Abstract
The synthesis and surface self-assembly behavior of two types of metal-porphyrin dimers is described. The first dimer type consists of two porphyrins linked via a rigid conjugated spacer, and the second type has an alkyne linker, which allows rotation of the porphyrin moieties with respect to each other. The conjugated dimers were equipped with two copper or two manganese centers, while the flexible dimers allowed a modular built-up that also made the incorporation of two different metal centers possible. The self-assembly of the new porphyrin dimers at a solid–liquid interface was investigated at the single-molecule scale using scanning tunneling microscopy (STM). All dimers formed monolayers, of which the stability and the internal degree of ordering of the molecules depended on the metal centers in the porphyrins. While in all monolayers the dimers were oriented coplanar with respect to the underlying surface (‘face-on’), the flexible dimer containing a manganese and a copper center could be induced, via the application of a voltage pulse in the STM setup, to self-assemble into monolayers in which the porphyrin dimers adopted a non-common perpendicular (‘edge-on’) geometry with respect to the surface.
Highlights
Metal-porphyrins occur ubiquitously in nature, e.g., as magnesium porphyrin pigments in the light harvesting system [1,2] and as iron porphyrins in the blood protein hemoglobin [3] and the oxidation enzyme cytochrome P450 [4,5]
Our group has extensively investigated the catalytic properties of Mn(III) porphyrins in the epoxidation of alkenes [12,13,14,15,16]. The majority of these studies have been carried out using supramolecular systems in solution, and in order to obtain more detailed information about the reaction mechanisms during catalysis, we have investigated the behavior of single porphyrin molecules adsorbed at a surface
A porphyrin dimer with a rigid aromatic linker was readily synthesized in five steps, and its bis-copper and bis-manganese complexes were obtained nearly quantitatively from the free base precursor
Summary
Metal-porphyrins occur ubiquitously in nature, e.g., as magnesium porphyrin pigments in the light harvesting system [1,2] and as iron porphyrins in the blood protein hemoglobin [3] and the oxidation enzyme cytochrome P450 [4,5]. Our group has extensively investigated the catalytic properties of Mn(III) porphyrins in the epoxidation of alkenes [12,13,14,15,16] The majority of these studies have been carried out using supramolecular systems in solution, and in order to obtain more detailed information about the reaction mechanisms during catalysis, we have investigated the behavior of single porphyrin molecules adsorbed at a surface. To this end, we constructed self-assembled monolayers of metal-porphyrins at a solid–liquid interface and investigated their properties with scanning tunneling microscopy (STM) [17,18,19,20].
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