ARTICLE pubs.acs.org/Langmuir
Self-Assembled Monolayers of Disulfide Cu Porphyrins on Au Surfaces: Adsorption Induced Reduction and Demetalation M. Veronica Rivas,† Lucila P. Mendez De Leo,† Mariana Hamer,‡ Romina Carballo,‡ and Federico J. Williams†,* †
Departamento de Química Inorganica, Analítica y Química Física, INQUIMAE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina ‡ Departamento de Química Analítica y Fisicoquímica, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Buenos Aires, Argentina. ABSTRACT: Metalloporphyrin molecules have a wide range of potential applications in diverse technological areas ranging from electronics to optoelectronics, electrochemistry, photophysics, chemical sensors, and catalysis. In particular, selfassembled monolayers of porphyrin molecules have recently attracted considerable interest. In this work we have studied for the first time the self-assembly of a novel Cu deutero porphyrin functionalized with disulfide moieties using electrochemical techniques, UV vis absorption spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and photoelectron spectroscopies (XPS and UPS). Experimental results indicate that the molecule adsorbs retaining its molecular integrity without forming molecular aggregates via the formation of Au S covalent bonds. Furthermore, the monolayer consists of a packed array of molecules adsorbed with the plane of the porphyrin molecule at an angle of around 30° with respect to the surface normal. Interestingly, adsorption induces reduction of the Cu center and its consequent removal from the center of the porphyrin ring resulting in porphyrin demetalation. Our results are important in the design of self-assembled monolayers of metallo porphyrins where not only blocking of the metal center by the functional groups that drive the self-assembly should be considered but also possible adsorption induced demetalation with the consequent loss in the properties imparted by the metal center.
’ INTRODUCTION Organic surfaces play a major role in material science thus a great diversity of methods have been employed to prepare organic thin films on solid surfaces. Functionalized self-assembled monolayers (SAMs) have attracted much attention because of their potential application in chemical sensors, biosensors, molecular switches, molecular electronics, etc.1 3 SAMs provide a convenient, flexible, and simple way to tailor the interfacial properties of metals, metal oxides, and semiconductors. SAMs are organic assemblies which organize spontaneously and are formed by the adsorption of molecular constituents from solution or the gas phase onto solid surfaces. The molecules that form SAMs have a chemical functionality with a specific affinity for the substrate. In particular organothiols are well suited for fabricating structurally well-defined adlayers of monolayer thickness on gold, silver, palladium, and platinum3 surfaces. It is well-known that organosulfur compounds react with gold surfaces4 forming S Au covalent bonds resulting in monolayer molecular assemblies. These ultrathin monolayers expose an organic surface with properties that can be tailored by varying the type of organothiol employed. Porphyrins are a ubiquitous class of naturally occurring molecules involved in a wide variety of important biological processes. The common feature of all of these molecules is the r 2011 American Chemical Society
basic structure of the porphine macrocycle, which consists of a 16-atom ring containing four nitrogen atoms, obtained by linking four tetrapyrrolic subunits with four methine bridges (see Figure 1 below). The size of the macrocycle is perfect to bind a great number of metal ions which can be inserted in its center forming metalloporphyrins. The diversity of their functions is in part due to the variety of metals that bind to the porphyrin ring. Porphyrin rings are found in a variety of important biological systems where they are the active site of numerous proteins, whose functions range from oxygen transfer and storage (hemoglobin and myoglobin) to electron transfer (cytochrome c and cytochrome oxidase) and energy conversion (chlorophyll). The involvement of porphyrins in many biological processes and the possibility to tailor their physical and chemical properties at the molecular level, including very large dipole moments, polarizability, nonlinear optical response, energy transfer, and catalytic properties, make porphyrins and metalloporphyrins extremely versatile synthetic base materials. Therefore porphyrinbased films on metal or semiconductor surfaces5 as well as orgReceived: June 3, 2011 Revised: July 28, 2011 Published: July 29, 2011 10714
dx.doi.org/10.1021/la2020836 | Langmuir 2011, 27, 10714–10721
Langmuir
Figure 1. Cu deutero IX porphyrin disulfide (CuPorSS).
anized assemblies of porphyrins on metal surfaces have attracted recent considerable attention,6 8 mainly because of the strong characteristic UV/vis absorption features of porphyrin chromophores9,10 plus its potential application in light harvesting devices,11 chemical sensors,12 and photocatalysts13 and as models for electron transfer reactions.14 The key in the implementation of hierarchical assembly of porphyrins for the construction of molecular-based devices is the ability to readily create assemblies that can be reliably organized and attached to surfaces with a controlled molecular structure. The most widely employed method to bind porphyrin molecules to metal surfaces has been the formation of SAMs via the derivatization of porphyrins with thiol groups despite the fact that the thiol functional groups can coordinate the central metal ion of the porphyrin, resulting in multilayer formation as well as the blocking of the catalytic center.10 Other strategies involve (i) preparation of SAMs containing imidazole-terminated adsorbates which bind covalently to a series of metalloporphyrins,15 (ii) covalent attachment of metalloporphyrins to dimercaptoalkane modified gold electrodes,16 (iii) formation of a pyridinethiol SAM followed by axial attachment of the metaloporphyrin,17 and (iv) funtionalization of metalloporphyrins with disulfide groups.18 In the work presented here the Cu deutero IX porphyrin (CuPorSS) macrocycle bearing a disulfide moiety shown in Figure 1 was synthesized as a platform for the rapid, high yield attachment of these molecules to gold surfaces. Adsorption of CuPorSS over Au surfaces results in the formation of thiol tethered porphyrin self-assembled monolayers. This surface assembly was characterized utilizing X-ray and UV photoelectron spectroscopies (XPS and UPS), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS), UV visible absorption spectroscopy, and electrochemical measurements. We found that CuPorSS forms a pack array of molecules that is bonded to the Au substrate via two covalent Au S bonds, with the thiolated legs close to perpendicular to the surface and the porphyrin ring at an angle of around 30° with respect to the surface normal. Furthermore, adsorption results in reduction of the Cu metal ion followed by its consequent removal from the center of the porphyrin ring. As a result these monolayers are not stable in aqueous solutions as Cu ions leached away from the Au surface.
’ EXPERIMENTAL SECTION Materials. Cu (II) deutero IX Porphyrin, cystamine.2HCl, N-acetylcysteamine, and (3-aminoporpyl)-trietoxysilan 99% (APTES) were
ARTICLE
obtained from Sigma-Aldrich. Tetrahydrofuran (THF), iso-butyl chloroformate, and triethanolamine were purchased from Merck. Ethylene chloride, methanol, and absolute ethanol were of analytical grade. Porphyrin Disulfide Synthesis and Characterization. The Cu porphyrin with the disulfide moiety was prepared from Cu deutero porphyrin as described in the literature with minor modifications.18 In short, the carboxylic acid functions of the Cu(II) deutero porphyrin were activated by treatment with iso-butyl chloroformate in THF in the presence of triethyl amine followed by addition of cystamine. In the same way, the metal free deutero IX porphyrin (PorSS) with the disulfide moiety has been synthesized. CuPorSS was characterized by matrixassisted laser desorption/ionization time of flight (MALDI TOF) resulting in the expected molecular weight of 688 g/mol. Furthermore PorSS was characterized by 1H NMR (300 MHz; CDCl3) resulting in the following signals: δ (ppm) = 3.28 (2t, 2xCH2CH2CONHR); 3.51; 3.59; 3.61; 3.63 (4s, 4xCH3); 4.25 (4t, NHCH2CH2S SCH2CH2NH); 4.27 (2t, 2xCH2CH2CONHR); 8.95 (2s, 2 H); 9.86; 9.90; 9.92; 10.11 (4s, 4 H meso). It should be noted that the NMR signals of the NH groups at the center of the porphyrin ring are lost due to the tautomerism observed in all porphyrins. Preparation of Au Substrates. Silicon (100) substrates were coated with a 20 nm titanium and 20 nm palladium adhesion layer and a 200 nm gold layer, thermally evaporated with an Edwards Auto 306 vacuum coating system at P < 10 8 bar. Au surfaces were cleaned electrochemically by cycling the electrode potential in 2 M sulfuric acid between 0 and 1.6 at 0.1 V s 1. The electrochemically active areas were calculated from the reduction peak of gold. Semitransparent gold substrates used for UV vis experiments were prepared according to Wanunu et al.8 Briefly, glass substrates were first cleaned with piranha solutions for 20 min, then treated with 1:1:2 H2O2:NH3:H2O at 70 °C for 20 min, followed by copious rinsing with water and methanol. The substrates were then shaken in a 10% solution of APTES in methanol for 3 h, rinsed and sonicated in methanol, and dried under a N2 stream. Glass substrates were coated with a 15 nm gold layer thermally evaporated and annealed (200 °C, 20 h). Prior to use the substrates were cleaned by UV/ozone (UVO Cleaner model 42, Jelight) for 15 min. Monolayer Preparation. Self-assembled monolayers of CuPorSS were prepared in liquid phase by immersing the gold substrate into a freshly prepared 100 μM solution in methylene chloride for 24 h at room temperature in the absence of light. A final and careful rinsing was made with methylene chloride before drying under N2. SAMs of N-acetylcysteamine were prepared in liquid phase by immersing the gold substrate into a freshly prepared 100 μM solution in absolute ethanol for 24 h at room temperature in the absence of light. A final and careful rinsing was made with ethanol before drying under N2. Photoelectron Spectrocoscopy. XPS and UPS measurements were performed under UHV conditions (base pressure 933.7 eV.39 Figure 4D shows the Cu 2p3/2 spectra corresponding to (i) CuPorSS thick films evaporated over Au surfaces, (ii) CuPorSS SAM over Au surfaces, and (iii) CuPorSS SAM after immersion in a water solution. The spectrum corresponding to the CuPorSS thick films shows a peak at 934.3 eV corresponding to the presence of Cu2+ atoms at the center of the porphyrin ring. This is the expected oxidation state for Cu metalloporphyrins given that Cu2+ is a d9 ion with a preference for a square planar geometry and little tendency to add axial ligands.40 On the other hand, the d10 Cu+ metal ion prefers a tetrahedral geometry,41 a state the rigid porphyrin ligand can never adapt. Furthermore, the relative good fit of the Cu2+ ion into the porphyrin center also explains the lack of any Cu+ porphyrin complexes. Moreover, it has been reported that reduction of Cu2+ in acidic solutions (pH
