Article pubs.acs.org/Langmuir
Conformational Change in Molecular Assembly of Nickel(II) Tetra(n‑propyl)porphycene Triggered by Potential Manipulation Soichiro Yoshimoto,*,† Teppei Kawamoto,‡,∥ Toru Okawara,§,⊥ Yoshio Hisaeda,§ and Masaaki Abe*,§,# †
Priority Organization for Innovation and Excellence, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan § Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 744, Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ‡
S Supporting Information *
ABSTRACT: Metal-coordinated porphyrin and related compounds are important for developing molecular architectures that mimic enzymes. Porphycene, a structural isomer of porphyrin, has shown unique properties in semiartificial myoglobin. To explore its potential as a molecular building block, we studied the molecular assembly of nickel(II) tetra(npropyl)porphycene (NiTPrPc), a metalloporphycene with introduced tetra n-propyl moieties, on the Au(111) electrode surface using in situ scanning tunneling microscopy. Because of the low molecular symmetry of NiTPrPc, the molecular assembly undergoes unique phase transitions due to conformational change of the n-propyl moieties. The phase transitions can be precisely controlled by the electrode potential, demonstrating that the latter can play an important role in the porphycene molecular assembly on Au surface. This new discovery indicates possible uses of this porphycene framework in molecular engineering.
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INTRODUCTION Porphyrin and related compounds are key components in important biological processes such as dioxygen transport, photosynthesis, and enzyme catalysis, where they constitute the active centers, that is, “hemes” in metalloproteins.1−3 They are also considered promising building blocks in designing molecular structures.4−7 Porphycene (Pc), a structural isomer of porphyrin, has attracted attention for creating new functional and molecular nanostructures.8−10 For example, when myoglobin (a wellknown O2 storage hemeprotein) was reconstituted with iron or cobalt porphycene, the O2 binding affinity was remarkably enhanced due to the lower symmetry (D2h) of the porphycene framework.11,12 Porphycene has also attracted interest in nanoscience due to the cis−trans tautomerization of the inner protons in its free base form (H2Pc) that could be induced by the scanning tunneling microscope (STM) tip.13−16 Recently, a 2D molecular assembly of free-base tetraphenylporphycene (H2TPPc) on Cu(111) in ultrahigh vacuum (UHV) was formed, showing highly ordered adlayers.17 Because of the low symmetry of the porphycene framework, molecular assemblies of porphycene compounds on surfaces offer us the ability to design and fabricate unique molecular architectures. The coordination of various metal ions to the porphycene framework could tune its properties, such as the intermolecular and molecule−substrate interactions. These metalloporphycene compounds are especially attractive in surface electrochemistry studies because their 2D molecular assemblies on a surface may enable the design of © 2016 American Chemical Society
new nanostructures for energy conversion by controlling the redox potentials of the coordinated metal ions.18 However, to the best of our knowledge, there has been no report about 2D assemblies of metalloporphycenes. Here we report for the first time a 2D molecular assembly of nickel(II) tetra(n-propyl)porphycene (NiTPrPc, Chart 1) on Chart 1. Chemical Structure and CPK Model of NiTPrPc
Au(111) under electrochemical conditions. The unique molecular arrangements in the NiTPrPc adlayer and their phase transitions were clearly revealed by electrochemical scanning tunneling microscopy (EC-STM).
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EXPERIMENTAL SECTION
NiTPrPc was synthesized as described in a previous paper.19 Au(111) single-crystal electrodes were prepared by Clavilier’s method.20 The Au Received: October 17, 2016 Revised: November 28, 2016 Published: November 29, 2016 13635
DOI: 10.1021/acs.langmuir.6b03782 Langmuir 2016, 32, 13635−13639
Article
Langmuir substrate was first annealed in a hydrogen flame and quickly cooled in ultrapure water saturated with H2 to avoid contamination. The cleaned Au(111) surface was dried in pure Ar gas and then immersed in a solution of NiTPrPc in benzene (∼100 μM) for 20−30 s. The NiTPrPcmodified Au(111) electrode was dried and transferred to either an electrochemical cell or an electrochemical STM cell. The electrochemical measurements were carried out in 0.1 M HClO4 under Ar at room temperature using an ALS/HCH model 704B electrochemical analyzer (ALS). For cyclic voltammetry (CV), a Pt plate was used as the counter electrode, and a reversible hydrogen electrode (RHE) was used as the reference electrode. EC-STM measurements were performed in 0.1 M HClO4 using a Nanoscope V system (Veeco Instruments). The STM images were obtained in the constant-current mode with a high-resolution scanner (HD-0.5I). The tungsten tips were etched in 1 M KOH and coated with nail polish to minimize residual faradaic currents. Two Pt wires were used as the quasi-reference and counter electrodes. All potential values are reported with respect to RHE.
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RESULTS AND DISCUSSION Figure 1a shows the enlarged typical CV profiles (20 times the current sensitivity in Figure 1b) of the NiTPrPc-modified Figure 2. (a) Large-scale (75 nm × 75 nm) and (b) high-resolution (20 nm × 20 nm) STM images of NiTPrPc adlayer on Au(111) in 0.1 M HClO4, observed at 0.70 and 0.40 V versus RHE, respectively. The tip potential and tunneling current were 0.40 V versus RHE and 1.5 nA for (a) and 0.25 V versus RHE and 2.0 nA for (b), respectively. (c) Close-up view and (d) the proposed model of squarely arranged NiTPrPc adlayer on Au(111) observed at 0.70 V versus RHE. The green and purple (or pink) shading of the NiTPrPc molecules indicate different molecular configurations of the n-propyl moieties.
In this image, the Au terrace was completely covered with highly ordered NiTPrPc arrays. Two different domains were found in the adlayer, with their boundary indicated by the dashed curve. Interestingly, each ordered domain displayed bright-colored rows, which were due to long-range modulations. These rows, separated from each other by a spacing of 6.8−9.2 nm on the terrace over the large scan area, indicate a change of the underlying Au atoms from 1 × 1 to √3 × 22 lattice configurations. This type of surface change in the topmost layer is called reconstruction.22 These characteristic rows found throughout the NiTPrPc molecular adlayer are formed due to the strong ability of NiTPrPc to donate π-electron to the Au(111) surface. A similar observation has been reported in the highly ordered Pt(II) octaethyl porphine (PtOEP) adlayer on Au(111).23 Figure 2b shows a high-resolution STM image obtained using a different tunneling current to elucidate the structural details of the NiTPrPc arrays. At the tunneling current of 2.0 nA, individual NiTPrPc molecules could be identified in alternating bright and dark rows, which are indicated by the red and blue arrows in Figure 2b. Further details of the internal structure, molecular orientation, and packing arrangement of the NiTPrPc adlayer are seen in a close-up view. Figure 2c clearly shows uniform molecular rows formed on Au(111), with the molecules arranged in a rectangular framework facing the same direction. The NiTPrPc molecule marked by the red arrow in Figure 2c appeared in a four-blade propeller shape, with a central dark spot and four bright spots at the corners. In contrast, the molecule indicated by the green arrow was observed as a bright ring with a dark spot at the center. The dark spot at the center of individual NiTPrPc molecules can be explained in terms of the electronic configuration of the dz2 orbital of the central metal ion. According to Hipps et al., cobalt(II) tetraphenyl porphine
Figure 1. Cyclic voltammograms of NiTPrPc-modified Au(111) electrode in 0.1 M HClO4 recorded at a scan rate of 100 mV s−1. (a) Enlarged CV profile in the electrical double-layer region and (b) CV profile in the overall potential region of 0 to 1.7 V. The black lines indicate the scan results of a clean Au(111) electrode.
Au(111) electrode in 0.1 M HClO4. In the potential range of 1.0 to 0 V, a clear redox wave was observed at 0.83 V. The current was lower than that obtained with a clean Au(111) electrode, suggesting that NiTPrPc molecules completely covered the electrode surface. The amount of electric charge consumed by the reductive peak was very small; therefore, this redox wave was not due to the faradaic current (electron-transfer reaction). According to a previous report, neither Ni2+ nor the porphycene framework in NiTPrPc undergoes any redox reactions in this potential region.21 Hence the small redox couple observed at 0.83 V would be attributed to a phase transition of the NiTPrPc adlayer on the electrode surface. The reaction must have occurred quickly and reversibly during the potential scans because there was almost no peak separation. Indeed, the observed currents observed at 0.83 V was proportional to the scan rate, indicating an electrochemical reaction (a weak chargetransfer reaction) of the NiTPrPc adlayer. When the potential was higher than 1.0 V, the oxidative current gradually increased, as shown in Figure 1b. (Note that the current sensitivity is different from that shown in Figure 1a.) Therefore, the upper limit of the potential was set at 1.0 V to avoid any chemical oxidation of the NiTPrPc adlayer. Figure 2a shows a typical large-scale STM image of the NiTPrPc adlayer formed on Au(111) at 0.70 V in 0.1 M HClO4. 13636
DOI: 10.1021/acs.langmuir.6b03782 Langmuir 2016, 32, 13635−13639
Article
Langmuir (CoTPP) molecules can be easily identified by a strong tunneling current resulting from orbital-mediated tunneling through the half-filled dz2 orbital, which yields a bright spot at the center of each CoTPP molecule.24 In the case of Ni(II) octaethyl porphine (NiOEP), the central Ni atom has eight electrons in the d orbitals (d8), and the filled dz2 orbital causes the central Ni(II) to appear as a dark spot instead.25,26 Therefore, the dark central spots observed in this study are attributed to the d8 electronic configuration. On the basis of the cross-sectional profile, the spacing between NiTPrPc molecules along the direction of arrow I in Figure 2c was measured to be 1.01 ± 0.05 nm, whereas that along arrow II was 1.21 ± 0.07 nm. The included angle of the molecular unit cell was almost 90°. A careful inspection revealed that the second NiTPrPc molecule in the row (indicated by the green arrow) was rotated by ∼90°. The irregular rotation was found mainly in rows of molecules with the bright ring, not those with the four-blade propeller shape. A corresponding structural model is proposed and shown in Figure 2d, in which NiTPrPc molecules with two different microorientations are displayed with distinct colors. The two molecular shapes that resemble squares and four-pointed stars can be tentatively explained by different configurations of the npropyl moieties on the porphycene ring. Density functional theory (DFT) calculation revealed several possible configurations for the four n-propyl moieties on the porphycene ring of NiTPrPc, three of which are displayed in Figure 3. According to
Figure 4. Potential-dependent STM images (30 nm × 24 nm) of NiTPrPc adlayer on Au(111) in 0.1 M HClO4 observed at (a) 0.60 and (b) 0.80 V versus RHE. The tip potential and tunneling current were 0.30 V versus RHE and 3.0 nA, respectively. (c) STM image of the boundary between the squarely and hexagonally arranged domains observed at 0.70 V versus RHE. The tip potential and tunneling current were 0.40 V versus RHE and 10 nA, respectively. (d) Proposed structural model for the hexagonally arranged NiTPrPc molecules observed at 0.80 V. The green and purple shading of the NiTPrPc molecules indicate different molecular configurations of the n-propyl moieties.
domain boundary across the lower part in Figure 4a completely disappeared. Therefore, the redox couple in the voltammetric profile of Figure 1 is assigned to this phase transition of the NiTPrPc adlayer. The two phases with squarely and hexagonally arranged NiTPrPc molecules can coexist at an intermediate potential, as shown in Figure 4c. In this situation, each NiTPrPc molecule in the squarely arranged domain was clearly observed as a ring with a dark center, while the orientations of the n-propyl moieties in the hexagonally arranged domain were unclear due to the fuzzy image. The different appearances of the two phases are associated with the interaction between NiTPrPc and the Au substrate. Reconstructed rows of Au were not seen at all in the hexagonally arranged domain. This indicates that the phase transition from square to hexagonal molecular arrangement in the adlayer induced reconstruction in the underlying Au substrate into the 1 × 1 structure to remain commensurate with the adlayer. In the squarely arranged adlattice, the lattice parameters were estimated to be 2.08 ± 0.07 and 1.45 ± 0.05 nm, which correspond to seven times the Au lattice constant in the [11̅0] direction and three times that in the √3 direction, respectively. Therefore, the adlattice outlined by the white rectangle in Figure 4c is tentatively assigned to the c(7 × 3√3) rect structure. A structural model for the hexagonal packing arrangement is proposed and is shown in Figure 4d. Taking the intermolecular distances into consideration, the NiTPrPc molecules at potentials >0.80 V were arranged approximately side-by-side with two different orientations, although the resolution in the STM image was not enough to construct a precise molecular-level model. When the potential was lowered to
