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Surface-Confined Metallosupramolecular Architecture: Towards a Novel Coordination Chemistry for the Design of Functional Nanosystems

Final Report Summary - MOLART (Surface-Confined Metallosupramolecular Architecture: Towards a Novel Coordination Chemistry for the Design of Functional Nanosystems)

The primary goal of the MolArt project was to develop a rationale for the design and use of single-molecule devices and supramolecular architectures in intimate contact with solid supports, with special emphasis on interfacial coordination systems. The respective model systems were typcially realized on atomistically well-defined interfaces, including 2D materials and hybrid interfaces. We notably studied and controled individual functional molecules, their surface anchoring and metal-directed assembly systematically with exquisite detail by molecular-level scanning probe microscopy and spectroscopy. The real-space direct observations were complemented by powerful x-ray spectroscopy techniques and computational modeling including first-principles and molecular dynamics methods, also used as stand-alone methods. In close cooperation with chemical synthesis groups de novo designed molecular tectons were explored to engineer low-dimensional molecular nanosystems and complex hierarchic architectures. Furthermore, biologically relevant species, from small amino acids or nucleobases up to entire protein complexes, and model compounds for prosthetic groups, such as metalloporphyrins, were explored regarding their self-assembly and functional properties.
We made significant progress in building intricate surface architectures with tailored organic molecular linkers (cyano, pyridil and other terminations). Two-dimensional nanoporous metal-organic coordination lattices were employed for electron confinement and the caging of supramolecular rotors and dynamers. Moreover, intricate mobility characteristics and insights in single-molecule thermodynamics were obtained using dedicated assembly protocols and nano-architectures. Furthermore, supramolecular arrays provided by 2D assembly were used as templates to create novel organometallic compounds and single-molecule magnetic systems. A series of intricate 2D metallosupramolecular lattices were achieved by flexible pyridyl linkers, notably yielding unprecedented surface-confined glassy networks. In addition, we pioneered lanthanide-directed assemblies, and fabricated novel types of adaptive coordination motifs, and unprecedented semiregular Kepler tilings as well as networks with dodecagonal symmetry, representing the first demonstration of quasicrystalline order in metal-organic superstructures. Finally, novel coordination and photoresponsive interfacial hybrid architectures were achieved, combining metallosupramolecular engineering, molecular switching and photovoltaic properties with 2D sheet and other materials.
The surface anchoring, chemical reactivity and utility for nanoscale devices of tetrapyrrole species has been broadly explored. In situ, on-surface metalation protocols were pioneered, including self-metalation of simple porphyrins and porphines, and the realization of porphyrin sandwich compounds with rare-earth centers. Such species proved to be rotatable and, when arranged in arrays, serve as templates to fabricate switchable fullerene-porphyrin donor-acceptor dyads. Intriguing single-molecule devices were created and operated with the scanning tunneling microscopy tip, whereby single proton transfer events in an open porphyrin macrocycle yield ultimately small atomistic four-level switches. Also the properties of metallotetrapyrroles providing coordinatively unsaturated sites were carefully investigated by probing adduct ligation, e.g. of diatomic gas molecules (NO, O2, ammonia and H2O). The pertaining characteristic electronic and structural changes reflect a so-called surface 'trans'-effect. By contrast, for weakly bound CO adducts an unprecedented cis-dicarbonyl 'rider'-ligation mode occurs, induced by the surface-induced molecular saddle-shape conformation.
Regarding the control and assembly of biologically relevant molecules, we notably explored a series of species, partially exhibiting pronounced chemical and conformational transformations upon thermal activation and striking dynamic phenomema. Cysteine endgroups were morever instrumental for the anchoring of genetically modified photosystem I protein systems between two gold electrodes, which approach allowed to realize integrated and selectively addressable nanoscale photovoltaic devices (electron pumps) wherein the biomolecular functional properties is retained. Further, related systematic introduced unprecedented insights in single-molecule junctions by correlating electronic transport and vibrational or switching characteristics assessed by Raman spectroscopy.
The activities included multiple methodological and instrumental developments, the latter notably including a molecular-junction spectroscopy set-up and a sophisticated electrospray ion-beam source for in vacuo surface-deposition of thermolabile functional species.