Nitric Oxide Adsorption Effects on Metal Phthalocyanines - The

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Nitric Oxide Adsorption Effects on Metal Phthalocyanines Tien Quang Nguyen, Mary Clare Sison Escan˜o, and Hideaki Kasai* Department of Precision Science & Technology and Applied Physics, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: April 20, 2010; ReVised Manuscript ReceiVed: July 07, 2010

The adsorption of nitric oxide (NO) on various metal phthalocyanines (MPc, M ) Mn, Fe, Co) has been studied using first-principles calculations based on density functional theory (DFT). In this study, we investigated the fully optimized geometries and electronic structure of MPc. We found that the electronic structures of metal atoms are essential in shaping the ground-state electronic structure near the Fermi level. These states are defined mostly by the d orbitals of the transition-metal atoms and, to some degree, by the states of nitrogen and carbon atoms of the inner rings. The numerical calculations showed that NO strongly chemisorbs to the metal atom with an end-on configuration and results in a change in geometric and electronic structures of MPc. The N-O bond lengths are slightly longer than that of the isolated NO molecule. The orbital energy levels are shifted with respect to the Fermi level. The HOMO-LUMO gap widens as compared to bare MPc. These changes are attributed to the hybridization of the π* orbital of NO and the d orbitals of the transition metal. Specifically, the interaction between dπ and the π* orbital is significant for MnPc-NO, while the hybridization of dz2 and the π* orbital plays an important role in CoPc-NO. 1. Introduction The discovery of biological functions related to nitric oxide (NO)1,2 has encouraged extensive research in chemistry, biology, and pharmacology. It is believed that NO is involved in many biological processes. First, it acts as a signaling molecule in the nervous system,3,4 as a weapon against infections,5 as a regulator of blood pressure, and as a gatekeeper of blood flow to different organs.6,7 Second, it is found that the concentration of NO in exhaled air can give very useful information on the status of the human body.8 In addition, a number of disease processes such as septic shock and destruction of insulinproducing cells have been attributed to the imbalance of NO concentration.9,10 Therefore, the need for the development of a sensitive and selective method for the detection and quantification of NO in these systems is very important. Phthalocyanine (Pc) is a kind of macrocycle compound having an alternating nitrogen atom-carbon atom ring structure (Figure 1). A high chemical and thermal stability as well as conducting properties of Pc has been attracting attention in various fields of science and industry.11,12 The Pc molecule is able to coordinate hydrogen or metal cations in its center by four isoindole nitrogen atoms. It has been found that many elements can coordinate to the Pc macrocycle. Therefore, a variety of Pc complexes exist, especially metal phthalocyanines (MPc). MPcs have square-planar structure. They can form highly ordered, thermally stable, and chemically inert structures. The geometric and electronic structures as well as the adsorption of some MPcs on metal substrates have been studied both theoretically13-15 and experimentally,16 albeit not extensive. It is known that MPc’s surface properties and electronic properties can be well modified by varying the metal at the center of Pc molecule or adding ligands onto the organic rings, making them attractive candidates for chemical sensors.17,18 Experimentally, some electrochemical nitric oxide microsensors based on nickel * To whom correspondence should be addressed. E-mail: kasai@ dyn.ap.eng.osaka-u.ac.jp.

Figure 1. Four initial adsorption sites of NO on metal phthalocyanine.

phthalocyanine have been developed.19,20 Although there have been several experimental and theoretical studies on MPc and its applications as a gas (O2, NO2) sensor,21,22 a fundamental understanding of gas chemisorptions onto these compounds is still lacking. Furthermore, the full optimization of molecular structure and the molecule binding to the aromatic ring was left unexplored. So far, Tran et al.23 studied the binding of NO and FePc thin films using computational methods. However, the role of the metal center in the interaction with NO as well as the trends in NO bonding was not included. Hence, the mechanism of NO-MPc interaction is still elusive. In this work, we present the results of the first-principles electronic structure calculations carried out for several MPcs (M ) Mn, Fe, Co) and the adsorption of a NO molecule on these systems. Here, we discuss in detail the optimized geometric structure, the trend of the O-N-M bond angle, the electronic structure, and changes in electronic properties of MPc before and after binding to NO. The mechanisms for these interactions are also carefully noted. These new insights pose significant implications in chemical sensor design employing MPc systems. 2. Computational Methods All calculations were performed within the DFT framework using the spin-polarized version of the Vienna ab initio

10.1021/jp1035426  2010 American Chemical Society Published on Web 07/20/2010

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Nguyen et al. TABLE 1: Calculated Distance (Å) between a Central Metal Atom and Its Nearest-Neighboring N Atom this work MnPc FePc CoPc

Figure 2. Total density of states and projected density of states of MnPc. The Fermi level is set to zero.

simulation package (VASP)24,25 with periodic boundary conditions in three directions. Nonlocal correction in the form of the generalized gradient approximation (GGA)26 was included for the exchange-correlation functional. The calculations used the projector augmented wave (PAW)27 potential to describe the electron-ion interaction. We adopted a supercell geometry in which a single k-point (located at the Gamma point) is used. The kinetic energy cutoff was set to 500 eV for simulations of all systems. In order to avoid interactions between the MPc in the cell with those in the neighboring cells, we introduced a unit cell of 21.3 Å × 19.7 Å × 16.0 Å. In addition, structural optimization was performed using the conjugate gradient method.28 The structures of MPc and NO-adsorbed MPc were fully optimized until the Hellmann-Feynman force exerted on an atom was less than 0.05 eV/Å. In order to find the most stable geometry of NO on MPc, we carefully set initial adsorption sites, as shown in Figure 1. For the first two cases (left side of Figure 2), the N-O bond is parallel to the phthalocyanine plane. The distance from NO to the MPc plane was set to 2 Å. For the last two cases (right side of Figure 2), the N-O bond was perpendicular to the phthalocyanine plane. The distance from O (site 3) or N (site 4) to the MPc plane was set to 2 Å. The adsorption of NO with sites other than a metal site such as N or C was also investigated. The simulations show that the NO molecule strongly adsorbs on the metal site and weakly adsorbs on all other nonmetal sites. The most stable position of NO on MPc is at the metal site, with the binding energies calculated based on the following expression: ∆E ) EMPc+NO - (EMPc - ENO). Here, EMPc+NO is the total energy of the MPc-NO system, EMPc is the total energy of MPc, and ENO is the total energy of the isolated NO molecule (in the gaseous state). 3. Results and Discussion 3.1. Metal Phthalocyanines. First, we conducted a full structure optimization for MnPc, FePc, and CoPc. We found that in these three systems, the MPcs assume a planar configuration. Our results show that the distance between the central metal atom and its nearest-neighboring N atoms (M-N1 bond length) is in the order MnPc (1.950 Å) > FePc (1.932 Å) > CoPc (1.925 Å). The M-N1 bond lengths in this work and those in theoretical29,30 and experimental31-33 works are shown in Table 1. Our calculations show very good agreement with previous studies. In Figure 2, the total density of states (DOS) and the siteprojected local density of states (LDOS) obtained by the PAW method for MnPc are presented (the Fermi level is set to zero). We note that the spin-up and spin-down components of total DOS, which lie below -4.0 eV and above 3.0 eV, are almost symmetric. This implies the minimal interaction between the

1.950 1.932 1.925

theoretical 29

1.931 1.92330 1.91730

experimental 1.93831 1.92832 1.92034

electronic states of the atoms in the outer ring and the d electron of the Mn atom. Similar observations are noted for the case of FePc and CoPc, which will be discussed further later. Here, we note that the difference in the electronic structure of MPcs mostly comes from the electronic states of metal atoms. Also, the states near the Fermi level are mainly defined by dxz, dyz orbitals of the Mn atom and, to some degree, by the states of pz orbitals of C1. Unlike the electronic states of C1 atoms, the electronic states originating from N1 and N2 atoms do not contribute to the electronic states near the Fermi level. The hybridization of the N1 pz orbital and the Mn dx2-y2 orbital forms bonding states below -2.0 eV and antibonding states above -1.4 eV. This creates a coordination bond inside of the inner ring of MnPc. From the DOS profile, the gap between the valence electronic states and the first empty band in MnPc is estimated to be 1.424 eV for the spin-up component and 0.213 eV for the spin-down component. The smaller gap is formed by the occupied state dxz (HOMO) and the unoccupied state dyz (LUMO). In the case of FePc, the total DOS and LDOS are shown in Figure 3. We can see that the electronic states, which lie below -3.0 eV and above 3.0 eV, are quite symmetric for both spin components. Thus, a similarity with MnPc is noted. The electronic states near the Fermi level are due to dxz, dyz, and dxy states of the Fe atom and, to some extent, to the states of C1 atoms. The electronic states originating from N1 and N2 atoms do not give any contribution to the states near the Fermi level. In fact, the N1 and N2 atoms exchange p electrons with Fe, forming the coordination bond inside of the inner ring of FePc (Fe-N1 bonds). The states contributed to by electrons of N1 and N2 atoms can be seen in the energy range from -3.0 to -1.0 eV. The energy gap of FePc estimated from DOS is 1.442 eV for the majority spin component and 0.199 eV for the minority spin component. The smaller gap is formed by the HOMO state (dπ orbitals) and the LUMO state (dxy). We further note that in the MnPc system, the dπ orbitals are not fully occupied due to the LUMO state, which belongs to the dyz orbital. In the case of FePc, the dπ states are fully occupied. The LUMO state is due to the dxy orbital. Next, for CoPc, the DOS and LDOS of CoPc are depicted in Figure 4. Similarly, the total DOSs for both spin-up and spindown components that lie below -3.0 eV and above 2.0 eV are symmetric. That implies lack of interaction between the d

Figure 3. Total density of states and projected density of states of FePc. The Fermi level is set to zero.

Nitric Oxide Adsorption Effects on Metal Phthalocyanines

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Figure 6. Mechanism of the interaction of NO with MePc. Here, d implies dxz or dyz. For MnPc-NO, the first process occurs. For CoPc-NO, the second process occurs. For FePc-NO, the mixing of both processes is expected to occur. This explains the differences in the M-N-O angle. Figure 4. Total density of states and projected density of states of CoPc. The Fermi level is set to zero.

Figure 5. The most stable configurations for NO adsorbed on MPc after full optimization.

TABLE 2: Binding Energy (eV) of NO with MnPc, FePc, and CoPc at Different Sites system

site 1

site 2

site 3

site 4

MnPc-NO FePc-NO CoPc-NO

-1.578 -1.899 -1.552

-1.566 -1.890 -1.508

-0.091 -0.588 -0.006

-1.738 -1.765 -0.348

electrons of the Co atom and the electronic states of the atoms in the outer ring of CoPc, as in the case of the two previous systems. In order to determine in detail the bands formed near the Fermi level, we analyzed the LDOS of CoPc, as shown in Figure 4. We note that the electronic states near the Fermi level are mostly contributed to by the dz2 orbital of the Co metal and the pz orbital of the C1 ring. The dπ and dxy orbitals are shifted lower to the Fermi level (compared to MnPc and FePc) due to the full occupation of the dxy orbital and partial occupation of the dz2 orbital. Our results are in good agreement with experiment, showing the dxy, dπ, and dz2 orbitals lying very close to each other (in energy), and they lie mainly in the energy range around the HOMO and LUMO states. Furthermore, the dz2 orbital was found to be half-occupied.33 The energy gap of CoPc was found to be 1.450 eV for the majority spin component and 1.400 eV for the minority spin component. Similarly, for MnPc and FePc cases, N1 and N2 atoms do not give any contribution to the states near the Fermi level. In fact, the p orbitals of the N1 and N2 atoms interact with the d orbital of the Co atom to create the coordination bond inside of the inner ring of CoPc. The interexchange of electrons occupying d orbitals and p orbitals of N1 and N2 definitely dominates the electronic structure of CoPc in the energy range of -2.4 to -0.4 eV. This is in good agreement with other DFT work.34 3.2. NO-Adsorbed Metal Phthalocyanines. Here, we discuss the geometric structure of NO adsorbed on metal phthalocyanines (MPc-NO). The most stable structure of NO on MPc after optimizing the whole structure of MPc-NO, as depicted in Figure 1, is shown in Figure 5. The binding energy for different initial adsorption sites is given in Table 2. Due to the relaxation of NO and MPcs, the final structures of NO adsorbed on MPcs are not the same as the initial settings. In fact, our simulations showed that the N terminal of NO prefers to bind with metal atoms rather than with the O terminal, which leads to a bent or linear structure of the M-N-O angle, and the M-N bond is almost perpendicular to MPc’s plane, as shown in

Figure 5. The binding energies of NO with MPc are -1.738, -1.899, and -1.552 eV for MnPc-NO, FePc-NO, and CoPc-NO, respectively. Interestingly, these values are quite close to the binding energy of NO with metal tape-porphyrin systems.35-40 This is attributed to the similarity in structure of phthalocyanine and porphyrin. The binding energies show that NO strongly chemisorbs to the MnPc, FePc, and CoPc (at metal sites). Also in all three systems, the difference in binding energy between site 1 and site 2 is small. This implies that, despite the strong interaction with MPc, the NO molecule can easily rotate around the M-N axis without a significant energy barrier. Indeed, the M-N-O angles in sites 1 and 2 also exhibit little difference. Furthermore, at the most stable position of NO, there is minimal increase (∼0.04 Å) in the N-O bond length. The N-O bond lengths are 1.192, 1.190, and 1.187 Å for MnPc-NO, FePc-NO, and CoPc-NO, respectively. NO adsorption causes the metal atoms to protrude from the MPc molecular plane toward the molecule while the organic rings buckle away from NO. The deviation of the metal atom from the Pc plane is about 0.33 Å for Mn, 0.31 Å for Fe, and 0.19 Å for Co. In all three cases, the metal-N bonds are almost perpendicular to the phthalocyanine plane. The order of metal-N bond lengths is Mn-N (1.624 Å) < Fe-N (1.707 Å) < Co-N (1.819 Å); the M-N-O bond angles display the reverse trend with Mn-N-O (179.94°) > Fe-N-O (147.34°) > Co-N-O (122.22°). These trends are consistent with those reported in experimental work.41 The orientation of the N-O bond in adsorbed systems can be explained as follows. For the case of MnPc, as analyzed previously, the electronic states near the Fermi level of MnPc are mostly defined by dπ orbitals of Mn metal; therefore, the hybridization between the π* orbital of the NO molecule and dπ orbitals of the Mn atom is significant. This interaction results in a linear structure of the Mn-N-O bond angle (179.94°). For the CoPc case, as mentioned above, the dz2 orbital mainly contributes to the electronic properties of CoPc near the Fermi level; thus, the hybridization between the π* orbital of NO and the dz2 orbital of Co is expected. This combination will result in a bent structure of the Co-N-O bond angle (122.22°). The explanation can be easily understood via the mechanism shown in Figure 6. Thus, the d orbitals play an important role in orienting the M-N-O angle when the NO molecule binds to the transition metals in MPc systems. For the case of FePc, the dz2 orbital of Fe is obviously the main factor for the bent structure of the Fe-N-O bond angle. However, we also expect the hybridization of the π* orbital and the dπ orbitals due to the contribution of the dπ orbitals near the Fermi level. Therefore, the Fe-N-O bond angle of FePc-NO should be intermediate between those of MnPc-NO and CoPc-NO, and this was verified by the trends in the M-N-O angle. Next, we discuss about the electronic structures of MPc-NO to determine how such systems are affected by NO adsorption. In Figure 7, we show the total density of states of the MnPc-NO

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Figure 7. Total density of states and projected density of states of the NO-adsorbed MnPc system. The Fermi level is set to zero.

Figure 8. Total density of states and projected density of states of the NO-adsorbed FePc system. The Fermi level is set to zero.

system, where the Fermi level is set to zero. The adsorption of NO on MnPc leads to the splitting of dπ states into bonding (dπ + π*) and antibonding states (dπ - π*), which are located at around -1.6 and 1.9 eV relative to the Fermi level, respectively. Upon NO adsorption onto the Mn metal, the dxy is filled and shifted to the position lower than the Fermi level. This may indicate that charge-transfer complexes between the NO molecule and MnPc are formed when electron-withdrawing or electron-donating gases interact with MnPc. As a result, the gap between the valence states and conduction states is opened. Indeed, the Bader analysis42 of charge distribution for MnPc before and after NO adsorption shows that the NO molecule gains 0.46e. Our simulations show that the energy gap is estimated to be 1.0 eV for the MnPc-NO system. It is significantly larger as compared to that of MnPc (0.213 eV). For the case of NO-adsorbed FePc, the total DOS is shown in Figure 8. Under the interaction of the NO molecule, the dπ and dz2 states are strongly modified. The filled dz2 state is shifted nearer to the Fermi level, while the filled dπ states are shifted further from the Fermi level. Moreover, the dxy state is filled and shifted down, right below the Fermi level. This leads to the formation of an energy gap between the filled dxy state and the unfilled dz2 state, which is estimated to be 1.0 eV. This energy gap is obviously larger than that of the FePc system. Thus, NO binding to FePc induces a significant change in the electronic structures of FePc, that is, the charge transfer between the NO molecule and FePc. In fact, an analysis of the atomic charge of the FePc system before and after NO adsorption using the Bader method shows that NO gains 0.36e. In the case of NO-adsorbed CoPc, the interaction of the NO molecule and CoPc strongly modified the electronic structures of CoPc. As shown in Figure 9, the hybridization of the dz2 orbital of Co and the π* orbital of NO leads to the splitting of the dz2 states into bonding and antibonding located away from the Fermi level. The dπ states (below the Fermi level) are a bit shifted down. This implies charge transfer between NO and CoPc. As a result, the energy gap between the valence band

Nguyen et al.

Figure 9. Total density of states and projected density of states of the NO-adsorbed CoPc system. The Fermi level is set to zero.

and conduction band is opened. Our simulations show that upon the adsorption of NO, the energy gap of CoPc increases from 1.4 to about 1.5 eV. The analysis of charge distribution of CoPc before and after NO adsorption using the Bader method shows that the NO molecule gains 0.18e. The electron charge transfer from MPc to NO in this study is consistent with experimental results, which show that the NO gases absorbed on MPc (which is a p-type semiconductor) turn out to be negatively charged and thus form acceptor states since they attract electrons from MPc.43 4. Conclusions We have used DFT method to investigate geometric structure and electronic properties of the metal phthalocyanines (MPc) and their complexes with the NO molecule. We found that the electronic states at the Fermi level in MPc are mainly contributed to by electron charge density of d orbitals of the metal atom and, to some degree, the pz orbital of carbon in the inner ring. The adsorption of the NO molecule on MPc induces significant changes in the geometry as well as the electronic structures of MPc. The orientation of NO on MPc depends strongly on the d electron state near the Fermi level. In the case of MnPc, Mn-N-O is linear, while in the case of FePc and CoPc, M-N-O is bent. This trend is in good agreement with the experimental works. The binding of NO to MPc also leads to the opening of the HOMO-LUMO gap in all cases. The energy gap increases from 0.213 to 1.0 eV in the case of MnPc, from 0.199 to 1.0 eV in the case of FePc, and from 0.121 to 1.5 eV for the CoPc case. The observed change in the geometry and the electronic structures can be explained in terms of the hybridization of NO π* orbitals and d orbitals of metal atoms as well as the charge transfer between the NO molecule and MPcs. This kind of interactions lead to the splitting of d states into bonding and antibonding states, shifting of HOMO and LUMO states to lower or higher states compared to the Fermi level, and forming the new HOMO-LUMO gap. The electron charge transfer from MPc to NO in this study is consistent with experimental results. The results of this work have shown that MPc is a potential sensor device for detecting NO gas. Acknowledgment. We are grateful to Dr. Hiroshi Nakanishi and Mr. Hirofumi Kishi, Osaka University, for their support on the present computations and for valuable discussions. This work is supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) through their Special Coordination Funds for the Global Center of Excellence (GCOE) program (H08). Some of the numerical calculations presented here were performed at the computer facilities of Cyber Media Center (Osaka University) and Yukawa Institute for Theoretical Physics (Kyoto University).

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