ARTICLE pubs.acs.org/JPCC
Theoretical and Experimental Study of Bonding and Optical Properties of Self-Assembly Metallophthalocyanines Complexes on a Gold Surface. A Survey of the Substrate Surface Interaction. Ingrid Ponce,‡ J. Francisco Silva,‡ Ruben O~nate,‡ Sebastian Miranda-Rojas,§ Alvaro Mu~noz-Castro,† Ramiro Arratia-Perez,*,† Fernando Mendizabal,*,§ and Jose H. Zagal‡ †
Doctorado en Fisicoquimica Molecular, Relativistic Molecular Physics (ReMoPh) Group, Universidad Andres Bello, Av. Republica 275, Santiago, Chile ‡ Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile Casilla 40, Sucursal Matucana, Santiago 9170022 Chile § Departamento de Química, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile
bS Supporting Information ABSTRACT: The formation of self-assembly monolayers (SAMs) based on a gold substrate and a thiolate ligand as “anchor” fragment of metallophtalocyanine has been employed as strategy toward the obtention of modified electrodes. In this Article, the formation of SAM’s involving iron and cobalt phtalocyanines anchored by 4-aminothiophenol (4-ATP) and 4-mercatopyridine (4-MP) to the Au(111) surface is explored by both experimental and theoretical studies for a better understanding of their bonding pattern and optical properties. The self-assembly metallophthalocyanines complexes on gold electrode exhibits an interesting charge donation from the 4-ATP or 4-MP toward both gold substrate and phtalocyanine, denoting an effective gold MPc interaction mediated by the titled anchor ligands. In addition, the optical properties of the self-assembled complexes supported on the gold electrode exhibit in conjuction with the well-described Q-band an interesting charge transfer from the Pc (π) toward the gold surface, as could be observed in the FePc-4MP-Au26 assembly.
1. INTRODUCTION Self-assembled monolayers (SAMs)1,2 have attracted significant attention in the literature because of their applications in many areas such as, biology, physics, optics, and chemistry3,4 because of their high selectivity and stability.5 SAMs consist of molecules having a headgroup and a tail group with functionality, where the headgroup has special affinity for a substrate, and the tail exhibits a functional group whose terminal end provides the functionality of the SAMs. Usually, the functional group is a redox center or a molecule that can undergo fast electron-transfer (ET) processes. In general, the headgroup of the SAM film promotes the spontaneous adsorption of the appropriated molecules from the solution onto a metal substrate. In this respect, the chemisoption of thiolates on gold represents the most important class of SAMs in electrochemical studies,6 leading to interesting modified electrodes,7 9 which continues to be a focus for research interest because of their importance in supramolecular nanotechnology, fundamentals studies of ET reactions, and so on.10 12 Gold exhibits strong affinity for sulfur leading to the formation of Au S bonds, involving an energy of 35 45 kcal/mol,13 where thiols can serve as effective “molecular anchors” to immobilize molecules that act as catalyst for ET reactions. Therefore, these r 2011 American Chemical Society
assemblies can be used as electrochemical sensors for several target molecules, among other applications. In particular, the advantages of SAM-gold electrodes using different metallophalocyanine complexes have been reported by Nyokong et al. for the electrochemical detection of several analytes.14 To immobilize metallophthalocyanine molecules (MPc) on gold surfaces, we have employed 4-aminothiophenol (4-ATP) and 4-mercatopyridine (4-MP), denoted for simplicity as L, to form a SAM structure on a Au(111) support (Scheme 1), followed by an axial coordination of the aromatic amino or pyridil group to the cobalt(II) phthalocyanine or iron(II) phthalocyanine (CoPc and FePc, respectively). STM experiments have provided direct evidence that a complete monolayer of 4-ATP or 4-MP can be formed. The complete self-assembly process produces a stable and highly ordered conducting film where the phthalocyanines remain parallel to the gold surface forming “umbrella” like structures15 due to the densely packed 4-ATP monolayer onto which CoPc or FePc molecules are coordinated. Received: September 9, 2011 Revised: October 12, 2011 Published: October 23, 2011 23512
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The Journal of Physical Chemistry C Scheme 1. Schematic Representation of the Studied Systems (M = Fe, Co, and Ni)
Figure 1. Structure of the studied systems (M = Fe, Co, and Ni).
Even in well-ordered systems such as SAMs, surface heterogeneities may lead to a local variation of surface coverage. For that reason, conventional electrochemical measurements based on the measure of substrate current may not uniquely isolate the contribution of the attached-to-substrate species. In this work, we have investigated experimentally and theoretically the stability of these molecules also anchored on SAMs formed on Au(111) to control the configuration of these molecules on the electrode surface.16 20 We focus our attention into the interaction between the MPc and the gold surface, mediated by anchor ligands (L = 4-ATP and 4-MP) for M = Fe and Co, describing theoretically the bonding energy and charge transfer between each fragment with the aim to gain more insight into the design of novel systems that can act as effective catalyst for ET reaction. In addition, we modeled the NiPc system that presents very low catalytic activity to compare it versus the active FePc and CoPc.
2. EXPERIMENTAL SECTION Materials. All reagents, FePc and CoPc complexes, 4-ATP 97%, 4-MP 95%, and dimethylformamide (DMF) solvent were obtained from commercial sources and used as received. The UV/vis measurements were carried out with a diode array Scinco S-3100 spectrophotomer. The MPc solutions were prepared with
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4-ATP and 4-MP 10 μM in DMF at variable proportions. The spectra were obtained at different times to observe changes in the absorption spectra as the MPc-4ATP and 4 4MP are formed. Furthermore, a concentration of 4.86 10 11 mol cm 2 for CoPc fixed on self-assembled 4-mercaptopyridine on gold was obtained. Details of the Calculations. The interaction of the metallophthalocyanine-L, MPc (Fe, Co, Ni; L = 4-aminothiophenol (4-ATP) and 4-mercatopyridine (4-MP)), moiety with the Au(111) surface was modeled by using a closed shell Au26 cluster, according to the literature.21,22 The Au26 cluster was built and optimized, by having three layers containing 14, 8, and 4 gold atoms (Figure 1), respectively. The geometry of the MPc-L was fully optimized in all calculations, leaving the Au26 atoms at fixed positions, depicting Au Au distances of 2.87 Å. The PBE (Perdew Burke Ernzerhof) functional was used in all calculations23 in conjunction with Stuttgart small-core pseudorelativistic effective core potentials (ECPs) for the metal elements Au, Fe, Co, and Ni, where 19 valence electrons (VEs) for Au, 16 VEs for Fe, 17 VEs for Co, and 18 VEs for Ni24 were considered. The orbitals associated with the ECP are all Gaussian type 31*G or 31**G; in addition, two f-type polarization functions were added for Au (αf = 0.20, 1.19). The C, N, and S atoms were also treated with ECP using a double-ζ basis set and adding one d-type polarization function,25 whereas for hydrogen, valence-double-ζ basis set plus one p-polarization function was employed.26 We also used the counterpoise correction to avoid basis-set superposition errors (BSSEs) in the calculated interaction energies. The excitation energies were obtained using the time-dependent (TD) perturbation theory approach,27 which is based on the random-phase approximation (RPA) method,28 as implemented in the Turbomole 5.9 program.29 With the aim of gaining more insight into the interaction between different fragments, namely, MPc, ligand (4-MP or 4-ATP), and gold surface (Au26), the Morokuma Ziegler partitioning scheme was employed,30 as coded in the ADF code31 by using the optimized geometries obtained from the Turbomole calculations, as described above. According to this scheme, the interaction energy is partitioned as follows: ΔEint = ΔEpauli + ΔVelstat + ΔEorb, where the ΔVelstat term accounts for the stabilizing electrostatic interaction, and ΔEorb stands for the stabilizing covalent character of the fragment fragment interaction. Triple-zeta STOs plus polarization function were employed as a basis set, where the scalar relativistic effects were taken into account through a two-component zero-order regular approximation (ZORA) Hamiltonian32 in conjunction with the PBE functional (TZP-ZORA/PBE) in the ADF calculation.
3. RESULTS AND DISCUSSION From a theoretical point of view, an understanding of the interaction between a molecule and a metal surface is important for the study of various adsorption processes such as heterogeneous catalysis, desorption, and monolayer on surfaces.33,34 Accurate electronic structure calculation methods have become increasing available. The interaction potentials of molecules on metal surfaces are well-known: the potential at short ranges near the energy minimum is dominated by orbital interactions, that is, Pauli repulsion and covalent bonding, which require a quantum description.35 At longer distances, the main contribution comes from electrostatic, induction, and dispersion (van der Waals) interactions.36 Previously, the adsorption of the thiol compounds 23513
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Table 1. Some Geometric Parameters of the Systems (Distances in angstroms and Angles in degrees) Denoting the Two Closest S Au Distances, S-Ring Distance, Napical-M, and the Tilt Angle of the Organic Ligand in Relation to the Surface systems
S Aua
S Aub
S-øc
Napical-M
tilt angled
Table 3. Natural Population Atomic (NPA) Analysis of the Systemsa systems FePc-4MP-Au26
FePc-4MP-Au26
2.77
2.96
1.80
1.93
169.6°
FePc-4MP CoPc-4MP-Au26
CoPc-4MP-Au26
2.75
3.03
1.79
2.06
166.8°
CoPc-4MP
NiPc-4MP-Au26
2.71
2.93
1.81
2.60
167.1°
NiPc-4MP-Au26
FePc-4ATP-Au26
2.64
2.99
1.82
2.09
156.6°
NiPc-4MP
CoPc-4ATP-Au26
2.59
3.07
1.79
2.39
140.0°
FePc-4ATP-Au26
NiPc-4ATP-Au26
2.61
2.93
1.81
3.52
148.0°
FePc-4ATP
a Closest Au S distance. b Second closest Au S distance. c ø means the aromatic ring of 4-MP or 4-ATP. d Defined as Au26 S-Cø.
Table 2. Interaction Energies (ΔEint) in kilocalories per mole between MPc-Thiolate and Au26 systems
CoPc-4ATP-Au26 CoPc-4ATP NiPc-4ATP-Au26 NiPc-4ATP a
Au
M
Pc
L
S
Nax
0.34
0.83
1.16
0.30
0.36
0.40
0.51
0.82 0.93
1.31 1.01
0.51 0.41
0.31 0.34
0.41 0.48
0.97
1.31
0.66
0.29
0.55
1.02
1.05
0.57
0.38
0.54
0.99
1.27
0.72
0.28
0.58
0.39 0.43
0.88
1.24
0.21
0.35
0.74
0.80
1.54
0.26
0.26
0.71
0.58
0.85
1.24
0.04
0.19
0.75
1.68
0.65
1.06 0.93
0.38 0.24
0.23 0.25
0.74 0.78
0.33
0.21
0.71
1.08
Free Au26 cluster is closed-shell and neutral; see the text.
ΔEint
4MP-Au26 FePc-4MP-Au26
38.7 29.7
CoPc-4MP-Au26
21.6
NiPc-4MP-Au26
35.7
4ATP- Au26
43.5
FePc-4ATP-Au26
35.2
CoPc-4ATP-Au26
30.6
NiPc-4ATP-Au26
41.1
has been investigated on the Au(111) surface using density functional theory.37,38 Here the MPc-L interaction with the gold surface (Au(111)) was modeled by using a finite-sized cluster truncated from the surface, as depicted in Figure 1. The Au26 cluster has been proposed as a reliable model of such surface.21,22 Geometrical Structure. Selected geometric parameters are summarized in Table 1. The obtained S Au distances are close to the typical bond length, denoting a longer distance in the 4-MP systems, compared with the 4-ATP counterparts, depicting a stabilizing S Au interaction of ∼40 kcal/mol.21,22 In all models, the most favorable site is found to be the one with the sulfur nucleus above a three-fold hollow with gold atom in the second layer. This sulfur nucleus is close to two gold atoms of the first layer forming a bridge site (Figure 1). The distance between the apical nitrogen of the anchor ligand (L) and the metal center (Napical-M distance) denotes typical coordination bond length for Fe and Co, denoting a single dative bond. In contrast, in the Ni case, a longer distance is observed, probably due to the formation of a weaker interaction in the Ni complexes. The tilt angle, denoting the coplanarity between the bridging ligand and the gold surface, shows values less than 180°, where the 4-ATP systems denote a marked inclination toward the gold surface. Gold Substrate Interaction. An analysis of the interaction energies between the MPc-L with the gold surface is given in Table 2, denoting the variation of the S Au distance with regards to the MPc moieties involved. The strength of interaction energies follows as: Ni > Fe> Co, which correlates with the S Au26 bond distances described above. It is important to note that the interaction values for the NiPc case are close to those obtained for the ligand Au26 case, that is, without the MPc moiety ( 38.7 kcal/mol for 4-MP and 43.5 kcal/mol for 4-ATP),
denoting the small extent of the NiPc gold substrate interaction mediated by both anchor ligands. To get deeper insight into the electronic charge rearrangement caused by the interaction of MPc moiety with the gold substrate, the natural population atomic (NPA) analysis based on the PBE density was performed for all the series here studied (Table 3). It is possible to observe a charge transfer from the negatively charged anchor ligands (4-MP and 4-ATP) toward the MPc and the gold cluster (Au26) when the MPc-L-Au26 assembly is formed. The charge on gold cluster is mainly due to a charge transfer from the sulfur and bridge ligands. The metal centers (Fe, Co, Ni) show a small change in their charges when the complexes are formed. The Pc ring increases its withdrawing capability when the complexes interact with Au26. These quantities change according to the type of bridge ligand and MPc. According to our results, the gold surface acts as an electronwithdrawing fragment, leading to a charge transfer from the 4-MP and 4-ATP linking ligands. The ligand gold surface interaction (ΔEint) is calculated to be about 38.7 kcal/mol for 4-MP and 43.5 kcal/mol for 4-ATP, with a charge transfer of ∼0.43e, resulting in a favorable interaction with a slightly ionic character (52%32), which retains ∼0.57e into the ligand. The inclusion of the metallophtalocyanine decreases the ligand gold surface interaction to a small extent with a slightly increase in the ionic character (54, 56, and 58% for Fe, Co, and Ni, respectively), denoting that the MPc remain attached to the gold surface via the 4MP and 4ATP ligands, when M = Fe, Co, and Ni. The interaction between the MPc and the ligand decreases when the system is allocated on the gold surface (Table 4), denoting that the ligand orbitals responsible of such and the ligand gold interaction are related, allowing a charge transfer between the MPc and the gold surface through the ligand. The FePc ligand interaction exhibits the highest interaction energy of the studied series, both with and without the gold surface ( 73.8 and 61.0 kcal/mol, respectively), where ∼51% of the stabilizing term concerns to the orbital stabilizing (ΔEorb) contribution. For the case of CoPc and NiPc, this interaction energy decreases because of a lower ΔEorb, representing 42 and 46% of the stabilizing energy, respectively, which in presence of the gold surface turns to 47 and 48%, respectively. The experimental and theoretical studies about the adsorption and desorption of alkanethiolate and thiolate monolayers on 23514
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Table 4. Role of the Gold Surface on the Interaction Energies (ΔEint), in kilocalories per mole, of the Interaction between MPc and the Ligand Interaction systems
ΔEint
FePc-4MP
73.8
FePc-4MP-Au26
61.0
CoPc-4MP
62.4
CoPc-4MP-Au26
27.7
NiPc-4MP NiPc-4MP-Au26
9.8 4.2
FePc-4ATP
75.1
FePc-4ATP-Au26
65.2
CoPc-4ATP
63.4
CoPc-4ATP-Au26
27.3
NiPc-4ATP
10.3
NiPc-4ATP-Au26
6.1
Figure 3. Absorption spectra at different times of CoPc-4MP in DMF solution. In addition, the absorption spectra of CoPc (solid black line) and 4-MP (solid red line) is given.
Figure 2. Absorption spectra at different times of FePc-4MP in DMF solution. In addition, the absorption spectra of FePc (solid black line) and 4-MP (solid red line) is given.
Au(111) have estimated that the energy involved in the process is about 30 40 kcal/mol. Our calculated adsorption energies are between 29 and 36 kcal/mol, which is in quite good agreement with the values reported in the literature.13 In addition, the interaction energy between the axial nitrogen bridge ligands and the metal center of the metallophthalocyanine was revisited (Supporting Information). It can be seen (Table SI-1 of the Supporting Information) that the two Fe complexes display higher interaction energies (60.0 and 51.8 kcal/mol) and shorter Nax-Fe distances. At the same time, in these two complexes the S Au distances are the longest. The interaction energies between N and the other two metals (Co, Ni) are smaller. In particular, those with Ni show the smallest magnitude. With the purpose to quantify the effect of having the bridge ligand protonated and neutral, we carried out a similar calculation with hydrogen bound to sulfur for all systems. The results are given in the Supporting Information (Table SI-2), denoting that the interaction energy with the Au26 cluster give magnitudes
Figure 4. Theoretical (red solid line, Gaussian half-width 0.5 eV) and experimental (dashed line) absorption spectra of the FePc-4MP, whereas black lines correspond to their assignments.
between 5.0 and 10.3 kcal/mol. The closest S Au distances are larger than those depicted by the thiolate complexes, denoting a decrease going from FePc to NiPc. The effect of negative charge on the bridged ligands decreases the interaction energy. For these complexes, the interaction energy falls in the range of weak van der Waals interactions. In the literature, there are studies about the adsorption of several molecules, such as, HSCH3 and HSCH2CH3 in gold clusters at the PBE level, where the reported S Au bond lengths were 2.81 Å (HSCH3) and 2.79 Å (HSCH2CH3), respectively.13b,40 The binding energies are smaller than our results described above, 10.1 and 11.1 kcal/mol, respectively. Optical Properties. The measured absorption spectra of FePc and CoPc in DMF upon the addition of 4-MP at different times are illustrated in Figures 2 and 3. When the FePc and 4-MP molar 23515
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Table 5. TD-DFT Singlet-Excitation Calculations for the Models system FePc-4MP
excitation
λcalc/nm
fa
λexp/nm
contributionb
A
600
0.192
660
113a f 120a (77)
LLCT (πfπ*)
B
571
0.206
600
114a f 120a (35)
MLCT (dxyfπ*)
115a f 119a (28)
MLCT (dyzfπ*)
117a f 128a (34)
LLCT (pzfπ*)
103a f 121a (22)
LMCT (πfdz2)
108a f 120a (25)
LLCT (πfπ*)
100a f 120a (16)
LLCT (σfπ*)
98a f 119a (66) 359a f 371a (34)
LMLCT (π*fπ*+dyz) LMCT (πfAu)
359a f 372a (10)
LLCT (π f π*)
359a f 371a (43)
LMCT (πfAu)
C D
FePc-4MP-Au26
344 335
0.198
310
0.597
E F
335 604
0.149 0.754
G
606
0.734
310 310 530
a
transition type
Oscillator strength. b Values are |coeff.|2 100.
Figure 5. Active molecular orbitals of the FePc-4MP-Au26, denoting the π f π* and π f gold surface transitions.
relationship is 1:1, we observe that the Soret band maximum shifts by 1 nm toward the red (316 nm); besides we observe a small shoulder around 350 nm. Moreover, the maximum of the Q-band (599 nm) shifts by 1 nm toward the blue (598 nm), and it is considered to be an independent signal. We also observe a new shoulder between 598 and 661 nm. When the CoPc and 4-MP relationship is 1:2 the FePc Soret band maximum (315 nm) shifts 4 nm toward the red (319 nm). The detriment of the observed bands in Figures 2 and 3 as a function of time is consistent with complex formation. It can be observed that the addition of the anchor ligand (which exhibits an absorption band at ∼350 nm) leads to a decrease in the Q-band (∼660 nm) and a slight shifts of the Soret bands (∼310 nm) of the isolated FePc or CoPc, denoting an effective complexation toward the MPc showing the appearance of a band at 435 nm. To gain more insight into the evolution of these bands when the SAM is formed, we calculated the allowed spin singlet transition for the FePc-4MP and FePc-4MP-Au26 based on the ground-state structures. The theoretical transitions of the model systems and experimental absorption spectroscopic data (Figure 4) are summarized in Table 5. The active molecular
orbitals in electronic transitions at the PBE level are shown in Figures 5. We were able to find two bands. The first calculated band around 600 nm, which is composed of two transitions that we have identified as A, is consistent with the Q-band of phtalocyanine macrocycle. The second transition at 600 nm, calculated at 571 nm (B), is composed mainly of 113a (π) f 120a (π*) and 114a (dxy) f 120a (π*) transitions. These bands correspond to ligand-to-ligand charge transfer (LLCT) and metal to ligand charge transfer (MLCT), respectively, as can be observed from the active MOs, which are depicted in the Supporting Information. The band located around 310 nm is composed of three transitions, namely, C, D, and E, where the D transition exhibits the highest oscillator strength (Figure 4), which denotes that this transition is the principal transition in the assigned band, showing mainly an LLCT character, denoted by the transitions 108a (π) f 120a (π*) and 100a (σ)f 120a (π). The calculated spectra corresponding to the FePc-4MP-Au26 system show similar transitions in the range between 500 and 900 nm, showing that the strong band around 600 nm is composed of two major transitions, namely, as F and G transitions, respectively. Their assignments are described in Table 5. Interestingly the electronic transition involves a charge transfer from the Pc (π) to the gold cluster, as depicted in Figure 5, in conjunction with the Q-band.
4. CONCLUSIONS The formation of SAM on a gold substrate and a thiolate ligand as an “anchoring” fragment of metallophtalocyanine leads to an interesting charge donation from the 4-ATP or 4-MP toward both gold substrate and phthalocyanine, denoting an effective gold-MPc interaction mediated by the titled anchor ligands (L). The thiolate gold substrate interaction is favored by ∼40 kcal/mol, which decreases when the MPc moiety is included, denoting the influence of the active center into the SAM structure. The interaction between the MPc and the ligand also varies when the modified SAM is formed, showing a slight destabilization with regards to the isolated MPc-L fragment. Both results revealed the influence of the MPc and gold substrate toward the formation of the overall structure, showing that the MPc is connected to the surface effectively by the anchor ligand, which decreases from the Fe system to the Co. In addition, the Ni counterpart denotes the smaller S Au interaction of the series, which is consistent with their low catalytic activity. 23516
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The Journal of Physical Chemistry C The optical properties of the self-assembly metallophthalocyanines complexes supported on the gold electrode in addition to the well-described Q-band involve an interesting charge transfer from the Pc (π) toward the gold surface, as depicted by FePc-4MP-Au26.
’ ASSOCIATED CONTENT
bS
Supporting Information. Interaction energies (ΔEint) in kilocalories per mole between thiole and Au26 and between nitrogen (Nax) and metal of Pc into Au26. Different views of the Au26 cluster. This material is available free of charge via the Internet at http://pubs.acs.org.
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