Article pubs.acs.org/JPCC
Structurally Tunable Self-Assembled 2D Cocrystals of C60 and Porphyrins on the Ag (110) Surface Francesco Sedona,* Marco Di Marino, Andrea Basagni, Luciano Colazzo, and Mauro Sambi Dipartimento di Scienze Chimiche, Università di Padova and Consorzio INSTM, Via Marzolo 1, 35131 Padova, Italy ABSTRACT: Due to the donor−acceptor nature of their supramolecular interactions, nanostructured porphyrin-fullerene self-assembled architectures show attractive properties that can be exploited in high efficiency solar cells. In this work, we show that six different ordered bicomponent porphyrin-fullerene (C60) networks are obtained by controlling the peripheral functionalization of meso-tetraphenylporphyrins (TPP) with amino groups and the stoichiometry of their aggregates with C60 on Ag (110). Such networks can be grouped in two general classes, depending on their structural habit: the so-called “stripes” phases, formed by alternating monomolecular stripes of C60 and TPP, and the so-called “pores” phases, where a fullerene net accommodates isolated TPP molecules in nanometer-sized pores. These phases are of general interest in the field of surface-supported electron donor−acceptor systems, since they represents a rare example of fullerene-containing surface-supported bicomponent supramolecular networks where the binary nanostructures are definitely more stable thermodynamically than the two separated single-component phases, thereby resembling three-dimensional TPP/C60 cocrystals. The thermodynamic stability in an extended temperature range has profound consequences on the degree of long-range order attainable in the selfassembly process.
1. INTRODUCTION Porphyrin−fullerene dyads are considered a prototype of donor−acceptor (D−A) systems, and the recent considerable interest on their supramolecular noncovalently bonded architectures is related to the possibility of using these selfassembled nanostructures in solar cells with increased power conversion efficency.1−5 In solid state supramolecular chemistry, the interaction between fullerene and porphyrin has been studied in detail: almost any free-base or metalated tetraphenylporphyrin (TPP) with different phenyl functionalizations in meso positions of the porphyrin macrocycle can be cocrystallized with almost any fullerene (or derivatized fullerene) from comixed solutions.6,7 The molecular packing of cocrystal structures, along with simulation studies, evidence that the van der Waals attraction between the curved π surface of fullerenes and the planar π surface of porphyrins (producing the so-called cofacial geometry) is the main driving force for their supramolecular recognition.8,9 In contrast with the big effort spent in the study of 3D fullerene−porhyrin supramolecular nanostructures, relatively few works have described two-dimensional architectures supported on a substrate. Itaya et al. have succeeded in forming a 1:1 cofacial fullerene−porphyrin supramolecular assembly of a C60 derivative with a highly ordered Zn-octaetylporphyrin array formed on a Au(111) surface at the solid−liquid interface.10,11 More recently, the cofacial orientation has been investigated at the molecular level by depositing fullerene on top of a Ce(TPP)2 double-decker monolayer.12 In addition, Bonifazi et al. have explored the lateral interactions (end-on geometry) between the © 2013 American Chemical Society
two molecules by dosing C60 on a porous monolayer formed by cyanophenyl-functionalized Zn-porphyrins.13,14 Recently, a theoretical study has outlined the interest of the TPP/fullerene end-on supramolecular geometry by calculating that the charge transfer excitation energy for this orientation is 0.6−0.75 eV larger than for the cofacial geometry. This difference can be very important because the open-circuit voltage (the maximum voltage) of a solar cell depends mainly on this parameter.15 Under this premise, it becomes very interesting to analyze the interaction between fullerene and porphyrins self-assembled on a substrate in the end-on geometry. In general, in order to obtain a surface self-assembled bicomponent (BC) system that involves fullerene with a second organic molecule, it is necessary to exploit the host guest approach, i.e., fullerenes are deposited on a preformed monolayer of the second component molecules. In some cases, the host molecules can form a porous rigid nanostructure that acts as a template for the subsequent deposition of fullerenes;16,17 in other cases, host molecules give rise to a flexible network: monolayers of porhyrins,13,14 αsexithiophene,18,19 acridine-9-carboxylic acid,20,21 Cl-subphtalocyanine,22 or pentacene23 reorganize after fullerene deposition and eventual mild annealing. This type of approach is necessary because it reduces the molecular mobility at the surface, leading to kinetically stabilized BC nanostructures and thereby avoiding phase separation in two single-component (SC) phases, that is Received: September 19, 2013 Revised: December 20, 2013 Published: December 23, 2013 1587
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Figure 1. Phase diagram of the TPP−C60 BC system on Ag (110). The first line shows high resolution STM images (13 × 13 nm2) of pure phases at the proper molecular fraction and second line (13 × 17 nm2) the coexistence of two phases at intermediate values of χ. In all images, the substrate [110̅ ] close-packed direction lies horizontally.
normally due to the high stability of the close packed hexagonal fullerene phase.24−26 We have recently demonstrated that two different binary TPP/C60 nanostructures can be obtained on Ag (110).27,28 As described in the Results and Discussion section, this system is very interesting in order to study the lateral interactions between C60 and TPP because, to the best of our knowledge, this is the first surface self-assembled BC system that involves fullerene where the binary nanostructure is clearly thermodynamically more stable than the two SC phases, as in the case of 3D cocrystals. The interest of the reported nanostructures is not only limited to the study of the role of TPP-C60 lateral interactions in the selfassembly process: indeed, with a proper functionalization of the porphyrin’s meso phenyl rings with −NH2 groups, we have previously demonstrated that it is possible to induce a thermally triggered reaction between the TPP amino-groups and fullerene, by this way fixing the supramolecular nanostructure with covalent bonds, so to obtain a long-range ordered and stable monolayer copolymer.28 In this work, scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) are employed in a very detailed study of the surface TPP/C60 self-assembly behavior on Ag (110). More precisely, we have studied how the BC nanostructures evolve on changing the C60 molecular fraction χ = C60/(TPP + C60) at the surface from 0 (100% of porphyrin) to 1 (100% of C60) and how the TPP peripheral functionalization with amino groups influences the self-assembly properties of the nanostructures. The study has been performed using the following:
(a) TPP: free-base tetraphenylporphyrin; (b) trans-TPP(NH2)2: 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin, where two trans phenyl groups have been substituted by p-aminophenyl groups; (c) TAPP: 5,10,15,20-Tetrakis(4-aminophenyl)porphyrin, where all four phenyl groups have been substituted by paminophenyl groups. By varying the TPP functionalization, we have obtained 4 SC and 6 BC phases. Due to the simple commensuration with the substrate of all of the unit cells, we have been able to fully characterize the geometry of the different nanostructures, a determining step in order to study the interactions between porphyrin and fullerene molecules and the driving forces for their self-assembly, which will be the subject of a forthcoming publication. In the following, the general name “porphyrin” collectively indicates all three tetraphenylporphyrin species when no ambiguity arises from this usage.
2. EXPERIMENTAL SECTION 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 Ar+ sputtering and annealing at 820 K until a clean surface with sufficiently large terraces was confirmed by STM imaging. Preparation of trans-TPP(NH2)2 molecules has been reported in a previous work,28 whereas TPP (Aldrich, 99% purity), TAPP (Porphyrin Systems, 98% purity), and C60 (Aldrich, 99% purity) are commercially available products. 1588
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Figure 2. Models of TPP/C60 phases.
SC or BC phases. From left to right, different nanostructures as a function of χ are shown: the two SC phases formed exclusively by TPP (left) or C60 (right) and the two BC phases: the so-called “stripes” phase at χ = 0.5, characterized by alternating monomolecular stripes of C60 and TPP, and the so-called “pores” phase at χ = 0.75 characterized by a network of pores, each formed by eight fullerenes that surround a single TPP molecule (see Figure 1c,f,g). The STM images in the second line of Figure 1 show how, if intermediate molecular fractions are adopted, the resulting surface is heterogeneous and two of the already reported phases coexist as segregated or intermixed islands. The molecular fraction at the surface has been determined with extensive STM analysis, checking the coverage of different phases. Moreover, the presence of only one or at most two phases depending on the molar fraction has been checked, on the macroscopic scale, by LEED analysis. In order to understand the lateral interactions between TPP and C60 molecules in the BC phases, it is important to analyze the structural similarities between the SC and the BC phases. Figure 2 shows the sketched models of the different SC and BC TPP− C60 phases. Every model is completed with the unit cell dimensions and with the matrix notation of the overlayer unit cell with respect to the substrate. TPP on Ag (110) at RT forms an ordered phase characterized by an oblique unit cell, whose shorter cell parameter (1.36 nm) is aligned with the [11̅3̅] substrate direction, which has been evidenced by a gray line in Figure 2a. At χ = 0.5, the system organizes in the BC stripes phase: the model in Figure 2b highlights how the stripes direction and the intermolecular distance along the stripes are the same as in the SC TPP phase. Formally, in order to form the stripes phase, the SC TPP phase widens the gap between neighboring stripes to accommodate a fullerene stripe in-between. At the other extreme, the BC pores phase (Figure 2c) shows interesting similarities with the SC fullerene phase: C60 on Ag (110) forms a slightly distorted hexagonal close-packed structure with a c(4 × 4) unit cell with respect to the substrate,30 where the distance between nearest neighboring C60 units is about 1.00 nm, very similar to the intermolecular spacing and symmetry that C60 adopts in the bulk. As evidenced by the dashed line on the STM image in Figure 1d, linear defects are present along the close packed direction aligned with the [11̅2̅] substrate direction. As reported in the model of the C60 phase (Figure 2d), molecules meeting at the linear defectevidenced by the dashed lineare separated by a shorter intermolecular distance of 0.96 nm. When χ = 0.75, the system organizes in the BC pores phase. In the model shown in Figure 2c, the gray line evidences how this phase is characterized by the presence of a zigzag motif formed by
The three different porphyrin molecules were deposited from a pyrolytic boron nitride crucible held at temperatures between 500 and 590 K, whereas C60 was sublimed at temperatures between 850 and 890 K from a tungsten crucible. Both crucibles were outgassed for a long time to avoid impurities during the depositions onto the substrate. The Ag specimen was kept at room temperature (RT) during sublimations. Large islands of ordered SC porphyrin phases have been obtained with no postdeposition substrate annealing, while extended areas of C60 SC and C60-porphyrin BC phases can be obtained only after annealing at about 380 K, with long annealing times (from 3 to 15 h, depending on the surface coverage) required for the development of BC phases. STM measurements were carried out at RT in constant current mode, using a Pt−Ir tip. The STM images reported in this work have been acquired with sample bias voltage varying between 0.5 and 1 V and tunneling current between 1 and 3 nA. STM images were analyzed with the WSxM software.29
3. RESULTS AND DISCUSSION Self-assembly of organic molecules is driven by subtle equilibria between weak and reversible lateral (intermolecular) and vertical (molecule−substrate) interactions. In this work, the systematic variation of the fullerene molecular fraction χ = C60/(TPP + C60) at the surface and of the number of porphyrin peripheral amino groups allows us to evaluate qualitatively the contribution of lateral interactions to the self-assembly process. The role of vertical interactions is still being investigated through the realization of the same BC porphyrin-fullerene phases on different Ag single crystal orientations. As far as vertical interactions are concerned, LEED and high resolution STM images show that all the ordered nanostructures obtained on Ag (110) are characterized by simply commensurate unit cells with respect to the substrate. This indicates that the corrugation of the vertical interaction potential is large enough to select specific adsorption sites for each molecular species. As will be shown below, this opens up the possibility of discretely tuning the structural habit of the molecular networks (stripes orientation and spacing, pore sizes, and so forth) by proper functionalization of the TPP building blocks. In the following, we first discuss the structural evolution of 2D TPP/C60 supramolecular networks as a function of the C60 molecular fraction; then we consider the effect of varying the TPP functionalization on SC TPP and BC stoichiometric phases. 3.1. Composition Dependence of TPP/C60−Ag (110) Networks. Figure 1 reports the phase diagram of the TPP/C60 multiphase system and summarizes how, by changing the molecular fraction at the surface, it is possible to obtain different 1589
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pores phases. At first glance, it seems that the amino groups do not change the lateral interactions between fullerenes and porphyrins. However, an accurate analysis of the different phases shows that even though the stripes and pores motif remains unaltered, the amino groups introduce several interesting modifications in the dimensions and in the orientation relative to the substrate of the different unit cells. If one looks at the models of SC porphyrin phases reported in the first column of Figure 4, then a clear trend shows up on adding amino groups to TPP units, starting from simple TPP through trans-TPP(NH2)2 to TAPP molecules. The SC phase of trans-TPP(NH2)2 is characterized by the presence of the same porphyrin stripes along the [113̅ ̅] substrate direction found in the unfunctionalized TPP SC phase (Figure 4a,d, highlighted by continuous gray lines). However, the presence of two peripheral amino groups in trans-TPP(NH2)2 forces the superstructure to increase the distance between two adjacent stripes, as evidenced by the comparison of the two unit cells, where the parameter along the stripe direction remains unaltered, while the second parameter increases from 1.50 nm for TPP to 1.66 nm for transTPP(NH2)2. In order to accommodate two more amino groups, as in the case of fully amino-functionalized TAPP molecules (Figure 4g), the system cannot maintain the unit cell parameter along the stripe direction, where the molecule are very close to each other (1.36 nm), and therefore it is forced to replicate the second unit cell parameter of trans-TPP(NH2)2 (dotted line in Figure 4d along the 5̅54 direction) in the mirror-symmetric direction, forming a quasi-hexagonal unit cell where nearest neighboring TAPP molecule are at a distance of 1.66 nm (Figure 4g). In summary, from the analysis of the three SC phases, it appears that the amino groups have a mainly steric effect that emerges from the comparison of the areas of the three unit cells, which increase from 2.02 nm2 for simple TPP to 2.37 nm2 for TAPP. This result is in line with the simulations performed by Barone et al. that compare self-assembled structures of TPP and transTPP(NH2)2 on Ag (111): in that case, the molecules interact laterally with each other through their phenyl moieties, which are engaged in T-shaped interactions. The presence of amino groups increases the distance between the center of the mass of nearestneighbor (NN) phenyl rings belonging to adjacent molecules from 5.2 Å (TPP) to 5.5 Å (trans-TPP(NH2)2).31 The analysis of the three different BC stripes phases is in line with this interpretation: TPP and trans-TPP(NH2)2 have the same unit cell, since in both cases porphyrins maintain the preferential [11̅3̅] direction for stripes alignment present in the respective SC phases and enlarge the interstripe distance in order to intercalate fullerene with porphyrin stripes. However, when TAPP and C60 are used, due to the presence of four amino groups on the former, the system completely reorganizes, forming a stripes phase with a newly oriented stripes direction (along the highly symmetric [11̅0] direction of the substrate) and characterized by a unit cell where the distance between NN TAPP molecules is 1.44 nm: slightly larger than 1.36 nm in order to accommodate two further amino groups. The three BC pores phases show interesting differences as well: the more evident is a new fullerene zigzag direction for trans-TPP(NH2)2 with respect to either the TPP or the TAPP cases (gray line in Figure 4c,f,i): the C60 zigzag motif for TPP and TAPP molecules is formed by a C60 triplet with intermolecular spacings of 1.00 nm and a C60 couple spaced at 0.96 nm, while for the trans-TPP(NH2)2, the spacings are inverted and the zigzag is
regularly staggered triplets of C60 molecules joined at bimolecular kinks, the same structural motif that can be found in the C60 SC phase across the linear defect, as evidenced by the gray line in Figure 2d: intermolecular C60 distances and line directions are exactly the same. From this comparison between SC and BC phases, it is evident that at low values of the molecular fraction, the system selforganizes in the stripes phase, where the alignment direction and intermolecular distances are mainly dictated by the interaction between the TPP molecules,31−33 whereas when the C60 fraction increases above 0.5, the pores phase appears on the surface and in this case intermolecular distances and alignment directions are dictated by the lateral interaction between fullerenes.24−26 As already anticipated, it should be stressed that the BC phases are thermodynamically more stable than the two SC phases. By first depositing and annealing the C 60 molecules and subsequently depositing the porphyrins at RT, it is possible to obtain a kinetically stabilized arrangement, with the two SC TPP and C60 phases organized in separated large islands of several tens of nanometers width. Starting from this situation in the submonolayer regime, after an extensive annealing at 380 K, the system rearranges as reported in the phase diagram of Figure 1, so that at equilibrium there is no trace left of the original copresence of the two SC phases. The stability of the binary phases allows us to obtain exceptional long-range ordered and lowly defective binary nanostructures, as reported in Figure 3, which can be seen as the 2D analogues of 3D porphyrin-fullerene cocrystals.
Figure 3. STM images (100 × 200 nm2) of large terraces of stripes (a) and pores (b) phases.
3.2. Changing the Functionalization of TPP mesoPhenyl Groups. Figures 4 and 5 report, respectively, the STM images and LEED patterns of the SC porphyrin and BC phases obtained by codepositing C60 with different porphyrins on the Ag (110) surface: the already described TPP molecules, the bisamino functionalized trans-TPP(NH2)2 species, and the tetraamino functionalized TAPP units. As much as pristine TPPs, also trans-TPP(NH2)2 and TAPP molecules give rise to stripes and 1590
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Figure 4. STM images and models of different SC TPP and BC TPP/C60 phases on Ag (110) obtained by changing the molecular fraction (rows) and the number of amino groups on the peripheral phenyl rings (columns). In all images, the substrate [110̅ ] direction lies horizontally.
stripes phases is led by the interporphyrin interactions, whereas for the C60-rich BC pores phases the interfullerene lateral interactions become decisive in determining the geometry of the nanostructures. Particularly interesting is how the system reacts to the introduction of a varying number of −NH2 groups on the TPP outer rim. From the self-assembly point of view, these functional groups have a simple steric effect: the system maintains the stripes or pores motifs unaltered, since they evidently optimize the lateral interactions between molecules, but decisive changes are introduced in the alignment and dimensions of the two patterns in order to accommodate the amino groups within the motifs. Overall, by controlling the stoichiometry and the functionalization of the molecular building blocks, the structural habit of the self-assembled supramolecular networks can be fine-tuned: molecular stripe directions and interstripe spacings, nanopore dimensions, and orientations can be mastered at the subnanometer level, while at the same time preserving a very high degree of long-range order, both with respect to the lateral dimensions of supramolecular ad-islands and to the particularly low degree of their defectiveness. A key role in this respect is played by the thermodynamic stability of the BC phases relative to the SC references, which is rarely found in fullerene-containing twodimensional BC systemsstripes and pores phases can be seen as 2D analogues of 3D binary cocrystals, with an essential and potentially useful difference found in the stacking geometry, which is cofacial in three dimensions and becomes end-on when the system is confined to two dimensions.
formed by a C60 triplet at 0.96 nm and a couple at 1.00 nm, resulting in a newly oriented zigzag motif. As before, the variation in the resulting nanostructure can be interpreted as a consequence of the steric effect of the TPP amino groups on the self-assembly process. Figure 6 shows the details of a single pore geometry for the three different BC pores phases and outlines how the length of the two pore diagonals is affected by the presence of amino groups. In the case of TPP, the two diagonals are shorter than 3 nm; when TPP is substituted with the larger TAPP, the system responds with a rigid shift of adjacent zigzag lines, thus increasing the length of the two pore diagonals to values larger than 3 nm. Finally, in the case of transTPP(NH2)2 the system self-assembles with a new zigzag line orientation, optimizing the pore diagonal lengths so to perfectly fit the elongated shape of trans-TPP(NH2)2 with a pore having a shorter (3 nm) diagonal.
4. CONCLUSIONS The TPPs reported in this work self-assemble with C60 on Ag (110) forming two general classes of BC phases, depending on the overlayer molecular fraction: the so-called “stripes” and “pores” phases. The structural habit of each class can be further modulated by functionalizing the TPP meso phenyl rings by a variable number of amino groups in para positions. Using STM and LEED analysis, we have been able to fully characterize the geometry of a total amount of four SC and six BC phases. The self-assembly strategy of the system can be understood by relating the BC phases to the different TPP and C60 SC phases on Ag (110). It appears that the molecular arrangement in the 1591
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Figure 5. Experimental and simulated LEED patterns of different SC and BC porphyrins/C60 phases on Ag (110). Due to the high number of spots on the reciprocal space, the LEED of two pores phases is not clear, as the others LEED patterns and can be ambiguously interpreted: for these phases, commensurate unit cells have been confirmed by accurate comparison with adjacent different phases on the same STM images.
Figure 6. Details of the geometry of pores phases for the three different porphyrins used in this work.
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“HELIOS” and Progetti di Ricerca di Ateneo, CPDA118475/ 11) and Regione del Veneto (SMUPR no. 4148). T. Carofiglio and E. Lubian are gratefully acknowledged for providing the trans-TPP(NH2)2 molecules.
AUTHOR INFORMATION
Corresponding Author
*Fax: +39 049 827 5161; e-mail:
[email protected].
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Notes
The authors declare no competing financial interest.
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REFERENCES
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ACKNOWLEDGMENTS This work has been partially funded by MIUR (PRIN 2010/11, Project No. 2010BNZ3F2: “DESCARTES”) and by the University of Padova (Progetto Strategico STPD08RCX5 1592
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dx.doi.org/10.1021/jp409367x | J. Phys. Chem. C 2014, 118, 1587−1593
