Scanning tunneling microscopy (STM) was used to observe and quantify single-molecule diffusion at the solution/solid interface and at the argon/solid interface. This work investigates the influence of the temperature, solvent, and STM tip on isolated molecular surface diffusion through analysis of the molecular trajectories in sequential STM images. The surface diffusion of Y[C6S-Pc]2 in phenyloctane was found to be thermally activated with almost no motion observed at 5 °C, whereas, above 30 °C molecular motion and/or adsorption/desorption are so rapid that it becomes difficult to track single molecules. The surface diffusion of molecules also depended on solvents; solvents with greater dipole moments (and presumably greater interaction with Au(111)) reduced diffusivity, while the absence of a solvent (i.e., argon/solid interface) increased diffusivity. At room temperature, the influence of the STM tip was quantified by varying the sample bias voltage, with the diffusion coefficient varying between 0.6 × 10–17 and 16 × 10–17 cm2/s. This is the first quantitative study of single-molecule (as opposed to vacancy) diffusion at the solution/solid interface. An important implication of this study is that even in the case of very strong adsorbate–substrate interactions, the STM tip can significantly mobilize surface molecules and thereby enhance the formation of self-assembled films. Moreover, because the tip-induced displacements are not unidirectional, one cannot diagnose tip-induced motion by analyzing the displacements at one set-point and scan rate. Particular care must be taken in any STM-based studies of self-assembly kinetics at the solution–solid interface.
This study explores directed noncovalent bonding in the self-assembly of nonplanar aromatic carboxylic acids on gold and graphite surfaces. It is the first step in developing a new design strategy to create two-dimensional surface metal−organic frameworks (SURFMOFs). The acid molecules used are tetraphenylethene-based and are typically employed in the synthesis of three-dimensional (3D) MOF crystalline solids. They include tetraphenylethene tetracarboxylic acid, tetraphenylethene bisphenyl carboxylic acid, and tetraphenylethene tetrakis-phenyl carboxylic acid. The twodimensional structures formed from these molecules on highly ordered pyrolytic graphite (HOPG) and Au(111) are studied by scanning tunneling microscopy in a solution environment. The process of monolayer formation and final surface linker structures are found to be strongly dependent on the combination of the molecule and substrate used and are discussed in terms of intermolecular and molecule−substrate interactions, bonding geometry, and symmetry of the acid molecules. In the case of linker self-assembly on HOPG, the molecule−substrate interactions play a significant role in the resulting surface structure. When the acid molecules are adsorbed on Au(111), the intermolecular interactions tend to dominate over the weaker molecule−substrate bonding. Additionally, the interplay of π−π interactions and hydrogen bonding that directs the surface selfassembly on different supports can be modified by varying the linker concentration. This is particularly applicable for the case of the acid molecules adsorbing on the Au(111) substrate. Precise control over predesigned surface structures and orientation of the nonplanar aromatic carboxylic linkers open up an exciting prospect for manipulating the direction of SURFMOF growth in two dimensions and potentially in 3D.
Single molecule microscopy can quantifiably probe the dynamics of reversible ligand binding to metalloporphyrin receptors at the solution/solid interface.
Scanning tunneling microscopy (STM) was employed to quantitively investigate in situ binding of 3-phenyl thiophene (PhTh) to Co(II)octaethyl porphyrin (CoOEP) supported on highly ordered pyrolytic graphite (HOPG) in fluid solution. To our knowledge, this is the first single-molecule level study of a complexation reaction between a metalloporphyrin and a sulfur base at the solution/solid interface and one of the few examples of thiophene coordination with a d7 transition metal. Real-time imaging experiments revealed that PhTh binds reversibly to HOPG-supported CoOEP at room temperature. The coordination process increases with increasing PhTh concentration. The nearest-neighbor analysis of STM images indicates that the complexation reaction is cooperative. Because PhTh does not bind to CoOEP in solution, the STM results strongly suggest that the presence of HOPG is crucial to observe ligand binding and cooperativity in this system. Periodic plane-wave density functional theory (DFT) computations corroborate that PhTh has low binding affinity toward CoOEP in solution but predict that the ligand can adsorb to CoOEP/HOPG through coordination with S atoms or interact through noncovalent π–π bonding with the porphyrin chromophore. Three possible structures were considered, and DFT theory was used to calculate binding energies and free energies. In solution and on the HOPG surface both a π–π configuration and a η1(S) configuration have similar computed energies. The η1(S) structure shows the largest stabilization in going from the vapor to adsorbed on HOPG. We also show that statistical analysis of nearest neighbors is more sensitive to cooperative binding than is fitting with the Temkin or Langmuir isotherm. The implication is that isotherm fitting alone is insufficient for identifying cooperative binding on surfaces.
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