8988
J. Phys. Chem. C 2008, 112, 8988–8994
Molecular Conformation, Organizational Chirality, and Iron Metalation of meso-Tetramesitylporphyrins on Copper(100) ´ cija,† Marta Trelka,† Christian Urban,† Paula de Mendoza,§ Eva Mateo-Martı´,| David E Celia Rogero,| Jose´ A. Martı´n-Gago,|,⊥ Antonio M. Echavarren,§ Roberto Otero,†,‡ Jose´ M. Gallego,*,⊥ and Rodolfo Miranda†,‡ Departamento de Fı´sica de la Materia Condensada, UniVersidad Auto´noma de Madrid, 28049 Madrid, Spain, Instituto Madrilen˜o de Estudios AVanzados en Nanociencia (IMDEA-Nanociencia), Madrid 28049, Spain, Institute of Chemical Research of Catalonia (ICIQ), AV. Paı¨sos Catalans 16, 43007 Tarragona, Spain, Centro de Astrobiologı´a (CSIC-INTA), 28850 Madrid, Spain, and Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Madrid, Spain ReceiVed: February 14, 2008; ReVised Manuscript ReceiVed: March 26, 2008
We report on the conformation and self-assembly properties of meso-tetramesitylporphyrin on Cu(100). The results show that the presence of the mesityl groups limits the interaction between the porphyrin ring and the copper surface, contributing to the high porphyrin mobility at room temperature. At low temperatures it is the substrate which determines the molecule orientation. The intermolecular interaction is also very weak, and only for high coverages do the porphyrins self-assemble to form large islands with two different mirror symmetric unit cells. The porphyrins can be Fe metalated by sublimation of Fe at room temperature on a porphyrin overlayer deposited on the copper surface. Introduction The controlled study of biomimetic systems is of paramount importance in order to understand the fundamental processes that govern biological systems. In this sense, ordered layers of organic compounds on solid surfaces can be used as model systems to study chemical reactivity or energy transfer mechanisms. Specifically, porphyrins and metalloporphyrins are key components in many biological processes, and Fe porphyrins in particular are of special interest as they are at the core of hemoglobin, the protein in the red blood cells responsible for the transport of oxygen.1,2 Morever, porphyrin derivatives have been used in chemical sensors,3 molecular wires,4,5 information storage memory devices,6 catalysts,7 solar cells,8 etc. The behaviors of different metalloporphyrin systems may be significantly altered by adsorption on metal surfaces. Therefore, the observation of their surface structures is of fundamental importance to fully understand the effect of structural and conformational changes on their reactivities. Since the first scanning tunneling microscopy (STM) observation of an ordered adlayer of a porphyrin derivative (5,10,15,20tetrakis-(N-methylpyridinium-4-yl)-21H,23H-porphine, TMPyP) in electrolyte solutions on the iodine-modified Au(111) surface,9 many other porphyrins have been studied by STM, both in solution and under ultrahigh vacuum (UHV) conditions,10–12 on a wide variety of substrates. It has since then been shown that adsorption on solid surfaces can alter porphyrin conformations, which may in turn influence their chemical and physical properties. For example, in the case of the most studied tetraarylporphyrin derivatives, the dihedral angle between the * Corresponding author. Telephone: + 34 91 3349090. Fax: + 34 91 3720623. E-mail:
[email protected]. † Universidad Auto ´ noma de Madrid. ‡ IMDEA-Nanociencia. § Institute of Chemical Research of Catalonia (ICIQ). | CSIC-INTA. ⊥ Instituto de Ciencia de Materiales de Madrid.
Figure 1. Top and side views of the minimum-energy gas-phase conformation (θ ∼ 61°) of meso-tetramesitylporphyrin (TMP). The graph shows how the molecular energy depends on the dihedral angle θ between the phenyl rings and the porphyrin core. The vertices of the two drawn rectangles mark the positions of the C atoms in the methyl groups that stay above (continuous line) and below (dashed line) the average porphyrin plane.
porphyrin ring and the meso aryl substituents, in the solid state, ranges around 60-90°,13,14 due to a competition between the steric repulsion of ortho substituents on the aryl with the peripheral hydrogens of the pyrrole units, and the effect of the π overlap with the porphyrin ring. The influence of the surface can modify this angle noticeably due to the π-metal interaction and the atomic corrugation and geometric structure of the surface.11,15,16 Regarding the self-assembly properties, the final twodimensional arrangement of an ordered porphyrin layer on a solid surface is the result of a competition between porphyrinporphyrin and porphyrin-substrate interactions. When the functional groups of the peripheral substituents allow the formation of strong directional bonds, these dominate the arrangement, generally giving rise to rather open structures;17,18
10.1021/jp801311x CCC: $40.75 2008 American Chemical Society Published on Web 05/21/2008
Self-Assembly of TMP on Cu(100)
J. Phys. Chem. C, Vol. 112, No. 24, 2008 8989
Figure 2. STM images, taken at 150 K, of the Cu(100) surface after deposition of ∼0.2 ML of TMP with the substrate held at 300 K. The top inset in (b) shows the atomic lattice of the Cu substrate, at the same scale as (b). The bottom inset shows an enlarged view of an isolated porphyrin, with an schematic drawing of the molecular structure superimposed. (a) I ) 0.20 nA; V ) -3.5 V; (b) I ) 0.25 nA; V ) -2.1 V.
otherwise, the molecules tend to form close-packed arrangements. In many reported cases it is the substrate which determines the molecular orientation. Here we report on the growth and two-dimensional selfassembly of an ortho-substituted tetraphenylphorphyrin, in particular, the meso-tetrakis(2,4,6-trimethyl)phenylporphyrin (also called meso-tetramesitylporphyrin, TMP) on Cu(100), with the purpose of studying the assembling behavior but also how the metal surface can modify the conformation of a more rigid porphyrin than previously studied. In this molecule, the rotation of the mesityl groups around the σ bonds is limited, both in solution and in the solid state, due to the presence of the methyl groups in the ortho positions, and this limits the extent of the distortion of the porphyrin core;19,20 the mesityl groups also contribute to the solubility of the porphyrins, since they are less able to undergo cofacial aggregation.20 Figure 1 shows the molecular gas-phase conformation of TMP calculated by molecular mechanics using the MM+ force field of the HYPERCHEM 7.5 molecular modeling package. In the optimized structure the mesityl groups rotate alternately an angle θ ∼ ( 61° around the σ bonds (the graph in Figure 1 shows how the energy of the molecule depends on this angle), resulting in a rectangular symmetry. In this way, four methyl groups (one in each mesityl group) stay above the mean porphyrin plane, forming a raised rectangle of 5.0 Å × 8.4 Å, while another four remain below this plane. In addition, the porphyrin core is nonplanar, but slightly saddle-shaped, with the pairs of C atoms forming the outer edge of each pyrrole ring alternately displaced by ∼0.3 Å above and below the mean porphyrin plane. Experimental Section The growth of the molecular films and the STM measurements were carried out in an UHV chamber, with a base pressure of
