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J. Phys. Chem. C 2009, 113, 7416–7423
Theoretical Investigation on the Electron Transport Path through the Porphyrin Molecules and Chemisorption of CO Nan Wang,†,‡ Hongmei Liu,‡ Jianwei Zhao,*,‡ Yanping Cui,§ Zhong Xu,§ Yuanfeng Ye,‡ Manabu Kiguchi,| and Kei Murakoshi| Key Lab of Analytical Chemistry for Life Science, MOE, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210008, People’s Republic of China, Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001 People’s Republic of China, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo, Japan ReceiVed: January 13, 2009; ReVised Manuscript ReceiVed: February 28, 2009
In this report, we studied the electron transport through cyclic π-conjugated molecules. The model system consists of metalloporphyrin with two thiol groups at either 9,11-substitution (P-connection) or 1,5-substitution (D-connection) which form chemical bonds with gold electrodes. We investigated 10 typical bivalent metals as the metal-molecule-metal junctions using first principle density functional theory and nonequilibrium Green’s function calculations. Due to the particular electron transport paths, all models in P-connection show similar I-V curves, indicating that the electron does not pass through the metal center in this configuration. In the D-connection, the electron takes the path through the metal center, leading to considerable difference in the I-V curves between the different metalloporphyrins. This means that the D-connected metalloporphyrin is potentially applicable in chemical sensor. We also studied a prototype for chemosensing the CO molecule theoretically at the same level. 1. Introduction Aviram and Ratner proposed that a side-substituted organic molecule could function as a molecular diode.1 Since then, the idea of using organic molecules as the functional units in electronic devices has received great attention. Recently, there are several recent reports on molecular wires,2-4 diodes,5-7 resonant tunneling diodes,8 and memory9 based on the linear conjugated molecules. Cyclic conjugated molecules are more promising than linear molecular materials because of the macro-π overlap.10,11 The electronic properties of the conjugated molecules can be tuned by changing the size of the rings or incorporating functional coordination groups (these groups may be ions or molecular clusters with size ranging from several angstroms to nanometers).12 This kind of molecule can potentially form new types of molecular devices.13 There is a particular interest in the change of conductance as the cyclic molecule binds or releases an ion.12 Such a chemical system has potential application as a molecular transistor,14 switch,15 or sensor.16 The rigid frame of the conjugated molecules can construct monomolecular circuits, such as a Wheatstone bridge or other complex logic circuits.17 Protoporphyrin (PH2) and its derivatives are the most promising materials for future applications in nanoelectronics due to their rigid geometric configuration, highly conjugated structure, and chemical stability.18 Most importantly, they can coordinate with simple metal ions which affect electron transport through the porphyrin molecule. In the past decade, there are extensive theoretical investigations of the geometric and elec* Corresponding author: phone and fax, +86-25-83596523; e-mail,
[email protected]. † Current address: Nanoscience, University of Cambridge, 11. J J Thomson Ave, Cambridge CB3 0FF, U.K. ‡ School of Chemistry and Chemical Engineering, Nanjing University. § Department of Applied Chemistry, Harbin Institute of Technology. | Division of Chemistry, Graduate School of Science, Hokkaido University.
SCHEME 1: Theoretical Simulation of the Transportation Behavior of Porphyrins in a Molecular Configuration with 9,11-Substitution (P-Connection)
tronic structures of porphyrin.19-22 However, there are few reports on the effect at metal coordination on the transport behavior so far. When the protoporphyrin is coordinated by a metal ion, electron transport via the ion is possible. This makes the electron transport more complicated for the conjugated metalloporphyrin than protoporphyrins. Specifying the electron transport path is of great importance in the design of the electronic devices and molecular sensors. In this paper, we focus on the first principles theoretical investigation of the transport behavior of protoporphyrin and 10 typical bivalent metalloporphyrins (MgP, MnP, FeP, CoP, NiP, CuP, ZnP, RuP, PdP, and PtP). We discuss the effect of the contact points on the porphyrin ring and model two different connections by functionalizing the porphyrin ring with thiol groups on either 9,11-substitution (P-connection, Scheme 1) or 1,5-substitution (D-connection, Scheme 2) to form the chemical Au-S bond. A significant difference in the transport behavior between the two connections indicates the importance of the metallic center in determining the electronic properties. This work may pave the way for the future design of transport related molecular devices, such as transistors and sensors.16
10.1021/jp900335p CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
Electron Transport through Porphyrin SCHEME 2: Theoretical Simulation of the Transportation Behavior of Porphyrins in a Molecular Configuration with 1,5-Substitution (D-Connection)
2. Computational Method The calculations followed two main processes. First we performed primary geometric optimization for the metalloporphyrin using PC GAMESS23 and Gaussian 0324 software. Two small gold clusters, each consisting of three atoms, were used as electrodes. These gold atoms form an equilateral triangle with sides of 2.88 Å. This arrangement was used in the simulation describing the attachment of the metal leads to the two gold clusters. Each sulfur atom connected to the gold cluster is confined at the center of the triangle, i.e., the hollow site. The relative positions of the gold atoms are frozen in each triangle, but the distance between the two clusters is relaxed during the geometric optimization. For the closed-shell systems, the optimization was carried out at the B3LYP level of theory with 6-31G* for C, N, and H, and LanL2DZ for Au. For open-shell systems, we employed UB3LYP in the calculation. The spin multiplicity was specified according to the lowest energy of isolated metalloporphyrins, which is 2 for MgP, 4 MnP, 3 FeP, 2 CoP, 1 NiP, 2 CuP, 1 ZnP, 3 RuP, 1 PdP, 1 PtP, respectively (Supporting Information). The HOMO is singly occupied MO (SOMO) for the open-shell systems. Further geometric optimization and transport calculations were carried out with first-principles computer code ATK,25-27 which is based on the Keldysh nonequilibrium Green’s functions (NEGF) technique,28 combined with density functional theory (DFT) with the local density approximation (LDA) and the Perdew-Zunger parametrization29 of the correlation energy of a homogeneous electron gas, calculated by Ceperley-Alder.30 The former optimized model molecules, except the two gold clusters (-S-M-S-) were translated into the gold junction with (111) surfaces, in which the distance between the sulfur atom and the gold plane was confined to 2.0 Å, within the range 1.90-2.39 Å used by most authors.4,31-34 Double-ζ with polarization (DZP) basis set was chosen for all atoms except gold, for which single-ζ with polarization (SZP) was used. The full optimization of the molecular structure was performed with a convergence criterion of 0.05 eV/Å. Due to the symmetry of the junction, the bias was scanned only in one direction from 0.0 to 2.0 V. 3. Results and Discussion 3.1. Geometric Features of the Metalloporphyrins. The molecular geometries of the free protoporphyrin and metalloporphyrins have been well-characterized.21,35-38 Most of the isolated metalloporphyrins exhibit a perfect planar D4h symmetry (see Supporting Information), except for CoP and RuP, in which the symmetry of molecules is reduced to D2h because of the Jahn-Teller effect. When they were confined to the metallic junction, the molecules appeared to undergo certain ruffling distortions from the metal electrodes. The optimization shows that the molecular symmetry of models in the P-connection was reduced to D2h and the molecule was elongated along the electric
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7417 field direction, exhibiting a rectangular shape. Table 1 gives some representative geometric parameters of the model molecules. The variation of R reflects the bonding interaction between the metalloporphyrin ring and the central metal cation. The metal cations in the first-row (Mn(II), Fe(II), Co(II), Ni(II), and Cu(II)) have a smaller radius and stronger coordination to the metalloporphyrin ring, which gives a smaller R value as the others have either a weak cordination (Mg(II)) or a large radius (Ru(II), Pd(II), and Pt(II)). Identical variation is also observed in the metalloporphyrin vacant hollow size as characterized by DA-B and DN-N. The calculated M-N bond lengths of the first-row metalloporphyrins are between 1.95 and 2.00 Å, notably shorter than in RuP, PdP, PtP, and MgP which are around 2.05 Å. This is in good agreement to X-ray diffraction data which showed M-N lengths of FeP (1.972 Å),39 CoP (1.949 Å),40 NiP (1.957 Å),41 CuP (1.981 Å),41 and ZnP (2.042 Å).41 Scheiner et al.38 also found similar M-N bond lengths for metalloporphyrins by the DFT method with VWNBP functional, 1.975, 1.980, 1.969, 2.029, and 2.062 Å for FeP, CoP, NiP, CuP, and ZnP, respectively, though no metal electrodes were included in their calculations. The trend in Table 1 can also be understood by considering the coordination that forms as a result of complexing of the nitrogen lone-pair electrons to the metal d-states and the steric hindrance between the metalloporphyrin ring and the metallic cation. In the case of the D-connection, the symmetry of the molecular models reduces to C2h, due to the bending of the Au-S bond. Table 2 shows representative geometric parameters optimized by the ATK program for all models. Comparing Tables 1 and 2, we find a similar variation of R and DN-N for both types of connections. Without metal ion substitution (PH2), or when the complexing ion has a weak interaction (MgP), both R and DN-N have larger values. For the metal ions with larger radius (RuP, PdP, and PtP), the parameters also increase. In contrast, first row transition metals cations (Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II)) have a small radius and strong coordination to the metalloporphyrin ring and, consequently, show smaller R and DN-N values. 3.2. Two MPSH Molecular Orbitals of the Porphyrins. Another feature of the junction is the molecular projected selfconsistent Hamiltonian42,43 (MPSH), which is the self-consistent Hamiltonian of the isolated molecule in the presence of an Au electrode, namely, the molecular part extracted from the whole self-consistent Hamiltonian for the scattering region. We analyzed the eigenvalues for all molecular models in the P-connection as shown in Figure 1. Moving across the periodic table from Mn to Cu, the energies of the metal d-orbitals tend to drop. This pattern is most evident in the dx2-y2 orbital. This trend is in good agreement with the variation of the HOMO. From the features in Figure 1, the metalloporphyrins can be broadly classified into four groups. In the first group, MgP, ZnP, NiP, PdP, and PtP, all the orbitals are separated from each other and the HOMO is much closer to the Fermi level of the system. FeP, CoP, and RuP belong to the second group, because their HOMO-1 and HOMO are degenerate and close to the Fermi level of the system. The third group contains only MnP, in which all four orbitals are very close to each other and to the Fermi level. From the spatial distribution of frontier orbitals of MnP in Figures 4 and 6 in the following, we find that the metal center greatly contributes to the four orbitals, especially the d orbital of Mn, that is dxy, dz2, dxz, dyz, and that the energy of the four orbitals is similar. Therefore, the four orbitals are almost degenerate. However, for the other molecules, the effect of the conjugated ring is significant. Therefore, the orbitals of these
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TABLE 1: Some Selected Geometric Parameters of the Molecular Models in a P-Connection parameter
PH2
MgP
MnP
FeP
CoP
NiP
CuP
ZnP
RuP
PdP
PtP
SDa
∠R/ deg ∠β/ Deg. ∠γ/ deg ∠δ/ deg ∠η/ deg ∠θ/ deg ∠λ/ deg DA-B/Å DN-N/Å
121.41 124.92 112.26 105.18 106.72 110.89 104.95 7.323 4.253
124.12 125.33 100.52 106.18 106.99 100.07 107.19 7.035 4.113
120.84 123.45 111.84 106.14 106.92 111.70 103.39 6.993 3.921
120.30 123.12 112.01 106.09 106.73 110.34 103.21 6.987 3.902
120.00 122.93 112.09 106.06 106.55 112.05 103.25 6.980 3.895
120.02 122.82 111.95 106.13 106.65 111.95 103.33 6.961 3.891
121.82 124.04 110.23 106.13 106.92 110.38 105.46 7.011 4.019
124.03 125.18 110.21 106.38 106.90 110.05 106.47 7.021 4.110
123.79 125.37 110.16 106.55 107.32 109.90 106.08 7.029 4.083
123.80 125.30 109.53 106.60 107.16 109.29 107.42 7.033 4.130
122.42 124.42 110.47 106.47 107.04 110.35 105.68 7.000 4.030
122.1 ( 1.7 124.2 ( 1.1 110.4 ( 1.0 106.3 ( 0.2 106.9 ( 0.2 110.5 ( 1.0 105.1 ( 1.7 7.01 ( 0.02 4.01 ( 0.10
a
The mean value is obtained by averaging the data of metalloporphyrins only.
TABLE 2: Some Selected Geometric Parameters of the Molecular Models in a D-Connection parameter
PH2
MgP
MnP
FeP
CoP
NiP
CuP
ZnP
RuP
PdP
PtP
SDa
∠R/ deg ∠β/ deg ∠γ/ deg ∠δ/ deg ∠η/ Deg. ∠θ/ deg ∠λ/ deg DA-B/Å DN-N/Å
125.73 121.61 111.73 105.95 105.98 111.73 104.61 6.877 4.312
126.33 124.41 109.91 106.69 106.85 109.81 106.75 6.875 4.168
122.78 122.98 111.20 106.76 106.62 111.42 103.99 6.859 3.948
122.32 122.44 111.50 106.61 106.60 111.49 103.79 6.852 3.940
122.38 122.31 111.53 106.52 106.52 111.56 103.86 6.839 3.942
122.06 122.02 111.47 106.51 106.44 111.63 103.95 6.832 3.939
123.88 123.01 110.23 106.74 106.68 110.38 105.97 6.868 4.091
125.90 123.95 109.70 106.79 109.93 109.62 106.95 6.879 4.191
125.60 125.06 109.51 107.13 107.16 109.46 106.75 6.901 4.103
125.89 124.91 108.86 107.10 106.96 109.02 108.07 6.889 4.157
124.78 124.14 109.96 106.87 106.77 110.12 106.29 6.858 4.049
124.2 ( 1.7 123.5 ( 1.1 110.4 ( 1.0 106.8 ( 0.2 107.1 ( 1.0 110.5 ( 1.0 105.6 ( 1.6 6.87 ( 0.02 4.05 ( 0.10
a
The mean value is obtained by averaging the data of metalloporphyrins only.
Figure 1. The metal cation substitution effect on the MPSH orbital level for the P-connection.
Figure 2. The metal cation substitution effect on the MPSH orbital level for the D-connection.
molecules are not degenerate. CuP is assigned to the fourth group and exhibits highly dispersed orbitals, one of which is located near to the Fermi level. The metallic center seriously affects the static properties of the metalloporphyrins in the P-connection. Figure 2 compares the MPSH of the molecular models in D-connection. In general, the D-connection shows a very similar pattern to the Pconnection. This is in agreement with the fact that the sidesubstitution does not much change the electronic structure of the metalloporphyrin. However, some minor differences do exist. For example, the HOMO and HOMO-1 become closer for PtP, and the LUMO and LUMO+1 become closer for NiP in the D-connection. These differences are probably caused by the electrode effect exerted via the Au-S bond, as also evidenced by the geometric characteristics (see Tables 1 and 2). The response of the energy levels of the MPSH orbitals to the junction bias is shown in Figure 3. The variation of the MPSH orbitals reflects the same classification given above. For MgP and PH2 which have either weak or no coordination, the orbital energy level is almost constant as bias increases. Due to the strong coordination effect for the rest of the metallopor-
phyrins, all four concerned orbitals decline with increasing junction bias. There are several characteristics to be noted. For the second group (FeP, CoP, and RuP), the HOMO and HOMO-1 are degenerate at all biases. For example, FeP has degenerate HOMO and HOMO-1 until 1.6 V. MnP has four orbitals very close to each other at 0.0 V. However, they suddenly separate under a bias at even a few milivolts. A similar pattern is also observed for the fourth group, CuP. The shadow in Figure 3 corresponds to the bias window (-Vb/ 2, Vb/2), where Vb is the applied bias. The integral over the bias window gives the current at that bias.44,45 Since the HOMO and LUMO may form the main resonant transmission peak in general, the shift of the HOMO and LUMO into the bias window at higher biases may increase the conductance. Another static feature of the molecular models is the spatial distribution that describes the mobility of the π-electron in the conjugated molecule. It may also give visual interpretation of the interaction between the metalloporphyrin ring and the metal center, but it cannot give direct insight into the transport. Figure 4 shows the spatial distribution of four MPSH orbitals close to the Fermi level. In the case of weak coordination, such as MgP,
Electron Transport through Porphyrin
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7419
Figure 3. The MPSH orbital level as a function of the junction bias for the representative metalloporphyrins wires in the P-connection.
Figure 6. The junction bias dependence of the spatial distribution of HOMO-1, HOMO, LUMO, and LUMO+1 for the representative metalloporphyrin molecules in the D-connection.
Figure 4. The junction bias dependence of the spatial distribution of HOMO-1, HOMO, LUMO, and LUMO+1 for the representative metalloporphyrin molecules in the P-connection.
Figure 5. The MPSH orbital level as a function of the junction bias for the representative metalloporphyrin molecules in the D-connection.
the orbitals never spread to the metal center. However, they may do so if strong coordination exists. In some extreme cases, such as MnP and NiP, the HOMO is even localized on the metal ions. These observations are good evidence for the diversity of interactions between the metalloporphyrin ring and the metal ions. Figure 5 shows the bias dependence of the MPSH for the series of porphyrins in the D-connection. Comparing Figure 3 with Figure 5, we find some differences. For PH2 and MgP, the HOMO-1 becomes closer to the HOMO in the D-connection
than in the P-connection. However, the difference is not very prominent. The most pronounced difference has been found in other groups. In FeP and MnP, the HOMO and HOMO-1 are located in the bias window at almost all biases. This might increase greatly the conductance of the molecular junction. A unique feature can be observed for CuP, the LUMO at which increases with bias until 1.0 V, but decreases thereafter. The LUMO is always located in the bias window, indicating that it may form the main transmission channel. The comparison between Figures 3 and 5 proves that the role of the metal center depends on the connection type. Figure 6 shows the spatial distribution of four MPSH orbitals near the Fermi level. Without the metal cation substitution, the molecular orbitals are spread over the whole protoporphyrin ring, regardless of the bias is applied. When a metal ion substitutes the metalloporphyrin center, the spatial distribution displays a different pattern. In particular, the metal center of FeP and MnP participates in the HOMO and HOMO-1, which is involved in the bias window at a bias of 1.0 V as presented in Figure 5. As a result, the HOMO and HOMO-1 make significant contribution to the current. Therefore, we might expect the current of FeP and MnP to be higher. 3.3. Transportation Behavior of the Porphyrins. Figure 7a gives the current as a function of junction bias. In terms of the current-voltage (I-V) curves, all metalloporphyrins as well as protoporphyrin are similar to each other. For some metals, such as Co(II) and Ni(II), the I-V curves are very similar, probably due to the similar ionic radius and capability in complexing the metalloporphyrin ring. In general, the protoporphyrin has the largest current at any time. Those metals (Mg(II) and Zn(II)) with weak interaction also exhibit larger current compared to the others. In contrast, if the metalloporphyrins have a strong interation, the molecule shows a smaller current. Despite the wide variety of metal ions used, the differences between the I-V curves are small. All the I-V curves of the metalloporphyrins are similar to that of the protoporphyrin. This result prompts us to reconsider whether the metal center plays an important role in the electron transport. Figure 7b shows the differential conductance as a function of junction bias. Due to the conjugation structure, the differential conductance at zero bias is much larger than the differential conductance for a linear molecule as calculated with the same method and basis sets. For example, the conductance of
7420 J. Phys. Chem. C, Vol. 113, No. 17, 2009
Figure 7. (a) The current-voltage curves and (b) the differential conductance-voltage curves of the series of porphyrins in the Pconnection.
dithiolbenzene is 28.5 µS at zero bias,46 8.8 µS for planar diphenylacetylene,47 and 2.1 µS for the oligo(phenylene ethynylene).46 This might be caused by having multiple electron transport routes in a cyclic molecule. All G-V curves show a similar feature, which is a decrease in conductance until ∼1.4 V, followed by an increase thereafter. Among these, MnP, FeP, and RuP exhibit a more pronounced sudden increase in conductance after ∼1.2 V, before which the conductance varies smoothly. Transmission spectra give more details of the electron transportation process, which may help us to understand the role of the metal center in the P-connection. Figure 8 gives the bias dependence of the transmission spectra. Despite having different metal cations, there are some general features among the metalloporphyrins. The most notable feature is the large transmission probability. Even at zero bias, the transmission
Wang et al. probability is still very high (∼0.2). This demonstrates the perfect conductivity of porphyrin-based molecules. Another feature is the sharp transmission wave with maximum peak height close to 1.0 which appears at about 1.5 V. This peak can be mainly attributed to the LUMO resonance transmission as indicated by the LUMO eigenvalues in Figures 1 and 2. The full width at half-maximum (fwhm) of this wave is about 0.5 V; therefore, in the case of low applied bias, the bias window does not include this transmission wave. However, when the bias is larger than 1.5 V, part of the LUMO resonance transmission wave can be involved, leading to an increase of current. In contrast, the transmission wave below the Fermi level is rather broad. It contains several waves contributed by the HOMO and other transmission tunnels. Their peak heights are almost the same as the LUMO resonance peak. Since these broad waves have widths as large as 1.0 V, the tails of these waves may extend over zero bias, and the current at low bias is mainly governed by them. Consequently, their shift under the junction bias directly affects the conductance. We also noted that all models show a decrease of the HOMO resonance peak with increasing bias. This implies that the conductance decreases initially but then increases when the bias is beyond a certain value, as the contribution from the LUMO resonance wave is then included. As mentioned above, although the various metal substitutions change the static properties of the metalloporphyrins, such as the energy levels of the HOMO and LUMO, and the spatial distribution of the orbitals, the electron transport is less affected. This suggests that electron transport is through the conjugated porphyrin ring rather than the metal center. Figure 9a gives the I-V behavior of the series of molecular models in D-connection. Unlike P-connection, the D-connection exhibits diverse I-V curves, depending on the metal center used. The protoporphyrin no longer shows the highest current. Which is now shown by those molecular models whose metal cations coordinate well to the metalloporphyrin ring (MnP, RuP, FeP, CuP, ZnP, and MgP). The I-V curves of the others (CoP, NiP, PdP, and PtP) are close to, or slightly smaller than, that of PH2. Another important feature in Figure 9a is the nonlinearity with applied bias concerned. Compared to Figure 7a, we see the I-V curves in D-connection deviate widely from the linear relationship. An arc can be observed for all the metalloporphyrins, whose currents are higher than that of protoporphyrin itself. This
Figure 8. The junction bias dependence of the transmission spectrum for the representative porphyrins wires in the P-connection.
Electron Transport through Porphyrin
Figure 9. (a) The current-voltage curves and (b) the differential conductance-voltage curves of the series of porphyrins in the Dconnection.
also indicates that the metal center plays a significant role in the electron transport route. Figure 9b gives the differential conductance as a function of the junction bias. The zero-bias conductance is almost 7 times greater in the D-connection (62.4 µS) than in the P-connection (0.9 µS). Samples with a larger zero-bias conductance have a greater range of conductance. For example, MnP decreases from 62.4 to 7.7 µS, and FeP from 42.6 down to -2.9 µS. In contrast, the metalloporphyrins with small zero-bias conductance (PdP, PtP, ZnP, and so on) vary only slightly. This observation shows again the effect of electron transport path on electron transport. More detailed information about how the metal center contributes to the transport can be seen from the transmission spectrum. Figure 10 gives the bias dependence of the transmission spectra for the series of the molecular models. Unlike in P-connection, the transmission spectrum in D-connection is sensitive to the bias applied. There are two significant differences between the P-connection and D-connection. First, the LUMO resonant peaks at ∼1.0
J. Phys. Chem. C, Vol. 113, No. 17, 2009 7421 V are very sharp in the D-connection. They show stable peak height but shifted peak position. Second, the variation of the HOMO resonance wave is more prominent in the D-connection than in the P-connection. Some of the HOMO resonance waves shift negatively, as in FeP. Some of them decrease dramatically, as in MgP, MnP, and PH2. An abnormal positive shift of the HOMO resonance wave was found for CuP when the bias was increased from 0.0 to 1.0 V. All these effects show the considerable effect of the metal center. In general, by means of MPSH orbital levels, spatial distribution, I-V curves, and transmission spectrum, we can retrieve information about the transport behavior of the porphyrin-based molecular wires. On the basis of the above discussion, we conclude that the electron transport paths vary in different connections. A detailed picture of this will be drawn below. 3.4. The Electron Transport Paths. When the molecule is attached between two electrodes, the molecular levels tend to align with the electrochemical potential of the Au electrode and establish an equal chemical potential. In the first configuration (P-connection), there are three types of possible electron transport paths. The first consists of two parallel routes through the conjugated chain of porphyrin. The second is a straightforward transport via the metal center. In this way, the electron will take the shortest path. The third seems to be more complex, as it shows central symmetry, and consists of a short path though the conjugated chain before reflection to the metal center via the coordination bond. The energy profiles of these three routes are schematically drawn in Figure 11a. Although the second route is shortest, it passes a vacuum barrier (of about 5.0 eV) between carbon ring and metal center, which may reduce the electron transport probability. In contrast, the energy profiles of route 1 and 3 are smooth. However, route 3 comprises several steps. When injected, the electron scatters up or down. It then encounters a vacuum barrier when it reaches the corner of the porphyrin ring. The electron must then be reflected to the metal center. The reflection efficiency is rather small as compared to the other transportation modes. Therefore route 3 is not dominant. The most probable transport path in the P-connection is scattering through the π-conjugation of the porphyrin ring, which shows less dependence on the metal center. The overall transport calculations agree with this proposal. When the D-connection is applied, only two types of electron transport routes exist. One route is via the π-conjugation of porphyrin ring, and the other is via the metal center. The PH2 in the
Figure 10. The junction bias dependence of the transmission spectrum for the representative porphyrins wires in the D-connection.
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Figure 11. (a) Schematic diagram of the electron transport paths as well as the potential energy profile for the P-connection. (b) Schematic diagram of the electron transport paths as well as the potential energy profile for the D-connection.
D-connection is a watershed, because it involves only the π-conjugation route. From the I-V calculations, we can see the conductivity of PH2 is higher than that of PdP, NiP, and PtP, even though they have two different electron transport paths. Therefore, the route through π-conjugation is more effective when the two paths remain competing. The potential profiles of both paths are illustrated in Figure 11b. Although energy wells exist, the attraction between the noble metal ion (Pd(II) or Pt(II)) and the π-electrons may reduce the transport ability of the system, leading to lower currents. 3.5. Molecular Sensing on the Basis of Electron Transport Routes. The electron transport path model is not only theoretically instructive but also useful for practical device design. In chemical sensing of a gas molecule, the sensitivity is highly dependent on the electron transport path. In this paper, we would like to present a theoretical prototype of a chemical sensor with a metalloporphyrin as schematically shown in Figure 12a. The CO molecule coordinates with the Fe center of the metalloporphyrin ring. The optimized structures show that the Fe-C and C-O bond lengths in the P-connection are 1.681 and 1.161 Å, respectively. They remain almost the same in the D-connection. The ∠FeCO angles of both configurations are 180°. When the P-connection is applied to the metalloporphyrin, the electron never passes through the metallic center as discussed previously. Regardless of the gas molecules adsorbed, the electronic signal should not change much. However, when the D-connection is used, the metallic center becomes active. The transportation behavior may be sensitive to the situation of the metallic center. Figure 12b gives the I-V curves for the P-connection of a FeP with and without an adsorbed CO molecule. As explained above, there is no obvious difference between two curves, confirming the electron transport path and indicating that chemical sensing is not feasible in this configuration. However, in the Dconnection (Figure 12c), the current is significantly reduced after chemisorption of the CO molecule. The zero-bias conductance of a bare FeP is 42.6 µS, but it is reduced to 0.74 µS after adsorption of CO, only 1.7% of the original value. Such a significant difference demonstrates the potential application of metalloporphyrin molecules in molecular sensing.
Figure 12. (a) Schematic presentation of the utility of the electron path design in the chemosensing. (b) The current-voltage curves of the FeP in the P-connection with and without CO adsorption. (c) The current-voltage curves of the FeP in the D-connection with and without CO adsorption.
4. Conclusions The electron transport properties of a series of metalloporphyrin molecules in different connections have been studied in detail. Due to the existence of multiple electron transport paths, the metal ion center plays different roles, depending on the connection. In a parallel connection (P-connection), in which two edges of the metalloporphyrin ring are parallel to the electrode surface, the electron does not pass through the metal center. It is therefore hard to modulate the electric behavior of the system by changing the metal center. However in a diagonal configuration (D-connection), in which two opposite corners of metalloporphyrin are connected to the electrodes, the electron can pass through the metal ion. Due to the participation of the metal ion in the electron transport path, the I-V curves can vary significantly. Therefore, the conductance can be modulated by the metal center.
Electron Transport through Porphyrin The present work provides strong evidence that a cyclic conjugated molecule may serve as an electronic circuit at the molecular level. Such a circuit is different from a macroscopic circuit because of the dependence on the electron transport path. Acknowledgment. This project was supported by the National Natural Science Foundation of China (No. 20821063 and 20873063) and National Basic Research Program of China (973 Program, 2007CB936302). We would like to thank Mr. William Winter, Dr. Colm Durkan, and Professor M. E. Welland FRS FREng from Nanoscience Centre, University of Cambridge for helpful suggestions. Supporting Information Available: The calculation of spin multiplicity of all metaloporphyrins. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (2) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones II, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (3) Li, Y.; Zhao, J.; Yin, X.; Yin, G. J. Phys. Chem. A 2006, 110, 11130. (4) Yin, X.; Li, Y.; Zhang, Y.; Li, P.; Zhao, J. Chem. Phys. Lett. 2006, 422, 111. (5) Ashwell, G. J.; Sambles, J. R.; Martin, A. S.; Parker, W. G.; Szablewski, M. J. Chem. Soc., Chem. Commun. 1990, 19, 1374. (6) Metzger, R. M. Chem. ReV. 2003, 103, 3803. (7) Stadler, R.; Geskin, V.; Cornil, J. AdV. Funct. Mater. 2008, 18, 1. (8) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (9) Reed, M. A.; Chen, J.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2001, 78, 3735. (10) Zhang, W.; Moore, J. S. Angew. Chem., Int. Ed. 2006, 45, 4416. (11) Li, Y. W.; Zhao, J. W.; Yin, X.; Yin, G. P. ChemPhysChem 2006, 7, 2593. (12) Liu, C.; Walter, D.; Neuhauser, D.; Baer, R. J. Am. Chem. Soc. 2003, 125, 13936. (13) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature (London) 2000, 408, 541. (14) Hecht, D. S.; Ramirez, R. J. A.; Briman, M.; Artukovic, E.; Chichak, K. S.; Stoddart, J. F.; Gru¨ner, G. Nano Lett. 2006, 6, 2031. (15) Steenwinkel, P.; Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G. Organometallics 1998, 17, 5647. (16) Dunbar, A. D. F.; Richardson, T. H.; McNaughton, A. J.; Hutchinson, J.; Hunter, C. A. J. Phys. Chem. B 2006, 110, 16646. (17) Ami, S.; Hliwa, M.; Joachim, C. Nanotechnology 2003, 14, 283. (18) Tsuda, A.; Osuka, A. Science 2001, 293, 79. (19) Ghosh, A. J. Porphy. Phthalocya. 2000, 4, 380. (20) Minaev, B.; Ågren, H. Chem. Phys. 2005, 315, 215. (21) Baerends, E. J.; Ricciardi, G.; Rosa, A.; van Gisbergen, S. J. A. Coord. Chem. ReV. 2002, 230, 5. (22) De Angelis, F.; Fantacci, S.; Sgamellotti, A.; Pizzotti, M.; Tessore, F.; Biroli, A. O. Chem. Phys. Lett. 2007, 447, 10.
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