Direct Visualization of Electronic Asymmetry within a Phenyl-Linked

Letter pubs.acs.org/JPCL

Direct Visualization of Electronic Asymmetry within a Phenyl-Linked Porphyrin Dimer Takashi Yokoyama* and Fumitaka Nishiyama Department of Nanoscience and Technology, Yokohama City University, 22-2 Seto, Yokohama 236-0027, Japan S Supporting Information *

ABSTRACT: Asymmetric electronic structures within a single molecule have been investigated by scanning tunneling microscopy and spectroscopy. Inside of a phenyl-linked porphyrin dimer, the asymmetric electronic structures are achieved by the incorporation of a cobalt ion in one porphyrin moiety. We find that a p-n junction between two π-conjugated segments is formed within the porphyrin dimer. The understanding of the local electronic structures of the intramolecular p-n junction should represent a fundamental step to realizing single molecular devices.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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(STS) at 4.8 K, revealing that the cobalt and free-base porphyrins work as donor and acceptor subunits, respectively. Figure 1b shows an STM image at 77.3 K of the Au(111) surface after the ∼0.7 monolayer deposition of the porphyrin dimers. The porphyrin dimers are ordered into 2-D islands through self-assembled aggregation on the Au(111) surface, whereas they appear to be mobile near the edges of the islands even at 77.3 K. The high-resolution STM image of a single porphyrin dimer shown in Figure 1c exhibits six paired lobes surrounding two paired oval protrusions. Similar to tBPsubstituted porphyrins, each lobe can be assigned as one of the tertiary butyl groups in the tBP substituent, and the paired oval protrusion can be assigned as the porphyrin ring.7−9 The appearance of the paired lobes (two tertiary butyl groups) suggests that the molecular plane of the phenyl rings is oriented to be closely aligned with the porphyrin mean plane, whereas ∼90° rotations of the phenyl rings are expected as an ideal conformation. The rotation directions of the phenyl rings are indicated by round arrows in the calculated conformation of the porphyrin dimer in Figure 1d, which have been identified from the height difference within a paired lobe, as indicated by large and small circles in the STM image. In addition, the dihedral angle between porphyrin and phenyl rings is estimated to be ∼20°, in good agreement with that of the tBP-substituted porphyrins on Au(111).7−9 The rotations of the phenyl rings are known to induce the saddle-shaped nonplanar deformation of the porphyrin macrocycle, which is characterized by alternately tilting of pyrrole rings above and below the porphyrin

ingle-molecule devices with advanced functions have been attracting much attention because the size of integrated circuit components will soon approach the scale of molecules.1 Aviram and Ratner proposed that molecular rectifiers would be realized by a donor-spacer-acceptor (DsA) molecular diode in 1974.2 The DsA molecular diode incorporates donor and acceptor π-conjugated subunits separated by an insulating spacer, which resembles a p-n junction of the semiconductor device. Although the observed rectification characteristics in early charge-transport experiments were associated with the complicated molecule−electrode contacts, the reliable diode behavior within single molecules has been measured recently by break junction experiments.3,4 In addition to the charge-transport properties,5 photoinduced electron transfer has been widely investigated for porphyrin linear arrays,6 which is associated with the photosynthetic systems. The asymmetric electronic structures within the porphyrin arrays have been introduced by substitution of different metals into the porphyrin macrocycles or of different subunits to control these phenomena. Scanning tunneling microscopy (STM) enables us to directly obtain the electronic structures of adsorbed molecules as well as the topographic information. Here we report on the energy and spatial resolved electronic structures of a phenyl-linked porphyrin dimer on Au(111). Figure 1a shows the chemical structure of the porphyrin dimer, in which the phenyl-linked porphyrin dimer is substituted with six ditertiarybutylphenyl (tBP) groups. The electronic asymmetry within the porphyrin dimer is achieved by the incorporation of a cobalt ion in one porphyrin macrocycle, where the two porphyrins are electronically separated by an insulating phenyl ring. The detailed electronic structures have been characterized by spatial- and energy-resolved scanning tunneling spectroscopy © 2014 American Chemical Society

Received: March 12, 2014 Accepted: March 26, 2014 Published: March 26, 2014 1324

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Figure 2. STM images (18 × 14 nm2) at 77.3 K of the self-assembled porphyrin dimers in (a) filled state (Vs = −1.0 V) and (b) empty state (Vs = +1.3 V). Several molecules are outlined in white for eye guidance. (c,d) Enlarged STM images of a single porphyrin dimer in panels a and b, respectively.

within the porphyrin dimer, whereas the contrast variations are not observed in the tBP substituents (the surrounding lobes). This should arise from distinct electronic structures between free-base and cobalt porphyrins. In addition, the contrast asymmetry is enhanced with decreasing bias voltages. Figure 2a,b shows filled- and empty-state STM images of the self-assembled porphyrin dimers at 77.3 K, respectively, in which several single molecules are outlined in white for eye guidance. In these STM images, it is evident that one of the two porphyrins appears brighter while another appears darker, and the image contrast of the tBP groups is almost independent of the bias voltages. Figure 2c,d shows the STM images in filled states at Vs = −1.0 V and in empty states at Vs = +1.3 V of a single porphyrin dimer, respectively, revealing that the image contrast of the porphyrin dimer is reversed by changing bias polarity. This result indicates that the highest occupied molecular orbital (HOMO) should be localized in the left-side porphyrin subunit, and the lowest unoccupied molecular orbital (LUMO) should be in the rightside porphyrin subunit, ensuring the formation of the intramolecule p-n junction. To clarify the bias dependence of the STM images, the local electronic structures of the porphyrin dimers were measured by STS at 4.8 K. In Figure 3a, we represent dI/dV tunneling spectra acquired at three specific locations within a porphyrin dimer, in which the green, blue, and red dots shown in the inset of Figure 3a represent the measurement locations of the corresponding colored spectra. In the red-colored spectrum obtained near the center of the right-side porphyrin, two peaks appear at −1.3 and +1.2 V, which should be originated from the π and π* conjugated states of the right-side porphyrin macrocycle. The green- and blue-colored spectra obtained at the leftside porphyrin exhibit the spectral peaks at −0.9 and +1.6 V, both of which are shifted upward in energy by 0.4 V compared with those in the right-side porphyrin. In addition, new peaks also appear at −0.2 V and close to 0 V in the green- and

Figure 1. (a) Structural formula of a porphyrin dimer, which is composed of a cobalt and free-base porphyrin dimer linked via a phenyl ring. The dimer is protected by six bulky tBP substituents. (b) STM image (40 × 40 nm2) at 77.3 K of self-assembled porphyrin dimers on the Au(111) surface (Vs = −3.0 V). The arrow indicates the [0−11] direction of the Au(111) surface. (c) High-resolution STM image (2.15 × 3.35 nm2) at 77.3 K of a porphyrin dimer (Vs = −3.0 V). Large and small circles indicate upper and lower butyl groups of the tBP substituents, respectively. (d) Corresponding conformation of the porphyrin dimer. Round arrows reveal rotation directions of the phenyl rings induced by surface adsorption, resulting in the saddleshaped deformation of the porphyrin macrocycles.

man plane.7 Consequently, the paired oval protrusions in Figure 1c should correspond to the upward-tilted pyrrole rings of the porphyrin macrocycles, whereas downward-tilted pyrrole rings appear as depression. Within the porphyrin dimer, the appearance of a V-shaped depression line indicates that the saddle-shaped porphyrins are linked with opposite orientations.9 In Figure 1c, the paired oval protrusions in the left-side porphyrin appear slightly brighter than those in the right side 1325

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Figure 4. (a) dI/dV tunneling spectra obtained at a center of the cobalt porphyrin at 4.8 and 77.3 K. Solid line denotes a Fano fit to the data. (b) STM topography at +2.0 V and dI/dV map at −0.01 V of the self-assembled cobalt and free-base porphyrin dimers. In the dI/dV map, bright dots represent the position of the cobalt ions.

macrocycles in the filled and empty-state STM images (see Figure 2c,d) should be associated with the distinct density distributions between the π and π* conjugated states. Compared with the π and π* conjugated states, the d-related states at −0.2 and close to 0 V were observed only within the cobalt porphyrin (left-side subunit). In Figure 3b, because the dI/dV map at −0.2 V appears to be similar to that in the π states at −0.9 V, the molecular orbitals at this state may result from the d-π interactions. In contrast, it is evident that the distributions at −0.02 V are restricted at the center of the cobalt ion. Such a strong resonant peak near the Fermi level (at 0 V) has been observed at magnetic atoms14,15 and molecules16−19 on metallic surfaces, which arises from many-body spin interactions,20 called the Kondo effect. Thus, the pronounced spectral peak obtained close to 0 V could be attributed to the Kondo effect, which indicates the existence of unpaired spins at the cobalt ion. The Kondo resonance peaks are known to exhibit a Fano line shape, characterized as the Kondo temperature (Tk), asymmetric parameter (q), and the energy shift (α) from the Fermi energy.15 Figure 4a shows the dI/dV spectra near the Fermi level obtained at the center of the cobalt porphyrin subunit, in which the peak shapes at 4.8 and 77.3 K have been well-reproduced by a Fano formula with Tk = 124.9 K, q = −12.6, and α = −12.1 mV with thermal broadening. Furthermore, as shown in Figure 4b, the dI/dV map at −0.01 V of the Kondo feature reveals the positions of the cobalt ions,

Figure 3. (a) dI/dV tunneling spectra obtained on a cobalt and freebase porphyrin dimer at 4.8 K. The colors indicate the position where the spectra were taken at corresponding colored dots shown in inset. In the spectra, pronounced speaks are obtained at −1.3, −0.9, −0.2, −0.02, +1.2, and +1.6 V, labeled as i−vi, respectively. (b) dI/dV maps at the spectral peak energies of i−vi in panel a. A model of the phenyllinked porphyrin dimer without tBP groups is superimposed in each map. During the dI/dV measurements, the feedback loop was stabilized at Vs = +2.0 V and It = 50 pA.

blue-colored spectra, respectively. They should be associated with the d-related states of the cobalt ion,10−12 and thus the left-side subunit should be identified as a cobalt porphyrin. The spatial characteristics of the molecular orbitals were probed by dI/dV mapping, reflecting the density distributions of electronic states. Figure 3b shows the dI/dV maps at different peak energies denoted as i−vi in Figure 3a. At −1.3 and −0.9 V, the density of the π conjugated states appears to be localized at upward-tilted pyrrole rings of the right- and left-side porphyrin macrocycles, respectively, although the relative intensity of the right-side porphyrin is weak at −1.3 V.13 Similar features were observed in the π* conjugated states at +1.2 and +1.6 V, but the enlarged protrusions, compared with those in the π states, should arise from extended conjugation in the unoccupied states. The slightly different appearance of the porphyrin 1326

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by grants-in-aid from the Japanese Society for the Promotion of Science.

which makes it easy to identify the cobalt or free-base porphyrins. In summary, the electronic asymmetry within a porphyrin dimer has been accomplished by the phenyl-linked cobalt and free-base porphyrin dimer. From our results, the cobalt porphyrin is characterized as a donor π-conjugated subunit, and the free-base porphyrin is characterized as an acceptor π-conjugated subunit. These subunits are electronically separated by an insulating phenyl ring, allowing the energy shift of the π and π* conjugated states by ∼0.4 V between the cobalt and free-base porphyrins. Thus, the diode properties could be expected if the charge transport through the porphyrin dimer is measured. In addition to a cobalt ion, the porphyrin macrocycles can incorporate many kinds of metal ions at the center, which should determine the energy levels in the π- and π*-conjugated states of the porphyrin subunit. To understand the influence of the central metal ions, we have also investigated the electronic structures of the phenyl-linked Cu and free-base porphyrin dimer on Au(111). In this case, whereas the energy level of the π*-conjugated states of the Cu porphyrin subunit was higher than that of the free-base porphyrin subunit by ∼0.6 V, the πconjugated states were almost identical in energy. (See the Supporting Information.). This indicates that the energy shifts between porphyrin subunits could be controlled by the rational selection of the incorporated metal ions.

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(1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Electronics Using Hybrid-Molecular and Mono-Molecular Devices. Nature 2000, 408, 541−548. (2) Aviram, A.; Ratner, M. R. Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277−283. (3) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; Hanisch, C. v.; Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. A Single-Molecule Diode. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8815−8820. (4) Diez-Perez, I.; Hihath, J.; Lee, Y.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. Rectification and Stability of a Single Molecular Diode with Controlled Orientation. Nat. Chem. 2009, 1, 635−641. (5) Sedghi, G.; Garcia-Suarez, V. M.; Esdaile, L. J.; Anderson, H. L.; Lambert, C. J.; Martin, S.; Bethell, D.; Higgins, S. J.; Elliott, M.; Bennett, N.; Macdonald, J. E.; Nichols, R. J. Long-Range Electron Tunneling in Oligo-porphyrin Molecular Wires. Nat. Nanotechnol. 2011, 6, 517−523. (6) Wasielewski, M. R. Photoinduced Electron Transfer in Supramolecular Systems for Artificial Photosynthesis. Chem. Rev. 1992, 92, 435−461. (7) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Mashiko, S. Nonplanar Adsorption and Orientational Ordering of Porphyrin Molecules on Au(111). J. Chem. Phys. 2001, 115, 3814−3818. (8) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Selective Assembly on a Surface of Supramolecular Aggregates with Controlled Size and Shape. Nature 2001, 413, 619−621. (9) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Mashiko, S. Conformation Selective Assembly of Carboxyphenyl Porphyrin Molecules on Au(111). J. Chem. Phys. 2004, 121, 11993−11997. (10) Scudiero, L.; Barlow, D. E.; Hipps, K. W. Physical Properties and Metal Ion Specific Scanning Tunneling Microscopy Images of Metal(II) Tetraphenylporphyrins Deposited from Vapor onto Gold(111). J. Phys. Chem. 2000, 104, 11899−11905. (11) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. Scanning Tunneling Microscopy, Orbital-mediated Tunneling Spectroscopy, and Ultraviolet Photoelectron Spectroscopy of Metal(II) Tetraphenyl Porphyrins Deposited from Vapor. J. Am. Chem. Soc. 2001, 123, 4073− 4080. (12) Bai, Y.; Sekita, M.; Schmid, M.; Bischof, T.; Steinruck, H.-P.; Gottfried, J. M. Interfacial Coordination Interactions Studied on Cobalt Octaethylporphyrin and Cobalt Tetraphenylporphyrin Monolayers on Au(111). Phys. Chem. Chem. Phys. 2010, 12, 4336−4344. (13) The weaker dI/dV intensity of the π-conjugated states at the right-side porphyrin than at the left side should arise from the height variations of the STM tip. The feedback loop was stabilized at Vs = +2.0 V to measure the dI/dV spectrum at each point. Under this condition, the right-side porphyrin appears brighter than the left side in the STM image (see Figure 2b), so that the larger tip-molecule distance at the right-side porphyrin should reduce the dI/dV spectral intensity. (14) Li, J.; Schneider, W.-D.; Berndt, R.; Delley, B. Kondo Scattering Observed at a Single Magnetic Impurity. Phys. Rev. Lett. 1998, 80, 2893−2896. (15) Madhavan, V.; Chen, W.; Jamneala, T.; Crommie, M. F.; Wingreen, N. S. Tunneling into a Single Magnetic Atom. Science 1998, 280, 567−569. (16) Zhao, A.; Qunxiang, L.; Chen, L.; Xiang, H.; Wang, W. W.; Bing, S. P.; Xiao, X.; Yang, J.; Hou, J. G.; Zhu, Q. Controlling the Kondo Effect of an Adsorbed Magnetic Ion Through Its Chemical Bonding. Science 2005, 309, 1542−1544. (17) Inacu, V.; Deshpande, A.; Hla, S.-W. Manipulation of the Kondo Effect via Two-Dimensional Molecular Assembly. Phys. Rev. Lett. 2006, 97, 266603. (18) Kim, H.; Son, W.-J.; Jang, W. J.; Yoon, J. K.; Han, S.; Kahng, S.J. Mapping the Electronic Structures of a Metalloporphyrin Molecule

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EXPERIMENTAL METHODS The experiments were performed in an ultrahigh vacuum chamber with a low-temperature scanning tunneling microscope. The Au(111) surface was prepared by Ar+ sputtering and annealing cycles. The porphyrin dimers were sublimated from a carefully degassed quartz crucible at 670 K. The substrate was held at room temperature during molecular deposition and was subsequently transformed to the cooled STM stage without thermal annealing. Electrochemically etched tungsten tips were used for the STM probe, which were prepared by Ar+ sputtering and electron bombardment heating cycles. All STM images were obtained in a constant current mode at 77.3 and 4.8 K. STS was performed from measurements of the differential conductance (dI/dV) as a function of a sample bias voltage (Vs) under open feedback loop with the lock-in detection (622 Hz, 10−30 mVrms sinusoidal modulation added to the bias).

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ASSOCIATED CONTENT

* Supporting Information S

STM/STS results of Cu and free-base porphyrin dimer on Au(111) at 77.3 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81 (0)45 787 2160. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Toshiya Kamikado, Prof. Shiyoshi Yokoyama, and Dr. Shinro Mashiko for synthesizing and providing the molecules. This work was financially supported 1327

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on Au(111) by Scanning Tunneling Microscopy and Spectroscopy. Phys. Rev. B 2009, 80, 245402. (19) Zoldan, V. C.; Faccio, R.; Gao, C.; Pasa, A. A. Coupling of Cobalt-Tetraphenylporphyrin Molecules to a Copper Nitride Layer. J. Phys. Chem. C 2013, 117, 15984−15990. (20) Kondo, J. Resistance Minimum in Dilute Magnetic Alloys. Prog. Theor. Phys. 1964, 32, 37−49.

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