ARTICLE pubs.acs.org/JPCC
Following the Metalation Process of Protoporphyrin IX with Metal Substrate Atoms at Room Temperature Ruben Gonzalez-Moreno,†,‡ Carlos Sanchez-Sanchez,‡ Marta Trelka,§ Roberto Otero,§ Albano Cossaro,|| ngel Martín-Gago,‡,^ Alberto Verdini,|| Luca Floreano,|| Marta Ruiz-Bermejo,^ Aran García-Lekue,3 Jose A and Celia Rogero*,† †
Centro de Física de Materiales (CSIC-UPV/EHU), Materials Physics Center MPC, San Sebastian, Spain Instituto de Ciencia de Materiales de Madrid (ICMM CSIC), Madrid, Spain § Instituto Nicolas Cabrera and Instituto Madrile~ no de Estudios Avanzados en Nanociencia (IMDEA-NANO), Universidad Autonoma de Madrid, Madrid, Spain Laboratorio TASC, Istituto Officina dei Materiali (CNR-IOM), Trieste, Italy ^ Centro de Astrobiología (CSIC-INTA), Madrid, Spain 3 Donostia International Physics Center (DIPC), San Sebastian, Spain
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‡
ABSTRACT: We have studied the in situ coordination reaction of porphyrin molecules, particularly protoporphyrin IX (H2PPIX), with copper substrate atoms in ultrahigh vacuum conditions with a combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy. We show that these protoporphyrin IX molecules deposited on Cu surfaces, as Cu(110) and Cu(100), form metalloprotoporphyrin IX (CuPPIX) by incorporation of Cu atoms from the surface already at room temperature. We have followed this reaction as a function of temperature and we have determined intermediate situations at lower temperatures where the physisorbed macrocycle rings present a tendency to establish hydrogen bonding between molecules.
’ INTRODUCTION The interaction of planar metal complexes, such as porphyrins, phthalocyanines, or corroles (tetrapyrrole molecules), with surfaces is especially interesting for designing novel catalysts, sensors, and other devices. Because of their photophysical properties they are good candidates for the construction of photonic devices, such as solar cells and organic light diodes.1,2 Depending on the required applications, the properties of these macrocycles can be tailored, for example, by changing the functional groups around the central core or the metal in the center of the core. On the other hand, the chemical reactivity of the metal core may affect the performance of the application devices, for example, by rapid oxidation of the metal, which makes it necessary to work with passivated molecules, or by loss of the metal by interaction with the surfaces.3 An alternative procedure for handling with these metalomolecules is to start with nonmetalated molecules (free molecules) and metalate them directly on a surface. The incorporation of selected metal atoms into porphyrins and phthalocyanines on the surface,414 also called metalation process, represents an advantage against the direct sublimation of metalomolecules.414 The routes reported in the literature for surface-mediated metalation in ultrahigh vacuum (UHV) conditions involve evaporation of r 2011 American Chemical Society
the metal atoms by vapor deposition in the appropriate stoichiometry before or after the molecular deposition, usually followed by annealing of the system formed by the substratemolecule metal atoms.414 Interestingly, in none of the reported cases was the formation of complexes with the substrate metal atoms detected, although such a mechanism would simplify the surface synthesis of metalloporphyrins and could likely improve some of their properties. Moreover, since the metal core and the substrate atoms are of the same nature, the moleculesubstrate contact can be enhanced. Metalation with the substrate has been suggested but it has never been proven so far. In this work we demonstrate that surface metalation takes place when the protoporphyrin IX (H2PPIX) molecule is deposited on Cu substrates. Depending on the chosen central metal atom, metalloprotoporphyrin IX molecules form biological complexes that are essential for life, for example, hemoglobin (Fe), which is responsible for oxygen transport in animals, and chlorophyll (Mg), which governs energy conversion in the photosynthesis process.15,16 Received: January 18, 2011 Revised: February 22, 2011 Published: March 22, 2011 6849
dx.doi.org/10.1021/jp200533a | J. Phys. Chem. C 2011, 115, 6849–6854
The Journal of Physical Chemistry C
Figure 1. (a) Top and (B) side views of the DFT-optimized ball-andstick model of H2PPIX.
The latter feature is one of the main reasons why this family of molecules is investigated within the context of photoluminescence applications. The H2PPIX molecule is the nonmetalated version of this family of molecules, where two hydrogen atoms replace the central metal one. Its structure is almost planar and the central macrocycle is surrounded only by short methyl and ethenyl groups and two propionic acids (Figure 1). In this work we use X-ray photoelectron spectroscopy (XPS) to investigate the chemical changes observed in H2PPIX when the molecule is interacting directly with metal surfaces, as Cu(110) and Cu(100). These surfaces have been chosen as models for further studies on more realistic ones for applications, as they could be metallic oxides. We study the transition from the physisorbed molecular configuration to the chemisorbed one as a function of the temperature. Thus, we follow the coordination reaction with the substrate atoms, demonstrating that, already at room temperature, H2PPIX molecules adsorbed on Cu surfaces can form Cu-protoporphyrin IX (CuPPIX) in strongly bound self-assembled monolayers (SAMs), as confirmed by scanning tunneling microscopy (STM) images.
’ EXPERIMENTAL DETAILS Growth of Molecules. The growth of the molecular films is performed in UHV, with a base pressure of
