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Ordered Monolayers of Free-Standing Porphyrins on Gold

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journal contribution
posted on 13.08.2014, 00:00 by Franziska L. Otte, Sonja Lemke, Christian Schütt, Nicolai R. Krekiehn, Ulrich Jung, Olaf M. Magnussen, Rainer Herges
The controlled attachment of chromo­phores to metal or semi­conducting surfaces is a prerequisite for the construction of photonic devices and artificial surface-based light-harvesting systems. We present an approach to mount porphyrins in ordered monolayers on Au(111) by self-assembly and verify it, employing STM, absorption spectroscopy, and quantum chemical calculations. The usual adsorption geometry of planar chromophores, flat on the surface or densely packed edge-on, is prevented by mounting the porphyrins upright on a molecular platform. An ethynyl unit as spacer and pivot joint provides almost free azimuthal rotation of the unsubstituted porphin. However, rotation of the larger tri­phenyl­porphyrin unit is sterically restricted: because the diameter of the substituted porphyrin is larger than the distance to its next neighbors, the phenyl substituents of neigboring molecules interact by dispersion force, which leads to an alignment of the azimuthal rotators.