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Porphyrinphosphonate Fibers on Mica and Molecular Rows on Graphite Matthias E. Lauer and Jurgen-Hinrich Fuhrhop* Freie Universita¨ t Berlin, FB Biologie, Chemie, Pharmazie, Institut fu¨ r Chemie/Organische Chemie, Takustrasse 3, D-14195 Berlin, Germany Received April 8, 2004. In Final Form: June 23, 2004 meso-Tetra(phenyl-p-phosphonate) porphyrin forms rigid and well-separated fibers of monomolecular thickness (2.8 nm) and lengths of several micrometers on mica at pH 13 (octasodium salt). The formation of these fibers could be observed directly by tapping mode scanning force microscopy (SFM) and was induced by capillary forces. Normal height images or images with a topographical inversion were observed depending on the distance of the SFM tip. Amplitude-distance curves indicated that a stable meniscus was formed on hydrophilic surface areas below a tip-sample separation of 20 nm. The meniscus let the original nanorods appear as ditches in the mica surface and enabled rearrangements. A partly protonated form of the same porphyrin (pH 11.5) gave rows of flat-lying porphyrins on graphite, which appear with molecular resolution in SFM images as well as two-dimensional platelets of monomolecular thickness.
Introduction Natural porphyrins, in particular protoporphyrin IX and the chlorophylls, consist of a rigid, apolar northern and a mobile, polar southern part with respect to their substitution patterns. This molecular characteristic prevents crystallization and favors, together with the hydrophobicity and rigidity of the porphyrin nucleus, fiber formation.1,2 Such noncovalent fibers are of interest as analogues of polymeric porphyrin wires for energy and electron transport.3 Symmetric meso-tetraphenylporphyrins lack this diversity of functional groups and have a high tendency to crystallize. Slightly water-soluble tetraphenylporphyrin (TPP) derivatives or analogues, for example, pyridyl4 or benzoic acid5,6 derivatives or such with long side chains,7 which are well-soluble in organic solvents, form well-organized Langmuir-Blodgett monolayers on smooth solid surfaces.8 Scanning tunneling microscopy (STM) gave images with molecular resolution of perfect 2D crystals. Fibers were achieved with TPP derivatives carrying long side chains with water-soluble groups and low pH.9a,b We prepared symmetrical TPPs with one or two phosphonate groups on the para- or the meta-positions of each phenyl group. Short flexible spacers were found to * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Inamura, I.; Uchida, K. Bull. Chem. Soc. Jpn. 1991, 64, 2005. (2) Martinez-Planells, A.; Arellano, J, B.; Borrego, C. M.; LopezIglesias, C.; Gich, F.; Garcia-Gil, J. Photosynth. Res. 2002, 71, 83. (3) Fuhrhop, J.-H.; Demoulin, C.; Boettcher, C.; Koening, J.; Siggel, U. J. Am. Chem. Soc. 1992, 114, 4159. (4) (a) Porteu, F.; Palacin, S.; Ruandel-Teixier, A.; Barraud, A. J. Phys. Chem. 1991, 95, 7438. (b) Endisch, C.; Bo¨ttcher, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1995, 117, 8273. (5) Fudickar, W.; Zimmermann, J.; Ruhlmann, L.; Roeder, B.; Siggel, U.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 9539. (6) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Xi, F.; Liu, H. W.; Xu, B.; Wan, J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10838. (7) Qiu, X.; Wang, C.; Zeng. Q.; Xu, B.; Yin, S.; Wang, H. J. Am. Chem. Soc. 2000, 122, 5550. (8) (a) Komatsu, T.; Yanagimoto, T.; Tsuchida, E.; Siggel, U.; Fuhrhop, J.-H. J. Phys. Chem. 1998, 102, 6759. (b) Schwab, A. D.; Deirdre S. E.; Collin R. S.; Elizabeth, Y. R.; Walter, S. F.; Julio, C. P. J. Phys. Chem. B 2003, 107, 11339. (9) Drain, C. M.; Batteas, J. D.; Smeureanu, G.; Patel, S. Dekker Encyclopedia Nanoscience and Nanotechnology; Marcel Dekker: New York, 2004; pp 3481-3501.
be necessary to achieve good solubility of the phosphonate esters in organic solvents and phosphonate salts in water. The porphyrinphosphonates were at first developed as building blocks for porphyrin towers on silicon or gold electrodes for electrochemical investigations. Zirconium(IV) was first applied as a cement between phosphonate groups. The formation of Zr4+PO32- salts was, however, found to be irreversible and led to broad, ill-defined rocks instead of the desired towers of monomolecular width.10a,b We therefore turned to the reversible assembly of the porphyrinphosphonate fibers by formation of sodium phosphonate stacks on hydrophilic mica surfaces and of flat-lying porphyrin rows connected by hydrogen bonds on graphite. Experimental Section Materials. The syntheses of aldehydes 1a,b and porphyrin 2 (see Chart 1) have been reported.9 Standard Porphyrin Solution for Atomic Force Microscopy (AFM) Samples. meso-Tetrakis[4-(3′-phosphonopropoxy)phenyl]-porphyrin 2 (11.4 mg, 0.1 mmol) was dissolved in 100 mL of 0.1 M NaOH. The resulting solution was stirred for 60 min at 80 °C, deposited in a freezer at 5 °C, and applied as a scanning force microscopy (SFM) probe after 3 weeks. Highly Oriented Planar Graphite (HOPG) Sample. A 20 µL sample of the porphyrin solution was dropped on a freshly cleaved HOPG surface, blotted off with filter paper after 45 s, and scanned in the tapping mode after 10 min at a scanning speed of 1.5 µm s-1. Mica Sample. The porphyrin solution was heated to 80 °C for 2 h and then filtered with a membrane filter (pore width, 0.5 µm; Schleicher & Schuell, FP30/0.2 PTFE). Muscovite (Ruby Red Mica Sheets) was split with a pair of tweezers, and 50 µL of the warm porphyrin solution was distributed evenly over the fresh surface. The fluid was collected in a corner of the mica sheet and quickly blotted off with filter paper. The probe was adjusted with a light microscope and either scanned repeatedly with the SFM tip in the tapping mode or stored for 3 days in a desiccator. Scanning Force Microscopy. SFM images were recorded using a MultiMode IIIa scanning probe microscope with Extender Modul (Digital Instruments, Inc., Santa Barbara, CA) that was operated in the dynamical modus. Olympus etched silicon (10) (a) Li, G.; Fudickar, W.; Skupin, M.; Klyszcz, A.; Draeger, C.; Lauer, M.; Fuhrhop, J.-H. Angew. Chem. 2002, 114, 1906; Angew. Chem., Int. Ed. 2002, 41, 1828. (b) Klyszcz, A.; Lauer, M.; Boettcher, C.; Gonzaga, F.; Fuhrhop, J.-H. Chem. Commun., in print.
10.1021/la049105w CCC: $27.50 © 2004 American Chemical Society Published on Web 07/30/2004
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Figure 1. SFM height images of porphyrin 2 in NaOH (10-4 M, pH 13) on HOPG: (a) overview, (b) detail of an island, (c) height diagrams of images a, b, and d, showing that all fibers have heights of about 2 nm corresponding to the diameter of porphyrin 2 and in image b dimeric fibers, and (d) rows of porphyrins in the disordered patches. cantilevers were mostly used with a typical resonance frequency in the range of 200-400 kHz and a spring constant of 42 N/m. All samples were measured at room temperature in air environment. The sample was first adjusted with an optical light microscope (NanoScope, Optical Viewing System). The microscope was then mounted on a vibration isolation table (Halcyonics, MOD-1) and under a glass bell for reduction of acoustical noise. The typical scanning speed was 1 line/s. Data analysis was performed after plane-fit, height measurements based on the cross-sectional profiles, and/or particle analysis. High-resolution phase images on HOPG were obtained in the tapping mode (constant height). The sample was scanned for 48 h with a Super Sharp silicon tip (low-frequency/ν ) 256.88 kHz). The cantilever was tuned to a target amplitude of 2 V at a drive amplitude of 44.17 mV. The tip was then approached, the scan size was reduced to 100 × 100 nm2, and the drive amplitude was decreased to 10 mV, which interrupted the tip-sample interactions. The set-point was decreased up to tip-sample interactions. The scan size was reduced to image size, feedback was switched off, and the scan rate was increased to about 32 lines/s. Further fine-tuning was done by optimization of drive amplitude. The final image was Fourier filtered. Inversion of the Topography (Liquid Bridge). An etched silicon cantilever (Olympus) with a measured resonance frequency of 244.4 kHz was mounted in the tip holder and placed above the sample. The age of the sample was 2 h. The cantilever was tuned to a target amplitude of 2 V at a drive amplitude of 162.9 mV. The measurements were done at a phase offset of -25°. The images in Figure 4a-d were obtained at a scan rate of 1.5 line/s. The set-point amplitude was 1.72 V. Image e in Figure 4 was taken after rearrangement at a set-point amplitude of 1.25 V; the scan rate was 1.5 line/s. Manipulation. To reduce the thermal drift, the HOPG surface was scanned for 48 h. A Super Sharp silicon tip with a resonance frequency of 303.808 kHz was mounted. The cantilever was tuned
Chart 1
to a target amplitude of 2 V, which could be achieved by setting the drive amplitude to 33.10 mV. The measurement was done at a frequency offset of 5%. The sample was scanned at a set point of 1.62 V. For manipulation, the drive amplitude was increased to 150% and an image was taken. The drive amplitude was decreased, and after a full scan the modified image was taken.
Results The phosphonatoporphyrin 2 was synthesized from the corresponding aldehyde 1a via the corresponding octaethylester which was hydrolyzed with calcium carbonate in dimethyl formamide (Chart 1). 3-Bromopropyl-diethylphosphonate was applied to obtain the phosphonates 1a,b from the corresponding phenolaldehydes. 8,9 The porphyrin para-tetraphosphonate 2 was dissolved in 0.1 M sodium hydroxide solution in water (10-4 M), and a 20 µL droplet was applied directly to a HOPG surface. After 45 s, the water was blotted off and the probe was examined by SFM in the tapping mode. Fibers with heights
Porphyrinphosphonate Fibers on Mica
between 1.4 and 2.8 nm and typical lengths of 500 nm were found, which were oriented parallel to each other at an angle of 60° corresponding to the HOPG crystal axes. The blotting procedure produced sediments perpendicular to the water flow, as indicated by the arrows (Figure 1a). At higher resolution, not only the common monomeric fibers were found but occasionally also parallel-lying dimers. The tip allowed the identification of two such fibers (arrow), even if their distance was as low as 7 nm. The image also showed that most of the fibers were surrounded by disordered and mobile patches of flat-lying porphyrins (Figure 1b). Occasionally these patches showed some crystalline order (Figure 1d). The patches proved to be mobile under the tip and acted presumably as the material source for fiber growth. The fibers were stable for up to 8 h and melted upon application of stronger tip forces to become part of the formless patches. The cross-section profile of Figure 1c indicates the heights of the fibers (1.42.8 nm) and patches (0.5 nm) as well as the distance between the monomers in the dimeric fiber. Upon diluting the same aqueous probe by a factor of 2, the pH dropped from 13.0 to 12.5 and platelets were formed exclusively. The thickness of the platelets was uniformly 2.9 nm, corresponding to the sheets of upright-standing porphyrins. These monolayers grew in ordered arrangements along terraces of the graphite subphase and showed sharp edges and small holes. They clearly have the character of 2D crystallites and are stable end products on the graphite surface under the given conditions. Interdigitated bilayers with a height of 4.5 nm were also observed (Figure 2a). Transmission electron microscopy (TEM) pictures taken on hydrophilized carbon showed overlapping sheets (Figure 2b), and occasionally occurring crystallites contained terraces with the same height difference of 2.8 and 4.5 nm (Figure 2c). These sheets without any curvature correspond to the final aggregation state of the porphyrinphosphonates on graphite and carbon and in solution (cryo TEM). The same deposition and blotting procedure of a 10-4 M porphyrin 2 solution at a lower pH of 11.5 produced rows of porphyrins and networks of associated fibers. Single molecules now became observable (Figure 3a). The wavelength of 2.8 nm within the corrugated structure corresponds exactly to the molecular width. We also observed rows of porphyrin dimers in a cubic arrangement (Figure 3b). All porphyrin pairs have a center-to-center distance of 2.8 nm. The stability of the chains made of single porphyrins is presumably caused by an ordering of the porphyrins on graphite by π-π interactions and by hydrogen bridges between partly protonated phosphonate groups and water. Two hydrogen bridges per monomer may occur in the monomeric rows, and three bridges in the corresponding dimer. The observed packing is not optimal; the monomer and dimer rows are separated by as much as 2 nm, whereas such a separation does not occur within the dimer row. Repulsive interaction between the phosphonate rows is presumably responsible for their separation by water channels. If the same solution of 2 was applied at room temperature under the same conditions to a freshly cleaved mica surface, no fibers or monolayer patches were detectable. Only ill-defined precipitates were then found. Heating of the solution to 90 °C, ultrafiltration, and application of a 20 µm droplet yielded, however, a closed hydrophobic monolayer of flat-lying molecules (Figure 4a). The picture on the left was obtained by tapping at a high tip-sample separation and/or low tip damping. It shows holes with a measured depth of 0.5 nm. With fresh, still moist preparations, we often observed an inversion of topog-
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Figure 2. (a) SFM height image of the same sample used in Figure 1 after 2-fold dilution and lowering of the pH to 12.5. Platelets with heights of 2.9 nm (monolayer) or 4.5 nm (interdigitated bilayer) were observed exclusively. (b) Crystallites show similar height steps of 1.6 nm each. (c) TEM of the same monolayer particles from an aqueous solution at pH 11.5 (0.5 × 10-5 M) on a hydrophilized carbon grid.
raphy, which occurred spontaneously upon lowering the damping constant of the tip. A water bridge was presumably formed between the tip and the mica or fiber surfaces and let the holes now appear with a 6-fold higher virtual depth of 3 nm. One may also say that the mica surface had virtually risen by 3.0 nm (Figure 4a,b).11 The monolayer then broke up completely to form a net of fibers (Figure 4c), which slowly expanded to cover the whole surface (Figure 4d). The height of the rigid fibers was a uniform 2.8 nm, and lengths of several micrometers were reached. Upon decreasing the tip-surface distance, a water bridge was again formed, which stabilized the whole image in an inverted topography (Figure 4e). The fibers with a height of 2.8 nm appeared as ditches with a depth of 8.5 nm. These fibers were the end products of selfassembly of porphyrinphosphonates on the hydrophilic mica surface. (11) Neves, B. R. A.; Leonard, D. N.; Salmon, M. E.; Russell, P. E.; Troughton, E. B. Nanotechnology 1999, 10, 399.
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Figure 3. SFM phase image of porphyrin 2 on HOPG after Fourier filtration: (a) rows of ordered single porphyrins with the cross-section profile and a model overlay on magnified detail and (b) the same for rows of dimers.
We then determined the amplitude-distance curve during the rearrangements of the monolayer of flat-lying porphyrin 2 molecules. In the approach phase, it showed steplike discontinuities indicating spontaneous shifts of vibration states between high- and low-amplitude branches (Figure 5; see Discussion). Steps were also observed in fluid polyvinyl films;12 here they are indicative of the formation of a meniscus, which occurs at a smaller tipsample separation. When contacting the surface, the amplitude dropped to zero and continuously rose again in the retraction mode. The amplitude signal increased by 30% over a distance of approximately 17 nm. At a critical distance, which was in the range of the maximal possible Kelvin radius, the meniscus broke. Then the slope of the amplitude-distance curve became almost orthogonal and reached the amplitude of the free vibrating cantilever. The presence of a meniscus (see Figure 7) seemed to be responsible for the observed rearrangements and the virtual height change reproduced in Figure 4. The inverted height profile is in some details even the better-resolved one, and the connecting meniscus separates close fibers obviously quite efficiently because the contrast of the image is enlarged by a factor of 4. Meniscus formation was strictly limited to hydrophilic areas on the surface and was not observed on the initial monolayer. We then applied the same procedure on a freshly prepared monolayer with a high density of defects (Figure 6a). Tapping under conditions of topographical inversion now produced strictly limited areas of fibrous nets (Figure 6c) again in normal and inverted topographies depending on the conditions of the measurement. The appearance of the fresh probe of the holey monolayer did not change upon resting for 1 day at room temperature (no figure, same appearance as Figure 6a). It had, however, dried out, and no inversion of topography could be induced by tapping with a
