STM Investigation of Temperature-Dependent Two-Dimensional

Oct 7, 2009 - STM Investigation of Temperature-Dependent Two-Dimensional Supramolecular Architectures of C60 and Amino-tetraphenylporphyrin on ...
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STM Investigation of Temperature-Dependent Two-Dimensional Supramolecular Architectures of C60 and Amino-tetraphenylporphyrin on Ag(110) M. Di Marino, F. Sedona, M. Sambi,* T. Carofiglio, E. Lubian, M. Casarin, and E. Tondello Dipartimento di Scienze Chimiche, Universit a di Padova and Consorzio INSTM, Via Marzolo 1, 35131 Padova, Italy Received July 22, 2009. Revised Manuscript Received September 15, 2009 Multicomponent supramolecular self-assemblies of exceptional long-range order and low defectivity are obtained if C60 and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on Ag(110) by sequential evaporation in the submonolayer range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. A ((2 -3, 6 ( 3) array consisting of supramolecular stripes of a 1:1 C60/TPP-NH2 2D adduct develops at 410 K (the low temperature, LT, phase). If the LT phase is annealed at 470 K, then a 3:1 fullerene/TPP-NH2 ((3 -5, 5 ( 5) nanoporous array (the HT phase) forms, with each pore containing a single porphyrin molecule. Phase separation occurs by annealing the HT phase at 520 K. Structural models are proposed and discussed on the basis of the experimental scanning tunneling microscopy results.

1. Introduction The supramolecular bidimensional (2D) ordering of [60]fullerene and its derivatives with organic counterparts selfassembled on suitable metal or semiconductor single-crystal surfaces is a topic of rapidly growing interest in the nanoscience community.1-3 A promising potential for applications in many different fields such as sensing, catalysis, nanoelectronics, and photochemical conversion of solar energy is foreseen, although the state-of-the-art research efforts in this area are still focused on fundamental issues concerning the often complex relationships between molecular structure and functionalization, surface structure, and intermolecular versus molecule/substrate interactions on one side and the resulting supramolecular architectures on the other. The strategy pursued to produce the multicomponent arrays usually consists of a two-step approach: (a) Surface self-organization of a molecular species able to interact with itself and with fullerene and its derivatives. Typically dispersion, dipolar, noncovalent directional (e.g., hydrogen bonds), host-guest, and/or electron donor (D)-acceptor (A) interactions are exploited, or a combination of two or more. (b) Subsequent fullerene dosage on the ordered organic layer acting as a template. Thermal treatment is sometimes used to improve the order.2 Calix-[8]-arene4 and corannulene5 are examples of templating layers that have been used or proposed for C60 ordering through *Corresponding author. E-mail: [email protected]. Fax: +39 049 827 5161. (1) Sanchez, L.; Otero, R.; Gallego, J. M.; Miranda, R.; Martı´ n, N. Chem. Rev. 2009, 109, 2081. (2) Bonifazi, D.; Kiebele, A.; St€ohr, M.; Cheng, F.; Jung, T.; Diederich, F.; Spillmann, H. Adv. Funct. Mater. 2007, 17, 1051. (3) Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A.; Chem.;Eur. J. 2009, 15, 7004. (4) Pan, G.-B.; Liu, J.-M-; Zhang, H.-M.; Wan, L.-J.; Zheng, Q.-Y.; Bai, Ch.-L. Angew. Chem., Int. Ed. 2003, 42, 2747. (5) Parschau, M.; Fasel, R.; Ernst, K.-H.; Gr€oning, O.; Brandenberger, L.; Schillinger, R.; Greber, T.; Seitsonen, A. P.; Wu, Y.-T.; Siegel, J. S. Angew. Chem., Int. Ed. 2007, 46, 8258.

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host-guest interactions. However, the most promising results to date are obtained when D-A interactions are exploited between porphyrins-based templating layers and fullerene as A, as recently reviewed by Bonifazi et al.,2,3 although direct charge transfer from/to the substrate might complicate this schematic picture. The rather strong D-A interactions between fullerenes and porphyrins6-9 prevent, at least in a certain temperature range, one of the most common obstacles in multicomponent selfassembly (MSA), namely, the phase separation of the two species to form surface-supported single-component islands. In fact, the rather strong tendency of fullerene to arrange in hexagonal or distorted hexagonal close-packed islands on most substrates has to be overcome if MSA has to be attained. Such a tendency is based on the 1.30 eV/molecule intermolecular interaction energy,10 which is predominantly of the VdW type, with dipolar contributions arising from the uneven charge density distribution on single and double C-C bonds.11,12 In addition, the modified surface energetics arising from the presence of the porphyrin network on the surface allows for efficient avoidance of the oftenfound surface reconstructions promoted by the presence of fullerene when deposited on the bare substrate.13-15 Finally, the extreme versatility given by organic functionalization of the (6) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 7090. (7) Boyd, P. D. W.; Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.; Chaker, L.; Brothers, P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc. 1999, 121, 10487. (8) Bhattacharya, S.; Nayak, S. K.; Chattopadhyay, S.; Manerjee, M. Spectrochim. Acta, Part A 2007, 66, 243. (9) Basiuk, V. A. J. Phys. Chem. A 2005, 109, 3704. (10) Nakamura, J.; Nakayama, T.; Watanabe, S.; Aono, M. Phys. Rev. Lett. 2001, 87, 048301. (11) Hou, J. G.; Jinlong, Y.; Haiqian, W.; Qunxiang, L.; Changgan, Z.; Langfeng, Y.; Bing, W.; Chen, D. M.; Qinshi, Z. Nature 2001, 409, 304. (12) Gritsch, T.; Coulman, D.; Behm, R. J.; Ertl, G. Phys. Rev. Lett. 1989, 63, 1086. (13) Murray, P. W.; Pedersen, M. Ø.; Lægsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. B 1997, 55, 9360. (14) Pedio, M.; Felici, R.; Torrelles, X.; Rudolf, P.; Capozi, M.; Rius, J.; Ferrer, S. Phys. Rev. Lett. 2000, 85, 1040. (15) Weckesser, J.; Cepek, C.; Fasel, R.; Barth, J. V.; Baumberger, F.; Greber, T.; Kern, K. J. Chem. Phys. 2001, 115, 9001.

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porphyrin macrocycle provides a powerful degree of freedom in designing and fine-tuning the “pitch” of the templating layer, by which the geometry of the MSA and the mutual distance between fullerene units within ordered arrays can be effectively controlled. Often such noncovalently interacting organic networks are flexible and able to respond to the presence of guest molecules by adapting their conformation to optimize the host-guest interactions. The resulting ordered MSA networks may additionally involve long-range, substrate-mediated interguest interactions that are ultimately responsible for the dimensions of the ordered domains.16,17 As a matter of fact, the extent of the long-range order is a key issue for potential applications of surface-supported MSA systems because a very high degree of coherence in the orientation, alignment, and packing of the building units is necessary as a basis for large-scale uniformity and coordination in the operations of functional materials. Long-range-ordered MSA arrays can be produced only if several conditions are simultaneously fulfilled: (a) high diffusion coefficients of all of the molecular species involved in the process at the given growth temperature; (b) an optimal balance between lateral intermolecular versus vertical molecule-substrate interactions; and (c) the careful design of intermolecular recognition through preferential noncovalent interactions between the different components of the assembly to avoid phase separation. In the few examples concerning C60/porphyrin MSAs so far reported in the literature (ref 2 and refs therein), Ag(111) has been mainly used as the substrate because it meets the first two requirements mentioned above. In this article, we show that MSA arrays of exceptional longrange order and low defectivity are obtained if C60 and 5-(4aminophenyl)-10,15,20-triphenylporphyrin (TPP-NH2) are assembled on Ag(110) by sequential evaporation in the submonolayer (ML) range of TPP-NH2 and fullerene on the substrate surface and subsequent annealing. Two different MSA phases are obtained and characterized as a function of the annealing temperature; these differ in the C60/TPP-NH2 ratio in the superstructure unit cells. Detailed structural models are developed on the basis of the experimental scanning tunneling microscopy (STM) results complemented by low-energy electron diffraction (LEED) and are discussed on the basis of the range of intermolecular interactions at stake in the investigated system.

2. Experimental Section 5-(4-Aminophenyl)-10,15,20-triphenylporphyrin was prepared starting from tetraphenylporphyrin by mononitration with fuming HNO3 in CHCl3, followed by the reduction of the nitro group to amino with SnCl2/HCl. Purification was carried out by column chromatography (silica, CH2Cl2/hexane 1:1 v/v eluent also containing 1% Et3N). All of the spectroscopic (UV-vis, 1H NMR) data were identical to those reported in the literature,18 and HPLC analysis performed after column chromatography showed no detectable traces of contaminants. The experiments were performed with an Omicron scanning tunneling microscope (VT-STM) operating in ultrahigh vacuum at a base pressure of 2  10-10 mbar. The Ag(110) crystal was cleaned by repeated cycles of 1 keV Arþ sputtering and annealing at 820 K until a clean surface with sufficiently large terraces was confirmed by STM imaging. TPP-NH2 molecules were deposited (16) Kiebele, A.; Bonifazi, D.; Cheng, F.; St€ohr, M.; Diederich, F.; Jung, T.; Spillmann, H. ChemPhysChem 2006, 7, 1462. (17) Sykes, E. C. H.; Mantooth, B. A.; Han, P.; Donhauser, Z. J.; Weiss, P. S. J. Am. Chem. Soc. 2005, 127, 7255. (18) Kruper, W. J.; Chamberlin, T. A.; Kochanny, M.; Lang, K. J. Org. Chem. 1989, 54, 2753.

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from a PBN crucible held at ∼590 K, and C60 (99% purity) was sublimed at ∼820 K from a tungsten crucible. The Ag specimen was kept at room temperature (RT) during sublimation. Both crucibles were outgassed for a long time to avoid impurities (e.g. solvent residues adsorbed on the TPP-NH2 powder) during deposition onto the substrate. The STM measurements were carried out at RT in constant-current mode using a Pt-Ir tip. The sample bias voltage (Vbias) is indicated for all STM images. STM length measurements are calibrated against the Ag lattice parameter along the [001] direction, which is set equal to 4.09 A˚. The error bars associated with length determinations represent 1 standard deviation, as determined from repeated measurements. One ML of TPP-NH2 is defined as a fully covering single molecular layer arranged in the ((2 -3, 5 ( 2) superstructure with respect to the underlying substrate (see below), corresponding to a surface density of 0.44 nm-2. Similarly, 1 ML of C60 corresponds to a fully wetting fullerene layer arranged in the c(4  4) superstructure found for C60 on Ag(110),19 with a resulting surface density of 1.06 nm-2.

3. Results Figure 1a shows a large-scale STM image (150  150 nm2) of approximately 1 ML of TPP-NH2 deposited on the Ag(110) surface at RT. Two ordered domains are clearly discernible on the substrate terraces, developing from a 2D molecular gas evident in the right part of the image. Each domain in the ordered phase consists of a series of parallel stripes of monomolecular width aligned with the [113] and [113] substrate directions, respectively, making an angle of 59° with each other. The inset in the upper right corner of Figure 1a shows a close-up of one of the two domains. The nearest neighbor (NN) intermolecular distance along the stripes is 1.4 ( 0.1 nm, and the distance between two adjacent stripes is 1.7 ( 0.1 nm. Figure 1b shows a high-resolution image of the TPP-NH2 superstructure. The unit cell (thick black line) has been confirmed independently by means of LEED measurements (not shown) and is in agreement with the abovereported experimentally determined intermolecular distances: unit vectors b1 = 1.36 nm and b2 = 1.66 nm at an angle of γ = 94.2° from each other define a ((2 -3, 5 ( 2) superstructure, whose model is sketched in Figure 1c.20 As verified by the integers in the matrix notation, the superstructure is simply commensurate with the substrate (i.e., all molecules in the overlayer occupy a locally equivalent adsorption site). The actual resolution in our STM measurements does not allow us to identify the site with certainty (i.e., the registry adopted in Figure 1c is only indicative). However, STM measurements are sensitive to the molecular orientation with fair accuracy. It appears that the molecule lies in the plane of the macrocycle parallel to the surface (except for the four peripheral phenyl rings, which are substantially tilted with respect to the molecular plane, as well known21) and with the axes bisecting the pyrrolic subunits aligned with the substrate’s [001] and [110] main azimuths, as shown in Figure 1c. A question now arises about the position of the single aminophenyl ring of each TPP-NH2 unit within the molecular array. Given the experimentally determined intermolecular distances (19) David, T.; Gimzewski, J. K.; Purdie, D.; Reihl, B.; Schlitter, R. R. Phys. Rev. B 1994, 50, 5810. (20) Matrix notation is given by adopting the usual convention by which the substrate (ai) and overlayer (bi) unit vectors are chosen in an anticlockwise direction starting form the shorter one: a1< a2 and b1< b2. In the case of the substrate, this means that if a1 is aligned along the [110] direction then a2 is parallel to [001], as indicated in all of the Figures, in particular, in Figure 3. (21) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899.

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Figure 1. (a) Large-scale STM image of the ((2 -3, 5 ( 2) superstructure of TTPNH2 on Ag(110) measured at RT (150  150 nm2, Vsample = þ0.66 V, I = 1.5 nA). (Inset) Close-up of one of the two domains (15  15 nm2, Vsample = -0.68 V, I = 0.07 nA). The substrate main directions are indicated in the lower right corner. The directions of alignment of the molecular stripes are highlighted. (b) High-resolution STM image (7  7 nm2, Vsample = þ0.03 V, I = 0.03 nA) and (c) molecular model of the (2 -3, 5 2) domain, drawn to scale with (b). The red circle highlights the proposed position of the amino-phenyl ring of a TPPNH2 unit. (See the text). The superstructure unit cell is indicated.

and molecular orientation, only two possibilities are viable: an interstripe arrangement, such as the one shown in Figure 1c where the amino-phenyl unit in a molecule (highlighted by a red circle) points to a molecule belonging to the adjacent stripe, or an intrastripe arrangement that would be obtained from the former by a 90° rotation of each molecule. We confidently discard the latter arrangement (where each amino-phenyl group would point to the next molecule in the same stripe) for steric reasons because quantitative structural models show that in this case the nitrogen atom of the amino group would lie at a bonding distance (∼0.16 nm) from one of the outer C atoms of the NN pyrrolic subunit belonging to the adjacent TPP-NH2 molecule. Furthermore, the interstripe arrangement implies that the N of the amino group falls at the right distance (∼0.38 ( 0.05 nm) from the NN phenyl ring belonging to a molecule of the adjacent stripe (Figure 1c) to be able to set up a N-H/π hydrogen bond. The distance is furthermore easily reduced by slightly rotating each TPP-NH2 molecule in Figure 2c by an amount that is well below the experimental precision. X-H/π contacts (X = N, O, S) occurring at a mean distance of X from the centroid of the aromatic ring of 2468 DOI: 10.1021/la9026927

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Figure 2. (a) Large-scale STM image of the ((2 -3, 6 ( 3) multicomponent C60/TPP-NH2 self-assembly (LT) phase on Ag(110) obtained at 410 K and measured at RT (90  90 nm2, Vsample = -1.0 V, I = 1.0 nA) (Inset) STM image (12  18 nm2, Vsample = -0.7 V, I = 1.0 nA) of a defective domain boundary in the LT TPP-NH2/C60 superstructure. Several C60 (TPP-NH2) molecules are highlighted with circles (crosses). (b) STM image (7  7 nm2, Vsample = þ1.0 V, I = 1.0 nA) and (c) molecular model of the (2 -3, 6 3) domain, drawn to scale with (b).

0.32-0.38 nm can be found in proteins and account for interaction energies of approximately 1.5-2 kcal/mol.22 Bringing the stripes closer than 1.7 nm would prevent the formation of such H bonds and would eventually also cause steric hindrance problems analogous to those mentioned for the intrastripe arrangement. In fact, preliminary experiments on pristine (nonaminated) TPP show that, in the absence of amino groups, molecular stripes align along the same [113] and [113] substrate directions but the interstripe distance is now reduced to 1.4 nm.23 Figure 2 shows the multicomponent self-assembly phase of C60 and TPP-NH2 that is obtained when submonolayer amounts of TPP-NH2 (∼0.5 ML) and C60 (∼0.2 ML) are dosed in sequence on the Ag(110) surface and annealed at 410 K for 80 min. To distinguish this superstructure from the one obtained at higher annealing temperature, this will be indicated as the low temperature (LT) phase. Island areas as large as 3000 nm2 are obtained, with a defectivity (mainly in the form of C60 vacancies) that is lower (22) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. 2003, 42, 1210. (23) Di Marino, M.; Sedona, F.; Sambi, M.; Carofiglio, T.; Lubian, E.; Casarin, M.; Tondello, E. Unpublished results.

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Figure 3. Structural relationship between pure TPP-NH2 ((2 -3, 5 ( 2) superstructure (Bravais lattice points marked in blue) and the multicomponent TPP-NH2/C60 ((2 -3, 6 ( 3) LT (red) and TPP-NH2/C60 ((3 -5, 5 ( 5) HT supramolecular self-assemblies (green). Colored arrows indicate the directions of alignment of the molecular stripes. (See the text). Black arrows show the main substrate surface azimuths.

than 1%. Two domains are detected, consisting of an array of long fullerene stripes (up to 30 C60 molecules) aligned along the same [113] and [113] substrate directions that were the azimuths of alignment of the single-component TPP-NH2 stripes of Figure 1. In addition, the NN distance between fullerene units along a stripe is 1.4 ( 0.1 nm (i.e., it perfectly mirrors the intrastripe NN intermolecular distance found in the pure porphyrin phase and is substantially larger than the NN interfullerene distance (1.0 nm) in the close-packed c(4  4) phase produced by fullerene alone on Ag(110)).19 As far as the interstripe distance is concerned, it is expanded from 1.7 ( 0.1 to 2.1 ( 0.1 nm on going from the singlecomponent TPP-NH2 superstructure to the composite C60/TPPNH2 phase, as shown schematically in Figure 3. The resulting superstructure (confirmed by LEED) is described by unit vectors b1 = 1.36 nm and b2 = 2.13 nm (in agreement with the abovereported experimentally determined intermolecular distances) at an angle of γ = 100.0° from each other, which defines a ((2 -3, 6 ( 3) superstructure whose model is sketched in Figure 2c. The C60/TPP-NH2 ratio in the unit cell is 1:1, and the average molecular surface density is 0.70 nm-2 with a porphyrin surface density of 0.35 nm-2 (i.e., decreased by 20% with respect to the single-component porphyrin phase). It appears that upon C60 deposition and annealing, regardless of the kinetic details of the restructuring, the net result is that the TPP-NH2 molecular rows keep their structural integrity along the directions of alignment (e.g., [113] in Figure 3) whereas the interstripe gap is enlarged to accommodate the fullerene molecules, which are kept apart from each other by prevalent interactions with six phenyl groups belonging to the four NN TPP-NH2 units. A direct experimental confirmation of the model can be obtained from STM images collected at the borders of ordered domains and/ or on defective regions of the self-assembly, such as the one reported in the inset of Figure 2a. Here, mainly because of a missing C60 row in the left multicomponent domain and the boundary between two domains, the position and the azimuthal orientation of several TPPNH2 molecules can be observed directly, thereby confirming the local molecular arrangement of TPP-NH2 units and also showing that the latter does not change on moving from single-component to multicomponent self-assembly. Langmuir 2010, 26(4), 2466–2472

Figure 4. (a) Large-scale STM image of the ((3 -5, 5 ( 5) multicomponent C60/TPP-NH2 self-assembly (HT) phase on Ag(110), obtained at 470 K and measured at RT (90  90 nm2, Vsample = -0.5 V, I = 1.0 nA). (Inset) High-resolution close-up of one of the two domains (15  15 nm2, Vsample = þ0.80 V, I = 0.70 nA). The black circle highlights a defect consisting of a double C60 vacancy. The arrow indicates a TPP-NH2 molecule. (b) STM image (7  7 nm2, Vsample = þ0.3 V, I = 0.3 nA) and (c) molecular model of the (3 -5, 5 5) domain, drawn to scale with (b).

If the LT TPP-NH2/C60 phase is further annealed at 470 K for 30 min, then a new multicomponent self-assembly phase is obtained (the high temperature (HT) phase), whose island dimensions are similar to those of the parent phase. A large-scale STM image of the resulting superstructure is reported in Figure 4a. It consist of a porous network of quasi-hexagonal symmetry consisting of close-packed fullerene rows aligned along the [331hh0] and [33 1h0 h ] substrate directions (only 2.3° from the [113] and [113] directions of stripe alignment in the LT phase, respectively; see the green superlattice in Figure 3), which alternate with sparser fullerene rows, wherein C60 units 2.3 nm from each other are separated by X-shaped nanopores, as appears in the close-up image in Figure 4b and the related structural model depicted in Figure 4c. The intrarow C60 NN distance in close-packed rows is 1.1 ( 0.1 nm (i.e., consistent with fullerene molecules in direct mutual contact at the van der Waals distance). The cavities have the right shape, dimensions, and orientation to accommodate a porphyrin molecule, as proposed in the reported model. The corrugation of the superstructure due to fullerene molecules prevents direct observation with sufficient resolution of the embedded TPP-NH2 units in the perfectly ordered areas of the superstructure. To prove the proposed building units arrangement DOI: 10.1021/la9026927

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within the supramolecular network, STM imaging close to point defects in the ordered domains gives valuable information, as in the case of the LT phase. The inset of Figure 4a shows a highresolution image of an area characterized by a point defect consisting of two missing fullerene molecules (highlighted by a circle). The dip associated with the vacancy has lateral dimensions that are comparable to those of the nanopores in the ordered areas, which suggests that tip convolution effects due to the high corrugation of the superstructure play a similar role in both cases and therefore the depth difference between regular pores and the point vacancy can be taken as meaningful. The average depth of regular pores is 0.38 ( 0.02 nm, and the depth of the C60 double vacancy is 0.58 ( 0.02 nm, which compares very well with the apparent height of the whole island with respect to the bare substrate (0.61 ( 0.02 nm). The 0.03 nm difference between the C60 double-vacancy depth and the island height can be taken as an estimate of the systematic error in pore depth measurements caused by tip convolution effects due to the high corrugation of the nanostructure. We now note that the depth difference between the regular pores and the fullerene vacancy (∼0.2 nm), which is an indirect measure of the apparent height of the porphyrin molecules nested in the pores with respect to the substrate, is measurably larger than the apparent height of pure TPP-NH2 domains on the silver substrate measured at comparable bias values on the single-component porphyrin phase (∼0.07 ( 0.02 nm; Figure 1). This suggests that the optimized interactions of the TPP-NH2 molecule in the pore with NN C60 molecules leads to an overall increase in the mean distance from the surface of the nested macrocycle with respect to the singlecomponent TPP-NH2 assembly (e.g., through modified conformations of the peripheral phenyl rings σ bonded to the central macrocycle). Finally, direct proof of the structural arrangement proposed in Figure 4c is provided in the inset of Figure 4a, where a single TPP-NH2 molecule (indicated by an arrow) can be singled out in a not fully developed (and therefore more accessible) pore close to the point defect. The lattice vectors of the HT nanoporous superstructure, confirmed by LEED, are b1 = 2.3 nm and b2 = 2.5 nm at an angle of γ = 121.7° from each other. The overall periodicity is described by a ((3 -5, 5 ( 5) matrix, with a 3:1 C60/TPP-NH2 ratio in the unit cell. The average molecular surface density is increased to 0.85 nm-2, and the porphyrin surface density decreases to 0.21 nm-2 (i.e., it is reduced by 52% with respect to the single-component porphyrin phase and by 40% with respect to the LT multicomponent phase). Evidently, the annealing treatment promotes an enrichment of the multicomponent self-assembly in C60: fullerene building units come into direct mutual contact, whereas TPP-NH2 stripes, which survived in the LT phase, are now disrupted and a peculiar network of porphyrin units, isolated from each other by fullerene “walls” of monomolecular width, develops on the substrate surface. The tendency toward fullerene enrichment of the assemblies, which ultimately leads to phase separation (possibly accompanied by an at least partial thermally induced desorption of TPP-NH2 molecules) is confirmed by increasing the annealing time and/or temperature. Figure 5 shows an STM image of the HT phase which has been annealed at progressively higher temperatures up to 520 K for several tens of minutes. Close-packed c(4  4) fullerene domains develop progressively from the HT phase (see the right side of the image), thereby attaining the highest possible C60 surface density compatible with the given substrate. At the end of the annealing process, the HT supramolecular phase disappears completely and the c(4  4) C60 islands represent the only ordered phase on the substrate surface. 2470 DOI: 10.1021/la9026927

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Figure 5. STM image (60  60 nm2, Vsample = þ0.8 V, I = 1.0 nA) of the C60/TPP-NH2 sample after 20 min of annealing at 520 K, measured at RT. The (HT) TPP-NH2/C60 f c(4  4) C60 phase evolution upon annealing the HT phase by a progressive increase in temperature is confirmed by STM and LEED analysis. The phase separation is complete after 80 min of annealing at 520 K.

4. Discussion In the present section, several interrelated aspects of the TPPNH2/C60 MSA arrays are comprehensively discussed on the basis of the reported results. 4.1. Structural Habit of the C60/TPP-NH2 MSAs. Linescans derived from Figure 5 (not reported) show that the apparent height of the HT and (4  4) phases is the same within experimental accuracy. Analogous measurements performed during the LT f HT phase transition similarly demonstrate that the two multicomponent phases have the same apparent height with respect to the substrate. This observation further confirms the models proposed for the two phases, where the C60 molecules are in direct contact with the silver substrate and are only laterally coordinated by TPP-NH2 units. In other words, there is no evidence of C60 adsorption on top of porphyrin building blocks within the multicomponent self-assemblies. Rather, C60 molecules are nested in the silver-exposing cavities of the TPP-NH2 2D network. It is known that C60 adsorption on Ag has ionic character, with sizable electron/molecule charge transfer (CT) from the substrate to the molecule. 24 On the basis of our results, it is reasonable to expect that CT from the metal to the electronaccepting fullerene, albeit modified by the presence of TPP-NH2 molecules, plays a role both in determining the overall structure of the multicomponent phases and in mediating the intermolecular interactions with respect to the fullerene-porphyrin interactions in the solid state6,7 and in solution.8 In fact, to provide direct C60/ Ag contact upon fullerene dosage on the preadsorbed TPP-NH2 phase, the latter has to rearrange by spacing out the porphyrin stripes (i.e., by decreasing the porphyrin surface density) to accommodate the C60 units (Figure 3). In other words, the TPP-NH2 superstructure does not act as a substantially rigid template that does not change its long-range structural habit upon (24) Magnano, E.; Vandre, S.; Goldoni, A.; Laine, A. D.; Curro, G. M.; Santaniello, A.; Sancrotti, M. Surf. Sci. 1997, 377-379, 1066.

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C60 dosage. Rather, the latter is changed in order to optimize simultaneously both the vertical molecule substrate and the lateral intermolecular interactions. Because substantial mass transport is required for the reorganization, the process appears to be thermally activated. (We offer more discussion on this topic below.) 4.2. Intermolecular Interactions. As far as the intermolecular interactions are concerned, on the basis of our results we can suggest a qualitative hierarchy in the investigated system. At least for the LT phase, the heteromolecular interactions seem to prevail on both kinds of homomolecular ones. When sequential TPPNH2 and C60 depositions are performed starting from substantially lower and either approximately stoichiometric (1:1) or porphyrin-rich doses, the LT multicomponent phase is exclusively obtained no matter how small the resulting islands are; singlecomponent fullerene domains are never observed. The next lateral interaction on a scale of decreasing strength is the intrastripe interaction between TPP-NH2 molecules along their two symmetry-equivalent alignment directions: as already remarked, while adjacent stripes are spaced out upon fullerene dosage, the intermolecular distance along the stripe is kept unaltered on going from the pure TPP-NH2 to the LT multicomponent phase. This preferential interaction is attributed to the π-π phenyl group interactions between NN TPP-NH2 units along the stripe,22 as suggested by Figure 2c. The interstripe distance is likely to be the outcome of the balance between the vertical molecule substrate interaction (which is strongly site-selective, leading to a simply commensurate superstructure) and the tentatively proposed interstripe N-H/π weak hydrogen bonds that determine the overall surface density of the porphyrin ML. (We remind the reader that the interstripe distance is shorter and hence the surface density is higher for nonaminated, pristine tetraphenylporphyrin (TPP).23) Further experiments are planned both on pristine and on diaminated TPP to ascertain systematically the role of the amino groups in the self-assembly of the single and multicomponent phases. 4.3. Dependence of the MSA Long-Range Order and Defectivity on Dosage. As mentioned above, the porphyrin surface density decrease on going from the TPP-NH2 phase to the LT multicomponent assembly implies substantial surface mass transport. As a consequence, the initial TPP-NH2 dose has important consequences for the final degree of long-range order and low defectivity attainable in the mixed phase. If the surface is fully covered by a ML of TPP-NH2 prior to C60 dosage, then there is no space available for the expansion of the porphyrin subnet, which ultimately leads to poorly ordered, defective small domains of the multicomponent phase. This means that the initial TPP-NH2 dose has to be chosen below the ML coverage so as to allow for the required ∼26% expansion. In addition, the higher the initial TPP-NH2 dose (and hence the larger the average TPPNH2 island dimensions), the larger the absolute diffusion lengths for individual TPP-NH2 molecules that are required to complete the phase transition and therefore the longer the required annealing times. We have indeed checked that starting from higher TPPNH2 coverage values and/or annealing for shorter times leads to smaller ordered multicomponent islands. 4.4. Fate of TPP-NH2 after Phase Separation upon Annealing. The transition from the LT to the HT phase implies a further decrease in the TPP-NH2/C60 ratio and the onset of prevailing direct C60-C60 interactions, which are exclusive after full phase separation at 520 K. Although in the LT phase the TPPNH2/C60 ratio is equal to the ratio found in the 3D solid-state meso-tetraphenylporphyrin cocrystallates6,7 and adducts in solution,8 the ratio modification at HT could be explained in general Langmuir 2010, 26(4), 2466–2472

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terms with the temperature dependence of the formation constant of the supramolecular phase. However, there are experimental details in Figure 5 and in analogous images acquired after annealing at HT that suggest a more complex picture. We first note that only C60 forms an ordered array (the c(4  4) phase) after phase separation. TPP-NH2 does not produce any ordered phase even after cooling at RT. The disorder makes a quantitative evaluation of the porphyrin coverage after annealing difficult. This difficulty notwithstanding, we can confidently say that at least a fraction of TPP-NH2 is lost upon annealing, probably through thermally induced desorption. In addition, the remaining disordered phase is qualitatively different from the 2D gas phase from which the ordered ((2 -3, 5 ( 2) superstructure of TTPNH2 on Ag(110) develops at RT: if one compares the disordered porphyrin fractions in Figures 1a and 5, then one important difference becomes apparent. While the 2D gas at RT in Figure 1a is very mobile, thus preventing the resolution of individual molecules, many such TTP-NH2 molecules can be resolved in Figure 5. Moreover, time-lapsed imaging sequences of the same surface area show that these molecules do not diffuse on the time scale of the STM acquisition, suggesting a much stronger interaction with the substrate after annealing at 520 K and subsequent cooling at RT than after direct dosing at RT. Literature data on phenylamine (aniline) adsorption on transition-metal and noblemetal single-crystal surfaces reported in the literature25-30 show that (a) the binding strength of the amino group to the surface is larger for Ag(110) than for Ag(111)25,26 and (b) on many (110)oriented surfaces phenylamine is found to adsorb mostly through the lone pair electrons of the nitrogen atom and undergo thermally induced dehydrogenation at the N atom.25-27 This suggests a strong interaction of the amino group with the surface. An analogous anchoring to the surface of our TPP-NH2 units through the amino group after annealing would explain both the absence of an ordered porphyrin phase after phase separation and the immobile TPP-NH2 molecules at RT. As indirect proof of dissociative N bonding to the surface, no immobile molecules after annealing at 520 K are observed if TPP is used instead of TPP-NH2.23

5. Conclusions In this article, we have shown that two different MSA phases can be obtained as a function of temperature by sequential evaporation of TPP-NH2 and C60 on Ag(110) followed by annealing, characterized by excellent long-range order and low defectivity. The single-component TPP-NH2 phase partially rearranges upon fullerene dosing and subsequent annealing at 410 K to form the LT MSA, which consists of a long-range-ordered array of supramolecular stripes of a 1:1 fullerene/TPP-NH2 2D adduct arranged in two symmetry-equivalent domains. Here, the interfullerene NN distance is controlled by the intermolecular distance in the porphyrin subnet through preferential heteromolecular noncovalent interactions. The HT TPP-NH2/C60 2D supramolecular array is perhaps the most peculiar finding in this work: a 3:1 fullerene/TPP-NH2 2D adduct in the form of an ordered fullerene nanoporous array (again arranged in two symmetry-equivalent domains), with each (25) Ramsey, M. G.; Rosine, G.; Steinmuller, D.; Graen, H. H.; Netzer, F. P. Surf. Sci. 1990, 232 266. (26) Rockey, T. J.; Yang, M.; Dai, H.-L. Surf. Sci. 2005, 589, 42. (27) Plank, R. V.; DiNardo, N. J.; Vohs, J. M. Surf. Sci. 1995, 340, L971. (28) Huang, S. X.; Fischer, D. A.; Gland, J. L. J. Vac. Sci. Technol., B 1994, 12, 2164. (29) Huang, S. X.; Fischer, D. A.; Gland, J. L. J. Phys. Chem. 1996, 100, 10223. (30) Schoofs, G.; Benziger, J. J. Phys. Chem. 1988, 92, 741.

DOI: 10.1021/la9026927

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pore containing a single porphyrin molecule isolated from its siblings. It would be very interesting to explore the possibility of including metalloporphyrin analogues of TPP-NH2 (e.g., FeTPPNH2) in such an array either through direct evaporation of the metalloporphyrin on the surface or by in situ metalation of TPP-NH231 already embedded in the supramolecular network to (31) Bai, Y.; Buchner, F.; Wendahl, M. T.; Kellner, I.; Bayer, A.; Steinrueck, H.-P.; Marbach, H.; Gottfried, J. M. J. Phys. Chem. C 2008, 112, 6087.

2472 DOI: 10.1021/la9026927

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test its reactivity toward small-molecule adsorption at the metal center. The nanoporous network is likely to influence the accessibility and consequently the reactivity of the coordinated metal atom, with potential effects in the fields of nanosensing and catalysis. Acknowledgment. We thank the University of Padova (Progetti Strategici 2008 - HELIOS) for financial support.

Langmuir 2010, 26(4), 2466–2472