In Situ Imaging and Computational Modeling Reveal That Thiophene Complexation with Co(II)porphyrin/Graphite Is Highly Cooperative

Abstract

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.

On the origin of cooperativity effects in the formation of self-assembled molecular networks at the liquid/solid interface

“…[21][22][23] The origin of this model stems from the elaborate work and thermodynamic models previously developed for the processes in solution. 20 More recently, the work of Hipps et al has shown that there is a small degree of cooperativity in the binding of small ligands such as methoxypyridine, 1-phenylimidazole or 3-phenylthiophene to a self-assembled network of cobalt(II) octaethylporphyrin (CoOEP) at the 1-phenyloctane/HOPG interface, [24][25][26] While adsorption overall appears to follow a Langmuir isotherm, which would suggest no cooperativity, comparing the fraction of adjacent bound CoOEP molecules with the fraction that would be obtained from a random distribution indicates that cooperativity is present. Additionally, the same group has presented a computational study to determine position-dependent desorption energies for coronene on Au(111) and HOPG as substrates by utilizing the nearest neighbour interaction energy model and showed that it performs well for systems showing weak adsorbate-substrate cooperativity.…”

“…Thus, while the concept of cooperativity has been observed as important in determining the type of network observed on the surface 36,37 and evaluated for several examples, 21,24,[26][27][28]38 a deeper quantitative understanding and thermodynamic investigation of the effect of cooperativity in the formation of SAMNs is still missing.…”