First observation of capping/uncapping by a ligand of a Zn porphyrin adsorbed on Ag(100) Federico J. Williams, Owain P. H. Vaughan, Kerry J. Knox, Nick Bampos and Richard M. Lambert* Department of Chemistry, University of Cambridge, Cambridge, UK CB2 1EW. E-mail: rml1@cam.ac.uk; Fax: +44 (0)1223 336362; Tel: +44 (0)1223 336467 Received (in Cambridge, UK) 17th May 2004, Accepted 7th June 2004 First published as an Advance Article on the web 28th June 2004 A significant first step towards creation of catalytically active porphyrin-functionalised metal surfaces has been achieved. The structure and properties of metalloporphyrins adsorbed at solid surfaces are of great topical interest due to the distinctive chemical and electronic properties of these versatile molecules. Self- assembled layers have been investigated as potential light- harvesting arrays, optical switches and photonic wires. 1 Supramo- lecular arrays are of interest with respect to the study of energy transfer and photosynthetic mechanisms; 2 two-dimensional assem- blies have attracted much attention for application as sensors or other types of molecular devices. 3 Here we are concerned with a different aspect of this field—the possibility of harnessing the intrinsic chemistry of a tethered metalloporphyrin to the com- plementary chemistry of the surface upon which it is adsorbed. The goal is to devise heterogeneous systems that are effective in the catalysis of delicate organic reactions (e.g. alkane hydroxylation, alkene epoxidation) that currently can only be carried out by means of homogeneous catalysis. The present communication describes a first step in this direction, namely the creation of a porphyrin- functionalised catalytically significant (silver) surface that exhibits ligand binding/unbinding reactions characteristic of the free metalloporphyrin. In solution, zinc porphyrins and the bidentate ligand DABCO are capable of reversibly forming a variety of supramolecular com- plexes. 4 Here, by means of STM, we show that this homogeneous chemistry can be mimicked by Zn-TBPP (Scheme 1) adsorbed at the Ag(100) surface. The structure of the porphyrin was deliber- ately chosen so as to minimise interaction between the macrocycle and the metal surface precisely with the object of avoiding the formation of dense highly ordered structures. Such dense layers would block the co-adsorption of reactants on the metal surface and hence would be catalytically inert towards the kinds of processes that are our goal—metal-mediated porphyrin-catalysed reactions. In other words, the structures we aim for are of just the opposite kind to those that are usually investigated. Experiments were carried out in an Omicron UHV STM. Zn- TBPP was deposited by evaporation under UHV (source tem- perature 573 K) onto the clean atomically flat Ag(100) surface held at 298 K. This resulted in nucleation and growth of 3D aggregates of Zn-TBPP molecules with dimensions of the order of 20 nm 3 20 nm. Subsequent annealing to 523 K for 60 min served to distribute the Zn-TBPP molecules across the surface. A typical result is illustrated in Fig. 1 which shows the STM image acquired at 298 K. No significant changes in image contrast as a function of tip bias or polarity were observed. The individual Zn-TBPP molecules are observed as symmetric four-lobed entities, each lobe corresponding to one of the four di-t- butylphenyl groups. 5 The molecules exhibit an apparent height of ~ 0.8 Å and an apparent diameter of ~ 20 Å, in agreement with observations on related systems. The adsorption geometry corre- sponds to the porphyrin ring being oriented parallel to the Ag surface (see top curve in Fig. 2(D) which is a line scan through a single molecule). Note that the molecules are distributed randomly on the terraces with no strongly preferred orientation with respect to the underlying metal lattice, despite the fourfold rotational symmetry of both adsorbate and substrate. Neither is there any apparent long range order. The same general behaviour was found over the entire range of coverage. This behaviour, which is exactly what is desired, is consistent with the di-t-butylphenyl groups at the meso positions lying essentially orthogonal to the plane of the macrocycle, thus inhibiting strong electronic interaction between the molecular p electron system and the Ag surface. Figs. 2(A)–(C) show a sequence of images of Zn-TBPP covered Ag(100) taken after exposure at 123 K to increasing amounts of DABCO. Fig. 2(A) shows the result of dosing the porphyrin covered surface with 50 L (1 L = 1.33 3 10 26 mbar s) of DABCO, (B) was acquired after a further exposure of 140 L and (C) shows the result of warming system (B) to 298 K. (Control experiments showed that DABCO does not adsorb on clean Ag(100) at 123 K.) The sequence (A)–(B)–(C) could be repeated many times on the same porphyrin covered surface. It is clear that treating the adsorbed porphyrin with DABCO results in a reaction that is reversible with temperature. The circles indicate three distinct types of features with similar lateral dimensions, but exhibiting different z-contrast. Typical height profiles of these features are shown in Fig. 2(D). All three entities are characterised by apparent diameters Scheme 1 Structures of 1,4-diazabicyclo[2.2.2]octane (DABCO) 1 and of Zn tetra-[3,5-di-t-butylphenyl]porphyrin (Zn-TBPP) 2. Fig. 1 STM image of Zn-TBPP molecules adsorbed on Ag(100) at 298 K, V = 0.75 V, I = 0.1 nA, 200 Å 3 200 Å. This journal is © The Royal Society of Chemistry 2004 DOI: 10.1039/b407374f 1688 Chem. Commun., 2004, 1688–1689