Building Self-Assembled Molecular Layers with Axially Substituted

Jul 7, 2010 - ‡Current address: Modumetal, 443 N. Northlake Way, Seattle, WA 98103. Received May 19, 2010. Revised Manuscript Received June 23, ...
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Building Self-Assembled Molecular Layers with Axially Substituted Titanium Phthalocyanines Tomohide Takami,† Aurora Clark, Richard Caldwell,‡ Ursula Mazur, and K. W. Hipps* Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, Washington 99164-4630. †Current address: WCU Program and Department of Physics, Konkuk University, South Korea. ‡Current address: Modumetal, 443 N. Northlake Way, Seattle, WA 98103. Received May 19, 2010. Revised Manuscript Received June 23, 2010 Adsorption on graphite (HOPG) by titanium phthalocyanine axially bonded to a catechol ligand (TiPcat), titanylphthalocyanine (TiOPc), and 1:1 mixtures of these are studied at the HOPG-octylbenzene interface. The surface structures of a two-component bilayer, and of the individual monolayers of TiOPc and TiPcat, were determined by scanning tunneling microscopy (STM). TiPcat self-segregates onto a monolayer of TiOPc when an equal molar mixture is used. The preferential formation of a TiOPc monolayer from a solution containing both molecules is attributed to the difference in adsorption energies between TiPcat and TiOPc on graphite. The transformation of the hexagonal lattice of the pure TiOPc monolayer into a pseudo-square lattice was induced by the adsorption of TiPcat molecules. DFT calculations of the catechol orientation are presented.

Introduction The structures and properties of metallophthalocyanines adsorbed at solid surfaces are of great interest due to their versatile chemical and electronic properties.1 Phthalocyanine (Pc) complexes have been investigated as potential light harvesting arrays,2,3 optical switches,4 and photonic wires.5 Titanium-centered phthalocyanines with tailored axial ligands have been exploited to introduce an element of asymmetry to the phthalocyanine, thus generating compounds with second-order nonlinear optical6 and photoinduced electron transfer properties.7 Catechol-like ligands are wellknown axial substituents for Ti(IV) complexes in general.8,9 Attaching an axial catechol ligand to Ti(IV) Pc generates an out-of-plane dipole moment and an octupole moment. The combination of dipoles and octupoles within the same molecule can yield interesting nonlinear optical properties.10 In addition, axial substitution with bulky groups such as substituted catecholates can reduce aggregation in phthalocyanine films.11 The current study of TiPcat (Figure 1) is an extension of our previous work on vanadyl phthalocyanines (VOPc)12-14 and that *To whom correspondence should be addressed. E-mail: [email protected].

(1) Drain, C. M.; Varotto, A.; Radivojevic, I. Chem. Rev. 2009, 109, 1630–1658. (2) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910–1921. (3) Friesen, B. A.; Nishida, K. R. A.; McHale, J. L.; Mazur, U. J. Phys. Chem. C 2009, 113, 1709–1718. (4) He, Ch.; Wu, Y.; Shi, G.; Duan, W.; Song, W.; Song, Y. Org. Electron. 2007, 8, 198–205. (5) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsay, J. S. J. Org. Chem. 2000, 65, 6634–6649. (6) Henari, F.; Davey, A.; Blau, W.; Haisch, P.; Hanack, M. J. Porphyrins Phthalocyanines 1999, 3, 331–338. (7) Ballesteros, B.; de la Torre, G.; Torres, T.; Hug, G. L.; Rahman, G. M. A.; Guldic, D. M. Tetrahedron 2006, 62, 2097–2101. (8) Albrecht, M. Chem. Soc. Rev. 1998, 27, 281–288. (9) Albrecht, M.; Janser, I.; Runsink, J.; Raabe, G.; Weis, P.; Fr€ohlich, R. Angew. Chem., Int. Ed. 2004, 43, 6662–6666. (10) Zyss, J.; Ledoux, I. Chem. Rev. 1994, 94, 77–105. (11) Palomares, E.; Martı´ nez-Dı´ az, M. V.; Haque, S. A.; Torres, T.; Durrant, J. R. Chem. Commun. 2004, 2112–2113. (12) Hipps, K. W.; Barlow, D. E.; Mazur, U. J. Phys. Chem. B 2000, 104, 2444– 2447. (13) Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 5993–6000. (14) Mazur, U.; Hipps, K. W.; Riechers, S. L. J. Phys. Chem. C 2008, 112, 20347–20356.

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Figure 1. Schematic and CPK drawings of titanylphthalocyanine (TiOPc) and catechol-substituted titanium phthalocyanine (TiPcat).

of others on titanyl phthalocyanine (TiOPc, Figure 1).15-19 Axial substitution of MPcs has been shown to produce adsorbate structures that are different and more complex than for the (15) Brena, B.; Palmgren, P.; Nilson, K.; Yu, Sh.; Hennies, F.; Agnarsson, B.; € Onsten, A.; Ma˚nsson, M.; G€othelid, M. Surf. Sci. 2009, 603, 3160–3169. (16) Kong, X. H.; Wang, M.; Lei, S. B.; Yang, Y. L.; Wang, C. J. Mater. Chem. 2006, 16, 4265–4269. (17) Kong, X. H.; Yang, Y. L.; Lei, S. B.; Wang, C. Surf. Sci. 2008, 602, 684–692. (18) Wei, Y.; Robey, S. W.; Reutt-Robey, J. E. J. Phys. Chem. C 2008, 112, 18537–18542. (19) Mannsfeld, S. C. B.; Fritz, T. Phys. Rev. B 2005, 71, 235405/1–235405/10.

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metal-free compound.14 Interpretation of the STM images of TiOPc and VOPc is controversial in that STM data do not give an unambiguous result as to whether the oxygen is up or down relative to the surface.13,14 We wanted to determine the effects of axial substitution on surface layer structure and submolecular STM images associated with a larger, electronically coupled axial ligand such as catechol where no orientational ambiguity is possible. In this report, we contrast the structures of TiOPc and TiPcat on graphite (HOPG) at the solid-octylbenzene solution interface. We will show that the catechol ligand is clearly seen in STM and that the surface structure of TiPcat on HOPG is very different from that of TiOPc when they each separately form monolayers. However, when they are coadsorbed, a complex set of interactions occur between the different adsorbates and the substrate and between the adsorbates themselves.

Methods Section Materials. Highly oriented pyrolitic graphite, HOPG (10 mm  10 mm by 1 mm thick), was purchased from SPI Supplies. Atomic resolution images of the clean hexagonal surface without Moire pattern were observed by STM. TiOPc was obtained from Aldrich (40,455-1, CAS 26201-32-1) and used without further purification. TiPcat was synthesized according to a modified procedure reported earlier.20,21 A 2:1 molar ratio of TiOPc and catechol (Aldrich C9510, CAS 120-80-9) was refluxed in chloroform (J.T. Baker 9180-01, CAS 67-66-3) at 65 °C for about 3 h. The reaction mixture was then cooled to room temperature, and the solvent was removed in vacuo. The resulting dark purple solid was dissolved in a small amount of methylene chloride (Aldrich 270997, CAS 75-09-2) followed by addition of methanol (Aldrich 322415 CAS 67-56-1) to precipitate a purple-blue product. The precipitate was purified in a Soxhlet extractor first by extracting the product with methylene chloride followed by precipitation with methanol. The solid material was washed in Soxhlet sleeve with methanol for a day. The final deep purple-blue product was dried, and the remaining trace contaminates were vacuum-sublimed at 250 °C (exceeding 250 °C will decompose TiPcat). Microanalysis yielded: C 68.29 (68.27 calc), N 16.69 (16.77 calc), Ti 7.14 (7.16 calc), H 3.12 (3.02 calc). UV-vis spectra of 10-4 M TiPcat in CH3Cl exhibited maxima at 349, 626, and 692 nm that compared well with reported literature values.20 The 13C CPMAS NMR spectrum of bulk TiPcat was acquired on a Bruker Advance 400 spectrometer with a Chemagnetics 7.5 mm two-channel MAS probe. The sample was spun at a rate of 8.0 kHz, with a relaxation delay of 4 s and a contact time of 2.5 ms, and showed four signals for the phthalocyanine moiety (152.0, 136.0, 133.0, and 129.0 ppm) and three for the axial catechol ligand (159.0, 123.0, and 110.0 ppm). The measured 13C chemical shifts for TiPcat agree favorably with reported spectra.20,21 Adsorption Procedure. Phthalocyanines were dissolved in chloroform. A 6 μL droplet of the prepared chloroform solution then was put on a freshly cleaved graphite substrate. The amount of phthalocyanines deposited on each substrate was controlled by the number of droplets and the concentration of the solution. Typically, two droplets (∼11 μL total) of a 1.6 μM solution was used to prepare a near-monolayer on the central region of the substrate. The droplets spread out onto an approximately circular region at ca. 5 mm diameter. This corresponds to an average coverage of 0.54 molecules/nm2. In the case of binary adsorption, each species was 0.8 μM with an equal molar mixture in solution. The deposited sample then was dried under Ar gas flow for 3 min. Finally, a droplet of n-octylbenzene (Alfa Aesar L03086, CAS 2189-60-8) was put on the sample surface. The surface was imaged through the octylbenzene layer in order to obtain stable STM (20) Barthel, M.; Hanack, M. J. Porphyrins Phthalocyanines 2000, 4, 635–638. (21) Barthel, M.; Dini, D.; Vagin, S.; Hanack, M. Eur. J. Org. Chem. 2002, 3756–3762.

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images. It is noted that STM imaging without using n-octylbenzene is possible but very difficult. With dried samples, the phthalocyanines on the sample surface frequently stick onto the scanning STM tip, causing unstable imaging. Moreover, for molecular coverage in excess of a monolayer, or less than 0.7 monolayer, one obtains unstable images because of phthalocyanine movement during the scan (see Supporting Information). Computational Methods. Geometry optimization was performed using the NWChem software program22 without symmetry constraints (in C1) for seven different conformations of the TiPcat using the B3LYP combination of exchange correlation functionals23,24 and two different basis sets to assess the impact of basis set upon the calculated geometries and energetics of catechol rotation. The first basis set utilized the Pople 6-31G** basis for all atoms which consists of 5s, 4p, 2d, 1f basis functions on the Ti, while C, N, and O atoms are described by 3s, 2p, 1d functions and H atoms are described by 2s, 1p functions.25,26 The second basis set utilized the LANL08þ effective core potential and basis on Ti,27,28 which replaces the 10 core electrons of Ti with an effective potential and describes the valence orbitals of Ti with 5s, 5p, 6d functions, while the aug-cc-pVDZ basis was used for C, N, O which has 4s, 3p, 2d functions and H atoms are described by a 3s, 2p basis. Note that these basis sets differ significantly from that used in prior work,29 wherein the def-SV(P) basis was used which describes Ti with 5s, 3p, 2d contracted Gaussian function and C, N, O with 3s, 2p, 1d and H atom with a 2s basis.30 Thus, the basis sets used in this work have a more complex description of the p and d space of the Ti and have more diffuse and polarization functions on the main group elements. The three conformations consisted of geometries that had the catechol unit: (1) eclipsed relative to the pyrrole N atoms, which we identify as 0° rotation; (2) staggered relative to the pyrrole N atoms, which we identify at 45° rotation; and (3) intermediate rotational values of 10°, 22.5°, 30°, 35°, and 40° rotation. Of these geometry optimizations, the 0° and 45° calculations were performed with no restraints whereas the remainder were fixed to maintain this rotational angle with the rest of the molecular structure being allowed to relax. Normal mode analysis was used to determine which structures were local minima. STM Imaging. STM tips were produced by electrochemical etching of 0.25 mm diameter platinum-iridium annealed wire (California Fine Wire Co., Pt 90% Ir 10%). The STM observations were carried out on a PicoPlus SPM (Molecular Imaging). The in-plane distances in the STM images were calibrated using the hexagonal lattice of graphite (0.246 nm), and the vertical scale was calibrated with the monatomic steps of graphite (0.34 nm). Typical set point parameters were -0.7 V bias on the sample and a tunneling current of 1.5 pA. SPIP31 and WSxM32 software were used for the STM image data processing and analyses. Drift compensation was carried out by comparing the consecutive images to determine the drift vector.

Results and Discussion Calculations. An important issue in this study is the extent of rotation of the catechol about the axis normal to the phthalocyanine. (22) “NWChem, A Computational Chemistry Package for Parallel Computers, Version 5.0”, Pacific Northwest National Laboratory, Richland, WA, 2006. (23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (24) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (25) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257–2261. (26) Rassolov, V.; Pople, J. A.; Ratner, M.; Windus, T. L. J. Chem. Phys. 1998, 109, 1223–1229. (27) P.J. Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (28) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput. 2008, 4, 1029–1031. (29) Muranaka, A.; Okuda, M.; Kobayashi, N.; Somers, K.; Ceulemans, A. J. Am. Chem. Soc. 2004, 126, 4596–4604. (30) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571–2577. (31) SPIP is the product of Image Metrology A/S, Lyngsø Alle 3A, DK-2970 Hørsholm, Denmark. (32) Horcas, I.; Fernandez, R.; Gomez-Rodrı´ guez, J. M.; Colchero, J.; GomezHerrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 013705/1–013705/8.

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Figure 2. Geometry and energy for TiPcat as a function of catechol rotation angle as determined with the B3LYP/lanl08/aug-ccpVDZ basis.

A previous calculation performed with a small basis indicated that the catechol was freely rotating at room temperature.29 If this were correct, the image seen in STM would likely be very symmetrical. On the other hand, a very large and rapidly increasing barrier would lead to well-defined bladelike images of the catechol, allowing its orientation to be unambiguously defined. Thus, we performed computational quantum mechanics on the catechol TiOPc species. As in prior work,29 the 0° structure was predicted by B3LYP to be the lowest energy conformation. Using either the 6-31G** or the mixed LANL08/aug-cc-pVDZ basis results in similar structures, with the largest difference being slightly longer (0.02 A˚) Ti-O bond lengths in the larger basis. The HOMO-1 of this conformation is characterized by delocalization of the highest occupied catechol molecular orbital with the dxy on the Ti and the binding nitrogens of the phthalocyanine rings. As the catechol is rotated toward the staggered conformation (see Figure 2), an activation barrier is observed near 10°, followed by a minimum near 22.5°. Here, deviations in the basis sets begin to be manifested such that the Pople basis predicts the 22.5° structure to be 2.6 kcal/mol higher in energy, while the LANL08/aug-cc-pVDZ basis predicts it to lie within 0.9 kcal/mol. Increasing rotation leads to a steady increase in the energy; however, the optimized 45° structure is predicted to be a stable intermediate (no imaginary vibrations) by B3LYP using either basis set, and both predict that it lies 10-12 kcal/mol higher in energy (ZPE corrections included) than the 0° structure, presumably due to the orbital delocalization exhibited in the HOMO-1 orbital. The staggered geometry is further characterized by motion of the phenyl rings such that one set of trans arms moves into the plane of the pyrrole ring, while the other set becomes more convex (see Figure 2). Unlike the results reported by Muranaka et al.,29 we find that the catechol cannot be freely rotating and is at most oscillating about the minimum energy (oxygen eclipsing nitrogen) structure. Attempts to optimize the transition state which must lie between the 22.5° and 45° minima on the rotational PES failed, presumably because the transition state has a rotational angle close to the 45° intermediate and with a similar energy such that the gradients that characterize the transition state are small and ill defined. Figure 2 plots the LANL08/aug-cc-pVDZ energetics of catechol rotation, with solid lines indicating optimized conformations, and dashed lines indicating the proposed path the results from scanning the rotational coordinate between optimized data points. Langmuir 2010, 26(15), 12709–12715

Figure 3. STM image of TiPcat molecules at the octylbenzenegraphite interface. Sample bias was -0.7 V, and tunneling current was 1.5 pA. The proposed unit cell is shown in the insert.

The prediction of these calculations is that the catechol will be preferentially aligned with the nitrogen atoms in the phthalocyanine ring but that the contour may be somewhat elliptical due to the low barrier to oscillations about the axis normal to the phthalocyanine ring. Because the low barrier to reorientation at 22.5° (0.26 eV) and the somewhat higher rotational barrier height to an equivalent 90° position (about 0.5 eV), it may be possible for tunneling electrons (at 0.7 V bias) to provide sufficient energy to occasionally drive the catechol over either barrier. Using the 6-31Gþ(dp) basis and optimized geometry, the dipole moments for TiOPc and TiPcat were calculated to be 3.36 and 2.05 D, respectively. Increasing the basis to the DGTZVP set resulted in values of 3.33 and 3.07 D. Moreover, the lowest two unoccupied MO’s of the TiOPc and TiPcat lay at -3.26 eV (degenerate) and -3.16 and -3.21 eV, respectively. The highest three occupied MO’s lay at (-5.39, -7.15, and -7.18 eV) and (-5.27, -5.60, and -6.42 eV), respectively. The HOMO-1 and HOMO-2 of TiPcat are both centered on the catechol. STM. Titanium phthalocyanine axially coordinated with a catechol ligand (TiPcat) can form an ordered layer on graphite. The constant current STM image in Figure 3 shows the various domains observed in a near monolayer of TiPcat at the HOPGoctylbenzene interface. The most important point about this image is that the catechol moiety is observed as a cigar-shaped feature, and it dominates this image. Further, there is a pronounced tendency for the catechols to align relative to each other. It is tempting to ascribe this to π-π interactions, but one must also recall that the axis of the catechol seen in the STM image is also that of the oxygen atoms and quadrupole interactions may be a significant contributor. Finally, one cannot rule out the possibility that the catechols are being oriented by the action of the STM tip since the energy of the tunneling electrons is sufficient to surmount the rotational barrier. This latter possibility seems unlikely since the scan direction was left-right (and right-left) relative to Figures 3-6. We were not successful in obtaining equal or better quality images at lower bias voltages or positive bias voltage values. This is very consistent with the calculations DOI: 10.1021/la1020127

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Figure 4. Left image (A): STM image of coadsorbed TiOPc and TiPcat molecules at the octylbenzene - graphite interface. The sample was prepared with a droplet of the mixed solution of TiOPc and TiPcat molecules dissolved in chloroform, dried in argon flow to evaporate the solvent, a droplet of octylbenzene was then added. The ratio of TiOPc and TiPcat molecules was 1:1, and the total concentration of the solution was 1.6 μM. 50 nm 53 nm. Upper domain showed hexagonal structure of TiOPc monolayer, and the lower domain showed TiPcat molecules adsorbed on TiOPc sublayer. Right image (B): High-resolution STM image of a single component TiOPc layer, showing hexagonal structure. In both images, sample bias was -0.7 V and tunneling current was 1.5 pA.

Figure 5. STM image of TiPcat on TiOPc monolayer on graphite with the image size 50 nm  50 nm. Sample bias: -0.7 V; tunneling current: 1.5 pA.

mentioned above since imaging of the cathacol is expected to occur through the HOMO-1 and HOMO-2 and lead to strong tunneling intensity in negative bias. The vacancies in the image allowed us to determine that the phthalocyanines were adsorbed directly on graphite with the TiPcat molecules forming about 0.9 monolayer in the case of Figure 3. At the bias voltage used to acquire this image, -0.7 V sample bias, the height of the molecules appears to be 0.35 nm. This is much shorter than the calculated distance from the center of the titanium to the center of the upper C-C bond on the catechol (0.60 nm) but is not surprising considering that the “height” (33) Scudiero, L.; Hipps, K. W. J. Phys. Chem. C 2007, 111, 17516–17520. (34) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197–7202.

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Figure 6. High-resolution constant current image of TiPcat on TiOPc monolayer on graphite. Molecular models of TiOPc are inset into the figure to clarify the monolayer packing and the position of TiPcat on the TiOPc sublayer.

in STM is really a measure of local tunneling probability.33,34 Unfortunately, the internal structure of phthalocyanine was not observed in this study; only the very high region associated with the catechol could be seen. This is unexpected since the Pc rings of a number of other MPcs and metalloporphyrins have been observed by STM.13,14,19,33-38 The inability to identify the Pc (35) Yoshimoto, S.; Tada, A.; Suto, K.; Itaya, K. J. Phys. Chem. B 2003, 107, 5836–5843. (36) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, X. M.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550–5556.

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Table 1. Unit Cell Parameters for Species at the HOPG-Octylbenzene Interface Observed in This Study A (nm)

B (nm)

R (deg) F (molecules/nm2)

TiPcat monolayer 1.29 ( 0.05 1.27 ( 0.05 90 ( 2 0.61 TiOPc monolayer 1.22 ( 0.05 1.22 ( 0.05 60 ( 2 0.77 0.51 TiOPc sublayera 1.50 ( 0.07 1.30 ( 0.07 90 ( 2 0.51 TiPcat overlayera 1.50 ( 0.07 1.30 ( 0.07 90 ( 2 b 0.267 TiPcat overlayer 1.96 ( 0.07 1.88 ( 0.07 90 ( 2 a High-density overlayer (Figure 5). b Low-density overlayer (Figure 4).

ring in TiPcat might simply be due the strong tunneling through the catechol coupled with the physical height of the ligand from the surface. We will also show that the TiPcat interaction with HOPG is significantly less than for TiOPc. A more general viewing of Figure 3 indicates that there are several small domains with orientations varying along the symmetry directions of the HOPG surface. The domains in Figure 3 include surface structures rotated by 60° and twin structures. There are also small regions that are misaligned with the underlying lattice as indicated by the alternation in apparent heights of adjacent molecules. If one ignores the catechol orientation, they all have a unit cell with parameters A = 1.29 ( 0.05, B = 1.27 ( 0.05, and R = 90 ( 2°. This structure gives a surface density of 0.61 molecules/nm2. Data on this structure and others discussed in this paper is collected in Table 1. We note that based on STM images the planar configuration of VOPc on Au(111)13 has a reported square lattice spacing of 1.42 ( 0.1 nm, while FePc on Ag(111) has reported lattice lengths of 1.45 and 1.26 nm.39 These correspond to surface densities of 0.50 and 0.55, respectively. Heitschold40 recently reported the lattice spacing of PdPc by STM as 1.4 ( 0.1 nm but found a value of 1.3 ( 0.05 by LEED. He says that the LEED value is more accurate. Thus, our values of 1.29 ( 0.05 and 1.27 ( 0.05 are reasonable. Figure 3 (inset) shows the critical dimensions along with a scale model unit cell showing TiPcat molecules as correctly scaled CPK models. For most of the domains seen in Figure 3, the orientation of the catechol lies at roughly 45° to the unit cell directions. As expected from the results of the calculations described above, there is significant uncertainty in the exact orientation of the catechol. In an attempt to obtain clearer images, we also tried to image TiPcat on Au(111). However, TiPcat was more mobile than on graphite, and better images than those shown on graphite were not obtained. Attempts to measure the catechol structure at low and very high coverage were not successful. STM images were unstable at less than 1 ML, and above 1 ML excess molecules caused unstable imaging because they frequently stuck on the STM tip. The dramatic effects of concentration variation can be seen in the Supporting Information. In contrast, when titanylphthalocyanine is adsorbed on graphite, a very different structure is observed (Figure 4B). Kong and co-workers have reported that titanylphthalocyanine (TiOPc) √ molecules form a hexagonal lattice on graphite having a 3 3  √ 3 3-R30° surface structure with a surface density of 0.71 molecules/nm2.17 When we imaged TiOPc at the HOPG-octylbenzene interface, we also observed a 3-fold structure similar to that reported by Kong and with the same lattice parameters. It is impossible to identify the individual TiOPc molecules in this surface layer, and the reader is directed to Kong’s paper17 for a (37) Nazin, G. V.; Qiu, X. H.; Ho, W. Science 2003, 302, 77–81. (38) Ogunrinde, A.; Hipps, K. W.; Scudiero, L. Langmuir 2006, 22, 5697–5701. (39) Takami, T.; Carrizales, C.; Hipps, K. W. Surf. Sci. 2009, 603, 3201–3204. (40) Gopakumar, T. G.; Lackinger, M.; Hackert, M.; Muller, F.; Hietschold, M. J. Phys. Chem. B 2004, 108, 7839–7843.

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possible explanation. This is in marked contrast to the situation on Au(111), where a well-defined image of the Pc ligand is observed.19,41 Figure 4A shows a different HOPG surface after exposure to a 1:1 molar mixture of TiOPc and TiPcat. The upper domain seen in the constant current STM image in Figure 4A is due to a monolayer of TiOPc on HOPG and has the expected hexagonal structure. The lower region in Figure 4A shows a domain where a low density of TiPcat molecules are adsorbed on an underlying TiOPc monolayer. In all images, we only observed TiOPc as the first layer. The average apparent heights of TiOPc and TiPcat from STM images were 0.08 and 0.35 nm, respectively. The top layer TiPcat molecules form a rectangular lattice with unit cell lengths of 1.88 ( 0.07 and 1.96 ( 0.07 nm while the angle between the unit cell vectors is 90 ( 2°. Unlike the case of the TiPcat monolayer, the catechols are aligned parallel to one of these unit cell vectors. However, one still observes a strong tendency for the catechols to align parallel to each other. Attempts to obtain higher resolution images of this low-density overlayer region were unsuccessful because the images became unstable with either higher current or lower voltage settings. Figure 5 displays a constant current STM image of a completely different sample surface prepared from a binary mixture of TiOPc and TiPcat in the same manner as the sample in Figure 4A. This area shows a higher density of TiPcat overlayer. In this image, the underlying TiOPc molecules can be identified with 4-fold molecular symmetry, very unlike the 3-fold symmetry of a pure TiOPc monolayer. The central TiO region in TiOPc was observed as a dark dip, which is in good agreement with previous studies of rectangular TiOPc structures.16,17 The sublayer structure is difficult to determine exactly, but it appears to have unit cell lengths 1.30 ( 0.1 and 1.50 ( 0.1 nm and the angle between the unit cell vectors is 90 ( 2°. This gives a surface density of 0.51 molecules/nm2. Data on this structure and others discussed in this paper are collected in Table 1. Thus, the TiPcat over layer induced ordering of the TiOPc sublayer results in a structure that is considerably less densely packed than uncovered TiOPc. This rectangular structure is also very similar in packing density to that seen for the square adlayer observed for TiOPc on Au(111): 0.52 molecules/nm2.19 The difference in this structure and that of Figure 4A is probably due to a difference in local sample concentration. Since the samples were made on different days, and because of the evaporation technique used, there can be differences in concentration both overall and in regions on the sample. Each of these structures was stable for days. The orientation of the TidO bond in the TiOPc sublayer is of interest. Kera et al. used Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS) to demonstrate that in submonolayer coverage of TiOPc on HOPG all TiOPc molecules are oriented laying flat with the oxygen atoms directed outward from the graphite surface.42,43 They showed that the orientation with the oxygen atom directed inward is metastable;if the film is annealed, the molecules with O directed downward overturn and form a uniform monolayer with the oxygen atoms directed outward. They also indicated that a stable bilayer was formed when the second layer had O oriented inward (oppositely oriented than the first layer).43 In this study, the central TiO was not observed. We do not, however, believe this is (41) Kanai, M.; Kawai, T.; Motai, K.; Wang, X. D.; Hashizume, T.; Sakurai, T. Surf. Sci. 1995, 329, L619–L625. (42) Kera, S.; Abduaini, A.; Aoki, M.; Okudaira, K. K.; Ueno, N.; Harada, Y.; Shirota, Y.; Tsuzuki, T. Thin Solid Films 1998, 327-329, 278–282. (43) Kera, S.; Abduaini, A.; Aoki, M.; Okudaira, K. K.; Ueno, N.; Harada, Y.; Shirota, Y.; Tsuzuki, T. J. Electron Spectrosc. Relat. Phenom. 1998, 88-91, 885–889.

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Figure 7. Properly scaled structural models of TiPcat on TiOPc rectangular structure observed in Figure 6.

evidence against Kera et al.’s interpretation. We ascribe the lack of tunneling current near the center of the molecule (in the dense sublayer) as due to a low density of electronic states at the energy of the tunneling electrons used in this study. For example, in previous studies, we reported STM images of vanadylphthalocyanine (VOPc) and interpreted them as O up, even though the isoindole rings of the Pc were well-defined but the VO was not observed.12-14,44 These results also should be contrasted with the images seen for CuPc and NiPc where the metal ions are not observed and for CoPc and FePc where the central metal ions are the tallest features in the image.34,35 On Au(111) in UHV, TiOPc molecules adopt a flat-lying geometry and pack with a square unit cell and the center of the molecule appears dark as in TiOPc sublayers of the type shown in Figures 5 and 6.19 While the TiO unit in TiOPc molecules adsorbed on herringbone structural sites sometimes can be seen in the STM, the authors agree with our analysis of VOPc. They conclude that both the dim and bright centered molecules have their oxygen atom pointed upward from the Au(111) surface.19 Consistent with these previous studies, we assigning the oxygen up configuration to the TiOPc sublayer induced by TiPcat adsorption. It is interesting to note that the large size of the catechol inhibits the second layer from adopting the dipole-dipole interaction preferred orientation seen in TiOPc dilayers. The TiO bond of the TiPcat second layer is directed outward from the surface in parallel with the TiO bond of TiOPc in the first layer. When TiOPc molecules partially cover a TiOPc monolayer on graphite (homobilayer formation), TiOPc packs with a pseudosquare unit cell and individual molecular shapes are observed that (44) Barlow, D. E.; Hipps, K. W. Ultramicroscopy 2003, 97, 47–53. (45) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391–5396.

12714 DOI: 10.1021/la1020127

correspond well to the shape of TiOPc.17 Submonolayers of TiOPc on graphite pack in a hexagonal unit cell and individual molecules cannot be identified, as shown in this study and by Kong et al.16,17 As mentioned above, TiOPc monolayers on Au (111) in UHV adopt a flat-lying geometry and pack with a square unit cell.19 As an example of extremely strong TiOPc-substrate interaction, TiOPc on Ag(111) shows tilting from the surface plane because of the large interaction between the oxygen atom in TiOPc and silver.18 Thus, it appears that TiOPc molecules can form rectangular structures when the interaction between the molecule and substrate is weak. Even for the same substrates, the adsorbate structure can be changed by changing the charge on the surface. For example, the highest reported packing density for TiOPc is a nonflat structure on Au(111) substrate induced by electrochemical methods.17 Thus, the fact that adsorption of TiPcat onto TiOPc induces a conversion from hexagonal to rectangular is not surprising since the dipole-dipole interaction between TiPcat and the TiOPc adlayer changes the balance of forces between surface interactions and adsorbate-adsorbate interactions. In dense bilayers, we observe TiPcat molecules to be 0.3 nm protrusions above the TiOPc layer. In these denser regions of the sample (e.g., Figures 5 and 6), TiPcat molecules also adopt a much denser packing than was seen in Figure 4A. In Figure 5, the standing catechol ligand on TiPcat is observed as a long oval shape, and the orientation of the catechol ligands is aligned generally (but not universally) alternately. We note that some of the TiPcat molecules in Figure 5 appear to have shortened or even bent catechols. We tentatively attribute this to tip-induced transitions of the catechol over the rotational barrier. From many images similar to Figure 5, and one unusually good image (Figure 6), it was determined that TiPcat molecules are sitting Langmuir 2010, 26(15), 12709–12715

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on the hollow sites of the rectangular TiOPc sublayer, therefore adopting the same unit cell parameters as the underlying TiOPc. The proposed structure is shown in Figure 7 with correctly scaled CPK models occupy the lattice positions. Finally, we observed only TiOPc molecules on graphite several times and confirmed that no images such as shown in Figures 4A, 5, and 6 appeared when only TiOPc is present.

Conclusions The catechol in TiPcat is predicted to have a 6 kcal/mol barrier to rotation to 22.5° and a larger 12 kcal/mol barrier to rotation by 90° about the axis normal to the Pc ring. The equilibrium position has the oxygen atoms eclipsing the ring nitrogen atoms. At the -0.7 V bias used in this study, electrons do have sufficient energy to induce hops over the rotational barrier. On the basis of the STM images, it appears that such induced transitions are not common. The HOMO and LUMO of TiPcat and TiOPc are localized on the phthalocyanine and oxygen atoms, but the HOMO-1 and HOMO-2 of TiPcat are have high density on the cathacol. When single-component adsorption takes place at the HOPGoctylbenzene interface, TiPcat molecules form rectangular structures while TiOPc molecules form hexagonal structures. The TiOPc hexagonal monolayer structure is denser than the TiPcat

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Article

rectangular monolayer structure. When TiOPc and TiPcat are coadsorbed from chloroform, TiOPc preferentially forms the first layer with TiPcat forming a second layer. One bilayer structure has TiPcat molecules adsorbed at the hollow sites of the TiOPc rectangular monolayer. Thus, TiOPc is more strongly adsorbed than TiPcat. As the coverage of TiOPc by TiPcat increases, the TiOPc structure is dramatically changed from hexagonal to rectangular, and the density of each layer is reduced from the values observed for the separate single molecule monolayers. This adsorbate mediation of surface structure is a useful phenomenon to control two-dimensional structure;especially if (as is the case here) only a fraction of a second layer is required to convert the underlayer structure. UHV experiments are underway to investigate the possibility of tip-induced rotation of catechol about the surface normal axis. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grants CHE-0555696 and CHE-0848511. Supporting Information Available: STM images for TiPcat at various coverage levels on HOPG. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la1020127

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