a dark violet precipitate formed. The precipitate was collected by filtration, washed with acetone, and passed through a silica

gel column with a 9:1 chloroform/pyridine eluent. The product was a violet solid. Yield 408 mg (56%), mp > 250 °C. 1H NMR (500 MHz, 360 K, DMSO-d /TFA-d 4:1, δ in ppm): 6 1 1.88 (t, 12H, -CH3), 4.24 (q, 8H, -CH2-), 8.44 (t, 4H, 5-pyridylH), 9.19 (d, 4H, 4-pyridyl-H), 9.33 (d, 4H, 6-pyridyl-H), 9.60 (s, 2H, 10,20-methine-H), 9.71 (s, 4H, 2-pyridyl-H), 10.64 (s, 2H, 5,15-methine-H). 1H NMR (500 MHz, 360 K, acetic acid-d4, δ in ppm): 1.97 (t, 12H, -CH3), 4.22 (q, 8H, -CH2-), 7.97 (t, 4H, 5-pyridyl-H), 8.71 (d, 4H, 4-pyridyl-H), 8.98 (d, 4H, 6-pyridyl-H), 9.42 (s, 4H, 2-pyridyl-H), 9.63 (s, 2H, 10,20-methine-H), 10.56 (s, 2H, 5,15-methine-H). IR (KBr, ν in cm-1): 3314 (ν-NH), 1685 (ν-CdN), 1586, 1565 (ν-CdC), 824, 802 (νPyridyl-CH). MS (EI, 80 eV, 350 °C) m/z: 730 ([M]+, 100%), 715 ([M-CH3]+, 12%), 365 ([M]2+, 9%). EA C48H42N8 (730 92 g/mol): calcd, C 78.88, H 5.79, N 15.33; found C 78.51, H 5.74, N 15.19. Zinc Complex of 2 (Zn-2). A stirred solution of 29 mg of 2 (25 µmol) and 4.58 mg (25 µmol) of zinc acetate in 5 mL of absolute pyridine was reluxed for 2 h. The absorption band of the free base at 630 nm had disappeared completely. The hot pyridine solution was poured into 20 mL of water and the precipitate was filtered off, resuspended in methanol/water 1:1, and centrifuged. This procedure was repeated twice and the centrifugate dried in vacuo. 2,8,12,18-Tetraethyl-3,7,13,17-tetrapyrid-3-yl-21,23-dihydroporphyrin-N,N′,N′′,N′′′-bis(bis-2,2′-bipyridylruthenium) (Ru-2). Then 7.31 mg (10 µmol) of 2,8,12,18-tetraethyl3,7,13,17-tetrapyridin-3-yl-21,23-dihydroporphyrin (2) and 13.01 mg (25 µmol) of cis-bis-2,2′-bipyridylruthenium(II)-chloride dihydrate were heated to 90 °C in 800 µL of perdeuterated acetic acid for 1 h. The solvent was removed and the residue was dried in vacuo and redissolved in 800 µL of perdeuterated methanol. After 1 h at 55 °C the solution was poured onto a Nucleosil RP 300-C8 (10 µm) column and eluted with 70% methanol

5038 Langmuir, Vol. 15, No. 15, 1999 containing 0.1% trifluoroacetic acid. The main fraction contained Ru-2 and was evaporated to dryness. A red powder (14 mg) was obtained. Ethyl 3-Oxo-3-pyrid-4-yl Propionate (11). Sodium hydride (60% dispersion in mineral oil) (60 g, 1.5 mol) was washed oil free with ether and dispersed in 600 mL of dry ether. Then, 92.5 mL (1.65 mol) of absolute ethanol was added dropwise over 30 min. Absolute ethyl acetate (210 mL, 2.145 mol) and 150 mL of freshly distilled 4-carboethoxypyridine (1.0 mol) were added, and the inhomogeneous reaction mixture was distilled to remove the ether. The resulting light yellow slurry was stirred under nitrogen at 100 °C for 18 h. After cooling, 172 mL of acetic acid was added and the mixture was neutralized with saturated sodium bicarbonate sulution. The mixture was extracted with CH2Cl2 (400 mL × 4). The extract was washed with water, dried over Na2SO4, and concentrated. The residue was purified by crystallization from cyclohexane. The product was a pale yellow solid. Yield 183.5 g (95%), mp 53-55 °C. Ethyl 2-Hydroxyimino-3-oxo-3-pyrid-4-yl Propionate (12). Compound 11 (96.6 g, 0.5 mol) in 200 mL of acetic acid was cooled to 15 °C and 48 g of sodium nitrite (696 mmol) in 100 mL of water was added dropwise to the bottom of the flask. The solution was cooled to 4 °C for 18 h. The resulting off-white precipitate was collected by filtration and washed with water (until the pH was approximately 7) and 50% ethanol. Yield 108.9 g (98%), mp 154 °C. 4-Acetyl-2-ethoxycarbonyl-5-methyl-3-pyrid-4-ylpyrrole (13). A 250 mL autoclave vessel was charged with 40 g (180 mmol) of isonicotinoyloxime 12, 33 mL (320 mmol) of acetyl acetone, and 10% palladium on carbon (500 mg, 4.7 mmol) in 150 mL of acetic acid. The inhomogeneous mixture was stirred under 150 atm of H2 at 30 °C for 18 h, at which time TLC showed no remaining 12. Charcoal was removed by filtration through Celite and the solvent removed by rotary evaporation. To the residue was added sodium bicarbonate solution, and the product was extracted with chloroform (200 mL × 3). The combined organic layers were washed with 100 mL of water, dried with sodium sulfate, and evaporated. Ether (80 mL) was added to cause precipitation of colorless crystals. Yield 35.3 g (72%), mp 169 °C. 2-Ethoxycarbonyl-4-ethyl-5-methyl-3-pyrid-4-ylpyrrole (14). Compound 13 (30.4 g, 112 mmol) was dissolved in 300 mL of absolute tetrahydrofuran. Sodium borohydride (10.5 g, 275 mmol) was added with stirring at 5 °C under nitrogen. Borontrifluoride etherate (47 mL, 372 mmol) was added dropwise at 5-10 °C. The mixture was stirred for 1 h at room temperature, and 2 N hydrochloric acid (200 mL) was added slowly to quench the reaction. After stirring overnight, the biphasic solution became almost clear. The solvent was evaporated, and the residue was neutralized with sodium carbonate solution. The pyrrole was extracted three times with 400 mL of chloroform. The combined organic layers were dried with sodium sulfate and evaporated, leaving behind a pasty mass, which was crystallized from methanol to give 27.4 g of a white powder (95%), mp 138 °C. 2-Benzoxycarbonyl-4-ethyl-5-methyl-3-pyrid-4-ylpyrrole (15). Compound 14 (25.8 g, 100 mmol) and 250 mg of sodium were refluxed with 120 mL of absolute benzyl alcohol at 25 mbar under nitrogen. Reversed phase chromatography with methanol/ water 8:2 gave a product with an Rf of 0.37. After evaporation of the benzyl alcohol, 1 mL of acetic acid, 100 mL of methanol, and 5 mL of water were added. The resulting light yellow precipitate was collected by filtration and suspended in 100 mL of methanol. Filtration and washing with 50 mL of 9:1 methanol/ water gave fine white needles. Yield 30.4 g (95%), mp 126 °C. 5-Acetoxymethyl-2-carbobenzoxy-4-ethyl-3-pyrid-4-ylpyrrole (16). Compound 15 (25.63 g, 80 mmol) in 400 mL of acetic acid and 20 mL of acetic anhydride was stirred under nitrogen for 1 h. Lead tetraacetate (39.0 g, 88 mmol) was added and the suspension was stirred at 40 °C for 16 h. A clear solution was obtained. The solvent was removed by rotary evaporation and the residue was basified with saturated sodium carbonate solution to pH 8. After dispersion with 400 mL of chloroform, a white insoluble solid (lead carbonate) was removed by filtration. The organic layer was separated, washed with brine, dried over sodium sulfate, and evaporated, leaving behind a yellow paste. Purification was done on a octadecyl functionalized silica gel

Donner et al. column with 8:2 methanol/water as the eluent (Rf ) 0.48). Yield 29.06 g (96%), mp 144-146 °C. Bis(5-benzoxycarbonyl-3-ethyl-4-pyrid-4-ylpyrrole-2-yl)methane (17). Compound 16 (18.9 g, 50 mmol) in 200 mL of trifluoroacetic acid was diluted with 400 mL of water. The solution was refluxed for 4 h under a positive pressure of argon, during which time the solution changed from light orange to dark red and a tar formed. The solvent was evaporated in vacuo and the resulting black pasty mass was lyophilized. The residue was redissolved in 200 mL of chloroform and the solution was basified with triethylamine. The mixture was filtered and evaporated. The resulting black oil was passed through a silica gel column (12 × 20 cm) with a 96:4 chloroform/methanol eluent. The first fraction was collected and evaporated. Crystallization from methanol gave colorless crystals. Yield 12.7 g (81%), mp 184 C. Bis(3-ethyl-4-pyrid-4-ylpyrrole-2-yl)methane (19). A mixture of 17 (9.37 g, 15 mmol) and 10% palladium on carbon (400 mg, 3.8 mmol) in absolute DMF (150 mL) was stirred under 150 atm of H2 at 30 °C for 18 h in an autoclave vessel. The resulting mixture of the air-sensitive diacid 18 and carbon in DMF was taken directly to the next step. After the addition of triethylamine (5 mL), the suspension was heated in an autoclave to 160 °C for 18 h and filtered through Celite. The filtrate was concentrated and the resulting brownish paste was redissolved in 15 mL of methanol. The desired product 22 began to crystallize by dropwise addition of water to a final concentration of 10%. The light yellow crystals were quickly filtered off and washed with methanol/ water 8:2. Yield 4.38 g (82%), mp 180 °C. Bis(3-ethyl-5-formyl-4-pyrid-4-ylpyrrole-2-yl)methane (20). To a stirred -15 °C solution of absolute DMF (3.85 mL, 50 mmol) in absolute chloroform (30 mL) under argon was added freshly distilled POCl3 (3.67 mL, 40 mmol) dropwise. The solution was stirred at room temperature for 1 h before a solution of 19 (3.56 g, 10 mmol) in absolute chloroform (60 mL) was added dropwise. The resulting brown mixture was refluxed for 30 min, at which time TLC showed no remaining 19. The mixture was cooled and saturated aqueous sodium acetate (10 mL) was added slowly to quench the reaction. The mixture was neutralized with saturated aqueous NaHCO3. The aqueous layer was extracted several times with chloroform (total 300 mL). The organic layer was dried over sodium sulfate and filtered and the solvent was removed. The residue was redissolved in ethanol (100 mL) and concentrated to remove chloroform. The dark brown precipitate was filtered and redissolved in 20 mL of methanol. 20 was crystallized by a dropwise addition of water to give a final water concentration of 10%. The white crystals were quickly filtered off and washed with methanol/water 8:2. Yield 2.96 g (72%), mp > 250 °C. 2,8,12,18-Tetraethyl-3,9,13,17-tetra-4-pyridylporphyrin1 (1). To a stirred -5 °C solution of 20 (413 mg, 1 mmol) in absolute acetic acid (40 mL) under argon was added acetic anhydride (300 mL) followed by HBr (2 mL of a 33% solution in acetic acid, 11.2 mmol). The solution was stirred for 10 min. A solution of 19 (178 mg, 0.5 mmol) in absolute acetic acid (5 mL) and acetic anhydride (45 mL) was added over a period of 30 min. To the resulting dark red solution was added additional HBr (1 mL of a 33% solution in acetic acid, 5.6 mmol) followed by 19 (178 mg, 0.5 mmol) in absolute acetic acid (5 mL) and acetic anhydride (45 mL). The mixture was allowed to warm to room temperature and stirred for 2 h before DDQ (227 mg, 1 mmol) in 20 mL of acetic anhydride was added. The resulting dark brown mixture was stirred for an additional 4 h, at which time UV/vis showed no more increase of the Soret band intensity (410 nm). The mixture was cooled on ice and ethanol (200 mL) was added slowly to quench the reaction. During this time the color of the solution changed from dark brown to yellow and a dark violet precipitate formed. The precipitate was collected by filtration, washed with acetone, and passed through a silica gel column with 9:1 chloroform/pyridine eluent. The product was a violet solid. Yield 431 mg (59%), mp >250 °C. 1H NMR (500 MHz, 360 K, DMSO-d /TFA-d 9:1, δ in ppm): 6 1 1.97 (t, 12H, -CH3), 4.40 (q, 8H, -CH2-), 9.16 (d, 8H, 3,5-pyridylH), 9.53 (d, 8H, 2,6-pyridyl-H), 10.04 (s, 2H, 10,20-methine-H), 10.83 (s, 2H, 5,15-methine-H). IR (KBr, ν in cm-1): 3311 (NH),

Ru(II) Complexes and Bilayers of Porphyrins 1684 (CdN), 1599, 1534 (CdC), 839 (pyridyl-CH). MS (EI, 80 eV, 350 °C) m/z: 730 ([M]+, 100%), 715 ([M-CH3]+, 15%), 365 ([M]2+, 19%). Zinc Complex of 1 (Zn-1). Then 10 mg of 1 and 5 mg of zinc acetate were refluxed in absolute pyridine for 2 h. The absorption band of 1 at 630 nm had then disappeared completely. The hot solution was then poured into 20 mL of water and then centrifuged. The residue was washed twice with methanol/water (1.1), again centrifuged, and dried in vacuo overnight at 60 °C.

Langmuir, Vol. 15, No. 15, 1999 5039

Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 312 “Vectorial Membrane Systems” and SFB 448 “Mesoscopic Systems”), the European TMR network “Artificial Photosynthesis”, and the Fo¨rderungskommision of the Free University (FNK). LA981791+