Scanning Probe Studies of Porphyrins and Ionic Liquids on Metal Surfaces
In this thesis, STM and nc-AFM disclose a tremendous richness of detail and a deep degree of comprehension on the molecular level. Both techniques yield real space information, which was applied to investigate the adsorption and reactivity of complex porphyrin derivatives and ionic liquids on different metal surfaces prepared under UHV conditions at or below RT. The publication [P1] addresses the adsorption behavior of three related cyano-functionalized tetraphenylporphyrin derivatives, namely Cu-TCNPP, Cu-cisDCNPP, and 2H-cisDCNPP on Cu(111) by STM as a function of temperature combined with DFT calculations in order to identify the role of the cyano groups and the central Cu atom. Cu-TCNPP forms a hexagonal honeycomb-type structure at RT that coexists with 1D molecular chains and completely transforms to a long-range ordered hexagonal honeycomb-type structure at 400 K. Cu-cisDCNPP forms flakes at RT, including mono-, bi-, and multiflakes, and then transforms to a hexagonal honeycomb-type structure at 400 K. 2H-cisDCNPP shows no ordered structure at RT, but orders after self-metalation upon heating to 400 K. Thus, all three molecules form the same long-range triangular porous hexagonal honeycomb-type structure, in which molecules adopt “saddle-shape” conformation. The observed structures share the same structure-forming element, that is, porous porphyrin triangles (distance of 3.1 nm) fused together via CN-Cu-NC interactions with Cu adatoms. Three porphyrin molecules are rotated by 60° to each other, forming regular triangular pores with Cu adatoms located at the corners (not visible in STM). Complementing STM, DFT calculations offer more insight into various energetic contributions leading to the thermodynamically stable hexagonal honeycomb-type structure. In addition, the internal structure of the unit cell indicates that cyano-phenyl groups (n = 2) in “cis” position are the minimum prerequisite to form a highly ordered 2D porous molecular pattern. To expand the understanding of the influence of linker groups (cyano- and isoindole (benzopyrrole) groups), publication [P2] addresses the adsorption behavior and structure formation of the novel cyano-functionalized benzoporphyrin 2H-TCNPTBP on Cu(111) by STM as a function of temperature. 2H-TCNPTBP, with four cyano- and four isoindole groups, shows the coexistence of three types of network structures, namely, a Kagome lattice structure, a quadratic pattern, and a hexagonal structure at RT. All three structures have molecules in a “saddle-shape” conformation. The Kagome and quadratic structures are porous with different pore sizes (diameters: ∼3 nm and ∼1.5 nm respectively), are stabilized by CN-Cu-NC bonds with Cu adatoms, and both have a molecular density of 0.18 molecules/nm2. The close-packed hexagonal structure has a molecular density of 0.42 molecules/nm2 and is stabilized with much weaker intermolecular H-bonds and dipole-dipole interactions of oppositely oriented cyano end groups. While the two Cu-coordinated porous structures (Kagome and quadratic) with same molecular density are stabilized by the energy gain due to the network formation (CN-Cu-NC), the hexagonal structure compensates the weaker intermolecular interactions (H-bond) by a factor of 2.3 higher molecular density. Heating to 450 K yields an immobile species with a “clover-shape”, which is attributed to the dehydrogenation and the formation of intramolecular C-C bonds between the isoindole and the phenyl groups. This finding suggests that cyano functionalization of benzoporphyrins results in unusual 2D self-assembled lattice structures. After gaining deeper insight into the role of linker groups, publication [P4] addresses the adsorption and self-assembly of exceedingly complex mixed benzoporphyrin derivatives 2H-diTTBP(x)BPs on Ag(111), Cu(111), and Cu(110) by STM as a function of temperature to provide further information on fundamental aspects. The mixture contains six different 2H-diTTBP(x)BPs with x = 0, 1, 2 (cis, trans), 3, and 4. On Ag(111), a long-range ordered 2D square phase was observed, which is stable up to 400 K. On Cu(111), an identical square phase coexists with a stripe phase, which disappears at 400 K. In contrast, on Cu(110) 2H-diTTBP(x)BPs adsorb as isolated molecules or dispersed short 1D chains (two to three molecules) along the < 11F0 > substrate directions, which remain intact up to 450 K. Notably, high-resolution STM images on all surfaces allow for the identification of different 2H-diTTBP(x)BPs. A “crown-shape” quadratic conformation on Ag(111) and Cu(111), an additional “saddle-shape” on Cu(111), and an “inverted” structure and quadratic appearance on Cu(110) were deduced. The different conformations are attributed to the different degree of interactions between the iminic nitrogen atoms of the isoindole and pyrrole groups and the substrate atoms on the three surfaces. Further, stabilization of the ordered 2D structures on Ag(111) and Cu(111) plus 1D short chains on Cu(110) is attributed to van der Waals interactions between the tert-butyl and phenyl groups of neighboring molecules. In addition, the thermal stability of the “crown-shape” conformation on Ag(111) and Cu(111) plus intact isolated “inverted” molecules on Cu(110), indicate that no reaction occurred. However, the loss of stripe phase on Cu(111) at 400 K implies a reaction, the nature of which is difficult to determine. A simple self-metalation with Cu atoms is unlikely, as metalated porphyrins exhibit island formation. Further, the presence of an intact “inverted” structure on Cu(110) indicates the absence of self-metalation. Moreover, self-metalation cannot be ruled out for molecules with four almost quadratically arranged protrusions. Overall, these findings show that the long-range 2D order or 1D short chains are stabilized by the outer periphery of the molecules rather than the substrate or the number of isoindole groups. Following the detailed characterization of the adsorption behavior of highly complex porphyrin derivatives on different metal surfaces, publication [P3] addresses the adsorption and reaction behavior of the ionic liquid [C1C1Im][Tf2N] on Cu(111) by nc-AFM, STM, and complementary XPS in combination with DFT calculations. Understanding the IL-substrate interface is of utmost importance for further developing the SCILL approach. [C1C1Im][Tf2N] on Cu(111) was deposited using two different preparation routes: either at RT followed by very fast cooling to 110 K, or directly at low temperature (< 160 K). At 200 K, IL films self-assemble into highly ordered islands (stripe phase) with intact cations and anions arranged next to each other. DFT calculations reveal two identically oriented cations nearly perpendicular to the Cu surface and two anions with different orientations, in line with the distinct contrast of anions observed in nc-AFM at 200 K and an adsorption energy (from DFT) per ion pair of 3.5 eV. Extended heating to 300 K triggers the stripe phase to evolve first into a hexagonal phase, and then into a porous honeycomb structure that coexists with many small, disordered islands. The hexagonal structure is a transition structure from the stripe phase to the honeycomb phase. The honeycomb structure is proposed as an oppositely orientated stacked “sandwich structure” of intact anions and cations. DFT calculations support the proposed structure and show that it is stable on the surface with an adsorption energy of 3.1 eV. The chemical composition measured by complementary XPS reveals that intact anions and cations are adsorbed next to each other at 200 K, and that no IL desorption occurs until 300 K. A large fraction of the IL is transformed into a new dissociated species at around 275 K and increases with time at 300 K, as evidenced by XPS. The decomposition products appear as disordered islands in nc-AFM and STM. Upon heating to 350 K, only small, disordered islands are observed by nc-AFM, and XPS indicates a complete decomposition of the IL on the surface. In summary, this thesis advances the molecular level understanding yielding novel detailed insights into the adsorption behavior of complex porphyrin derivatives and ionic liquids on different single crystal metal surfaces, with a focus on the specific roles of molecule-molecule and molecule-substrate interactions. The fundamental understanding obtained from the investigated systems in publication [P1-P4] might grant protocols as nanoscale templates for developing molecular devices or catalytic concepts.