Article pubs.acs.org/Langmuir
Controlling Porphyrin Nanoarchitectures at Solid Interfaces Jonathan P. Hill,*,†,‡ Yongshu Xie,§ Misaho Akada,† Yutaka Wakayama,† Lok Kumar Shrestha,† Qingmin Ji,† and Katsuhiko Ariga†,‡ †
WPI-Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ‡ JST-CREST, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan § Key Laboratory for Advanced Materials and Institute for Fine Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai, PR China
ABSTRACT: Two complementary examples of porphyrin nanoarchitectonics are presented. The fabrication of binary molecular monolayers using two different porphyrin molecules, tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin (1) and tetrakis(4pyridyl)porphyrin (2), by deposition in ultrahigh vacuum was demonstrated. Two unusual heteromolecular monolayer structures were observed, with one exhibiting good separation of 1 molecules within the monolayer. Also, a synthetic nanoarchitectonic approach was used to prepare self-assembled molecular nanowires at a mica substrate. The nanowires could be observed to grow using atomic force microscopy (AFM), and the network structures of the nanowires could be influenced by manipulation using the AFM probe tip.
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INTRODUCTION The assembly or self-assembly of small organic molecular species at an interface is an important subject for potential nanotechnological applications.1−5 This is because of the diverse properties of molecules and the ease with which they may be synthetically modified in order to determine or at least influence the nature of their properties and the form of their self-assemblies. Furthermore, nanoarchitectonic principles6 may be applied in order to nanostructure the resulting systems further by the careful selection of the substrate, which affects molecule−surface interactions, or in repetitive processes such as layer-by-layer assembly.7,8 In all of these cases, one of the critical considerations is the structure of any initially deposited layers because these can strongly influence the properties of the final structure such as in thin film organic transistor films where molecular orientation within the films is a determinant of their effectiveness.9 Also, highly structured monolayers composed of electroactive organic chromophores may be useful as highcapacity molecular memories in future devices10−12 or as arrays of single-molecule switches13,14 so that control of the monolayer structure has become crucial in research on molecular electronics and for single-molecule applications.15,16 Therefore, the ability to exercise control over the molecular © 2012 American Chemical Society
nanostructures at substrates has become a burgeoning area of investigation.17−20 Organic chromophores such as porphyrins and phthalocyanines have been intensively investigated from the point of view of their molecular conformations,21−23 self-assembly,24−26 and physical properties when adsorbed on various substrates.27,28 This is because of their inherent chemical stabilities and ease of deposition from solution or at low pressure. In addition, they possess convenient nanometric dimensions that enable rapid molecular resolution imaging while also being reversibly redoxactive with several available stable oxidation states, which may imply uses in memory or switching elements of molecular devices.29 We and others have previously investigated the selfassembly of redox-active porphyrins under ultrahigh vacuum conditions and by simple solution processing methods.30−35 There have also been several reports of interesting intermolecular chemical processes occurring that might lead Special Issue: Interfacial Nanoarchitectonics Received: November 14, 2012 Revised: December 30, 2012 Published: December 31, 2012 7291
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could be erased using the same procedure. The observation of the regrowth of the nanowires was made by returning to the initial imaging conditions. Electron Microscopy. Scanning electron microscopy images were obtained using a Hitachi model S3600 FE-SEM instrument (operating voltage 5 kV). Samples were cast on a passivated silicon substrate followed by coating with Pt (Hitachi E1030 ion sputterer). Scanning transmission electron microscopy was also conducted on a Hitachi S3600 FE-SEM instrument on samples cast on carbon-coated copper grids.
to the development of methods for forming highly ordered polymer nanostructures of predetermined or controllable form.36−38 Notable examples involving porphyrins include polymerization through meso positions or meso substituents reported by Grill and co-workers39 and intermolecular polymerization through the formation of C−Cu bonds reported by Raval and co-workers.40 In this work, we have used two methods to control porphyrin nanostructuring at surfaces. First, we examine the formation of molecular monolayers involving two species (a method that has become popular in recent years) where a phenol-substituted porphyrin, 5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin,41−43 was allowed to interact with 5,10,15,20tetrakis(4-pyridyl)porphyrin44 at a metal substrate and the resulting formation of binary monolayer structures was assessed using scanning tunneling microscopy in ultrahigh vacuum (UHV-STM). Second, we have applied a dual synthesis/surface abrasive approach to control the structure of self-assembled porphyrin wires on a mica surface. This latter approach involved the synthesis of a trimeric porphyrin derivative45 bearing substituents known to interact with the mica substrate under various conditions, including high humidity.
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RESULTS AND DISCUSSION The chemical structures of the porphyrins used in this work are shown in Scheme 1. 5,10,15,20-Tetrakis(3,5-di-t-butyl-4Scheme 1. Chemical Structures of Compounds 1−3 Used in This Work
EXPERIMENTAL SECTION
General. Solvents and chemicals were obtained from Tokyo Kasei Chemical (TCI) Co. Ltd., Wako Chemical Co. Ltd., or Aldrich Chemical Co. Ltd. 5,10,15,20-Tetrakis(3,5-di-t-butyl-4hydroxyphenyl)porphyrin (1) and 5,10,15,20-tetrakis(4-pyridyl)porphyrin (2) were synthesized and purified according to literature methods.46,47 Synthesis of the porphyrin trimer (3) was carried out according to the methods of Tong and co-workers and has been reported previously.45,48 Scanning Tunneling Microscopy. Porphyrins were dried in vacuo before use in the STM experiments. Atomically clean single crystals of Au(111) used as substrates were prepared by standard Ar+ sputtering and annealing (700 K) cycles. Typically, submonolayer coverage of the substrate was obtained by sublimation for 10 min at 320 °C from a Knudsen cell to the metal substrate over an intervening distance of 30 cm in ultrahigh vacuum (1 × 10−8 Pa), followed by characterization using STM at room temperature or 150 K. Details of the deposition and measurement conditions are given in the respective figure captions. Atomic Force Microscopy. AFM measurements were carried out under ambient conditions. Samples were prepared by dropping a suspension of the preassembled molecules (100 μL, ∼10−4 M) in dichloromethane/methanol onto a freshly cleaved mica surface at room temperature (27 °C) and relative humidity RH = 62%. The relative humidity of the atmosphere was constantly monitored during AFM measurements. AFM images were obtained using a Seiko Instruments SPA400-SPI4000 equipped with a calibrated 20 μm xyscan-range and a 10 μm z-scan-range PZT scanner. All AFM images were taken in dynamic force mode at a scanning frequency of 0.6 Hz. Rectangularly shaped silicon cantilevers (tip radius ∼10 nm, SI-DF20, Seiko Instruments Inc.) with a spring constant of 11 N/m and a resonance frequency of 122 kHz were used for imaging in air. Supersharp tips (tip radius 2 to 3 nm, SI-DF20S, Seiko Instruments Inc.) were used for the images shown in Figure 4. Details of the deposition and measurement conditions are given in the respective figure captions. Nanowire Erasure. For nanowire erasure, a rectangularly shaped cantilever was used (SI-DF20). In dynamic force mode, an area for erasure was selected (either 50 or 100 nm) and the substrate was rotated to present the nanowire perpendicular to the scanning direction of the probe tip. Then the scan rate was increased to 4 Hz. The scan rate was increased until damage to the nanowire could be observed, followed by scanning at that rate until no structure could be observed (i.e., the nanowire was erased). Residual nanowire fragments
hydroxyphenyl)porphyrin (1) is a phenol-substituted porphyrin bearing antioxidant substituents, which has been studied by us previously.41−43 1 has been observed to assemble into a variety of structures when it is deposited on Cu(111) surfaces. The oxidation of 1 to a quinone form has been observed to result in the stabilization of its monolayers at metal surfaces as a result of the conformational adaptation of this molecule.30 1 tends to be distorted into a (more planar) calixpyrrole-like conformation when deposited on, for instance, Cu(111). Coincidentally, the distorted state is almost identical in structure to the twoelectron oxidized quinone state of 1, thus causing a significant stabilization of the molecules’ structures at surfaces. Figure 1 7292
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shows a monolayer of 1 on Au(111) illustrating the hexagonal close-packed structure that is a result of the molecules’ distortion to the more planar form. However, 2 has been studied by several research groups49−51 for the possibility to form extended arrays involving coordinative interactions between the peripheral 4-pyridyl groups of 2 and codeposited metal atoms/cations even in systems involving more than one organic ligand species deposited on the substrate.52,53 We were interested to observe if hydrogen bonding between phenols and 4-pyridyl groups could be used as a mediating interaction in the formation of regularly arrayed porphyrins. This might then allow the formation of heterometallic arrays of porphyrins. When deposited alone on Au(111) at ambient temperature, 2 leads to monolayer formation (Figure 2a) whose structure could be improved by annealing at 50 °C. Despite the formation of a more homogeneous monolayer after annealing, there still exist defects in the structure that appear as rows of vacancies as shown in Figure 2. In fact, the vacancies are due to the possibility of the variation in the conformation of 2 because the porphyrin molecule seems to adopt a saddle shape perhaps as a result of the interaction with surface metal atoms that is more likely to maximize the packing efficiency. For 2, there appear to be two possible conformations: one with a planar porphyrin and one with a saddle conformation. The structures crystallize in 2D domains as can be seen in Figure 2c. Saddle-
Figure 1. STM image of hexagonally packed 1 on Au(111). Spacefilling representations of molecules are also shown. 20 nm × 20 nm, It = 75 pA, Vs = +1.0 V.
Figure 2. STM imaging of tetrakis(4-pyridyl)porphyrin (2) on Au(111) deposited onto a substrate at room temperature (K-cell temperature 340 °C) and then annealed for 4 h at 50 °C. (a) Image size 180 nm × 180 nm, It = 100 pA, and Vs = +1.0 V. (b) 50 nm × 50 nm; stripes of vacancies are clearly visible in the lower half, It = 85 pA, Vs = +1.0 V. (c) Image containing two different domain structures, 25 nm × 25 nm, It = 80 pA, Vs = +1.0 V (panels d and e). (d) Model of the densely packed domain. (i) and (ii) and adjacent arrows indicate rows of molecules with similar conformations corresponding to those given in panel f; 10 nm × 10 nm, It = 80 pA, and Vs = +1.2 V. (e) Model of the structure about the vacancies; 5 nm × 5 nm, It = 70 pA, and Vs = +1.1 V. (i) and (ii) refer to conformations given in panel f. (f) Space-filling representations (ChemDraw 3D) of the molecular conformations involved in panels d and e. (i) 2 with planar tetrapyrrole and 4-pyridyl substituents subtending ∼60° between the macrocyle and substituent planes. (ii) Saddle conformation of the porphryin. 7293
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Figure 3. STM images of the structure obtained by the consecutive deposition of 2 followed by 1 on Au(111). Deposition temperature (K cell): 2 (335 °C) and 1 (320 °C). Deposition time: 2 (4 min) and 1 (8 min). (a) 70 nm × 70 nm, It = 85 pA, and Vs = +1.0 V. (b) 50 nm × 50 nm, It = 80 pA, and Vs = +1.0 V. (c) With space-filling models of 1 added; 40 nm × 40 nm. (d) STM/model of the structure with the arrangement of molecules indicated. Note the uniform alignment of the saddling of 1 demarked by the bright points in the STM profile. 15 nm × 15 nm, It = 90 pA, and Vs = +1.1 V. (e) Mixed monolayer with two different conformations of 1 together with 2. Colored spots denote molecular conformations. 10 nm × 10 nm, It = 80 pA, and Vs = +1.1 V. (f) Model of the structure shown in panel e.
2 presumably because of the difference in size of 1 and 2. Figure 3c shows an STM image with 1 highlighted, revealing their separation within this 2D supramolecular structure. Figure 3d shows a high-resolution image of the structure with a model superimposed. Essentially, molecules of 1 and 2 occupy rows. Rows of 2 are densely packed whereas in the adjacent rows of 1 molecules appear to be not in contact so that regular spaces appear between the molecules. This is most likely due to the existence of hydrogen bonds between the rows. From the superimposed model, it appears that a molecule of 2 interacts (hydrogen bonds) with two molecules of 1 through 4-pyridyl groups at its 5 and 10 (sometimes known as cis) positions. The remaining 4-pyridyl groups at 15,20 positions do not interact with phenol groups although there exists the possibility of CH−π interactions with porphyrin β-pyrrole protons of adjacent molecules of 2. In this structure, both molecules of 1 and 2 may be forced to occupy a saddle conformation although this is not completely clear because of the lower resolution of these images. Regarding the saddle conformation of 1 in Figure 3d, it is unusual that the axis of saddling appears identical for all 1 molecules contained in the mixed structure
shaped molecules whose 4-fold rotational symmetry has been broken by the unsymmetrical rotation of its 4-pyridyl substituents can crystallize independently to form a gridlike lattice containing vacancies. Alternatively, the two conformations can cocrystallize in a mixed structure with alternating diagonal rows of saddle-shaped and planar molecules as shown in Figure 2d. A model of the structure where two adjacent rows of saddle conformation 2 result in rows of vacancies in the mixed structure is shown in Figure 2e. Space-filling representations of the two possible different conformations of 2 are shown in Figure 2f. When 1 and 2 are deposited consecutively on the same substrate, coassemblies of the two molecules can be obtained by varying the temperatures and deposition times of the two molecules. Under optimum conditions, a very well ordered structure can be obtained with molecules of 1 regularly positioned in a matrix composed of 2 as depicted in Figure 3. This regular structure can surprisingly be found over large areas of substrate of up to at least 100 nm2 as illustrated in Figure 3a,b. At larger observation length scales (Figure 3c), it is difficult to observe details in regions of the structure containing 7294
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Figure 4. Imaging of nanowires formed from compound 3. (a) SEM image of 3 on passivated silicon. Sample sputtered with ∼10 nm Pt. (b) AFM image (supersharp tip) of nanowire arrays of 3 on a mica substrate indicating the very regular separation of the nanowires and their homogeneous dimensions. (c) Profile of nanowire array. L = distance between respectively colored points; ΔZ = height difference between points. (d) AFM image (supersharp tip) of 3 nanowires on a mica substrate. Image size: 1 μm × 1 μm. (e) Energy-minimized (Chemdraw 3D, MM2) models of 3 assuming the amphiphilic segregation of substituents (i) in extended confirmation (molecular long dimension ≈ 5 nm) and (ii) in a conformation appropriate for maximizing the interaction with a hydrophilic surface (length perpendicular to substrate ≈ 3.1 nm). (f) Model of nanowire formation. Cartoon of the molecule with arrows indicating the rotational flexibility of the dendrons, which is necessary for the effective packing of the molecules.
(by observing the bright lines that are due to the protruding pyrrole β positions). In monolayers composed purely of 1, this feature is generally arranged at random. In the case of this mixed monolayer, it is likely that the unsymmetric interaction (4-fold symmetry is again broken in these rows of arrays) imposes structural constraints leading to a nonrandom arrangement of this feature. Another mixed structure of 1 with 2 could be observed and is shown in Figure 3e,f. This molecular monolayer is actually composed of three nonidentical species because of the involvement of a second conformation of compound 1, which we have reported previously.41 As can be seen in Figure 3e, molecules of 1 (red circle) have a similar conformation to that in Figure 3d. Molecules of 2 are also present (green circle). Blue circles denote an unsymmetrical conformation of 1 caused by the uneven rotation of its phenyl substituents. The structure is rationalized as the model shown in Figure 3f. Clearly, this is a very rare occurrence because it involves the presence of two different conformations of the same molecule (1) being present at the same time as a second chemical species (2). In this case also, 2 appears to occupy a saddle conformation. It is unclear what interactions govern the formation of this monolayer although the intermolecular proximity of the phenol groups suggests that, in this case, hydrogen bonding interactions might exist, especially because twisting of the phenyl groups in the blue conformation of 1 ought to favor the close approach of the
hydroxyl groups. Hydrogen bonding involving these sterically hindered phenols has been observed only when the phenyl groups are orthogonal, which relieves the steric effect of the bulky t-butyl groups.54 Apart from the ordered structure, another unusual feature can be found, in particular, where the ratio of 2 to 1 is large. Rows of molecules of 1 can form a separating layer or boundary between domains of 2. In that case, molecules of 1 appear not to mix with the monolayer and are segregated at the domain boundaries. The mixing of 1 and 2 into a heteromolecular monolayer is in contrast to the phase separation behavior observed by Buchler and co-workers,55 who observed that differing molecule−molecule and substrate−molecule interactions can lead to phase separation in binary codeposited porphyrin systems. In the mixed monolayer case described here, we believe that the heterointermolecular interactions (probably hydrogen bonding) promote the mixing of 1 with 2 whereas the underlying substrate has less of an effect on the final monolayer structure although other substrates may result in different structures. Porphyrins are some of the best suited molecules for deposition and study using UHV-STM because of their high chemical stabilities, variable redox states, and chromophoric natures. They also tend to form very well ordered 2D structures with potential in various applications. However, the use of highvacuum techniques is disadvantageous from the point of view 7295
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Figure 5. Time-resolved images of 3 nanowire growth. (a) Spontaneous formation of nanowires at several sites. White arrows indicate origins. All images are 5 μm. (b) Self-assembly in action. A small aggregate of 3 (indicated by white arrows; single molecules are not observable) is observed to join the end of a growing nanowire. All images are 500 nm. (c) Increasing complexity in 3 nanowire formation. t = (i) 0, (ii) 20, (iii) 30, (iv) 40, (v) 60, and (iv) 120 min. All images are 5 μm.
that large molecules are difficult to deposit (even if they are suitably stable), and it would be additionally desirable to be able to control the structure of supramolecular formation not only by physical means but also by utilizing molecular design principles. For these reasons, we designed and synthesized compound 3,45 which possesses three porphyrin units connected through a central benzene group in a 1,3,5substitution pattern.45,48 The large dimensions and high molecular weight of this unit make its deposition in UHVSTM inconvenient although its molecular structure suggests interesting self-assembly and physical properties. To study this molecule, we selected atomic force microscopy as the imaging technique to observe structures assembled at a surface by deposition from solution. To solubilize the porphyrin trimer, several substituents were introduced, including nine triethylene glycol monomethyl ether groups, which are known to interact strongly with hydrophilic substrates. These substituents thus make molecules of 3 trigeminal amphiphiles (by analogy with geminal amphiphiles56). Molecules of 3 self-assemble into highaspect-ratio nanowires when deposited on mica substrates. Similar structures could also be observed on passivated silicon but were not found if hydrophobic substrates such as metals or highly oriented pyrolytic graphite were used. Initial observations of 3 deposited on mica reveal that large aggregates, probably vesicles, are present in solution. These aggregates are disrupted once deposited, gradually forming nanowire structures (some disordered materials remains at the periphery of the sample). Notably, these nanowires could be observed to grow by sequential observation using AFM. Typical images of the nanowires and a representation of the probable molecular conformation of 3 deposited on hydrophilic substrates are shown in Figure 4. A model of the structure consistent with the dimensions of the nanowires on the molecular level is also shown. Figure 4a shows an SEM image of the nanowires as deposited at a passivated silicon surface.
Observation of the porphyrin nanowires using SEM was made inconvenient by the necessity of depositing a relatively thick >10 nm layer of platinum as a staining agent. Later, it became obvious that this is due to the small dimensions of the nanowires. AFM imaging (panels b−d) reveals the very homogeneous dimensions of the nanowires and their high aspect ratios. After the deconvolution of the dimensions from the AFM tip radius, we estimate that the nanowires are 3.2 nm in height with a breadth of around 8 to 9 nm. Where formed in parallel arrays, nanowires are separated by an ∼5 nm trough. The nanowire height is consistent with the flexibility of the molecule and the hydrophilicity of the oligoethylene glycol chains, which tend to interact strongly with the mica substrate. This results in the energy-minimized length of a molecule of 3 being reduced from 5 nm to around 3.4 nm as illustrated in Figure 4e. The interaction of the oligoethylene glycol chains with the mica substrate should also be enhanced under ambient conditions because of the high humidity in Japan at the time of the measurements because mica is known to be coated with up to a 1 nm adsorbed layer of water. Humidity affects the rate of nanowire growth (higher humidity leads to more rapid growth) because this in turn affects the hydration of the mica substrate. In this work, the humidity was monitored, revealing that low humidity (
