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Understanding the interactions between molecules on surfaces is crucial for advancing technologies in sensing, catalysis, and energy harvesting. In this study we explore the complex surface chemistry resulting from the interaction of Co(II)octaethylporphyrin (CoOEP) and iodine, I2, both in solution and at the phenyloctane/HOPG interface. In pursuit of this goal, we report results from electrochemistry, NMR and UV-Vis spectroscopy, X-ray crystallography, scanning tunneling microscopy (STM), and density functional theory (DFT). Both spectroscopic methods of analysis confirmed that at and above the stoichiometric ratio of one CoOEP to one I2 the reaction product was metal centered CoIII(OEP)I. X-ray crystallography verified that a single iodine is bonded to each cobalt ion in the triclinic, P-1 system. The surface chemistry of CoOEP and I2 is complicated and remarkably dependent on the iodine concentration. STM images of CoOEP and I2 in phenyloctane on highly oriented pyrolytic graphite (HOPG) at low halogen concentrations (1:<2 Co:I ratios) presented random individual Co(OEP)I molecules weakly adsorbed onto a hexagonal (HEX) CoOEP monolayer. Images of 1:2 Co:I ratio solutions, showed phase segregated HEX CoOEP and pseudo-rectangular (REC) Co(OEP)I incorporating one solvent molecule per Co(OEP)I. The REC structure formed in long parallel rows with the number of rows increasing with increasing solution I2. In this case, the presence of CoOEP on the surface was attributed to the spontaneous reduction of Co(OEP)I by the graphite substrate. DFT calculations indicate that the REC Co(OEP)I:PhO form is energetically more stable than the HEX form of Co(OEP)I on HOPG. Experimental STM images and DFT calculated adsorption energies and STM images support our interpretation of the observed structures.
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Cooperativity and coverage dependent molecular desorption in self-assembled monolayers: computational case study with coronene on Au(111) and HOPG
One of the common practices in the literature of molecular desorption is the comparison of theoretically (mostly using DFT) calculated single molecule adsorption energies with experimental desorption energies from studies like temperature programmed desorption (TPD) etc. Comparisons like those do not consider that the experimental desorption energies are obtained via ensemble techniques while theoretical values are calculated at the single molecule level. Theoretical values are generally based upon desorption of a single molecule from a clean surface, or upon desorption of an entire monolayer. On the other hand, coverage dependent molecule–molecule interactions add to and modify molecule–substrate interactions that contribute to the experimentally determined desorption energies. In this work, we explore the suitability of an additive nearest neighbor model for determining general coverage dependent single molecule desorption energies in non-covalent self-assembled monolayers (SAMs). These coverage dependent values serve as essential input to any model attempting to reproduce coverage dependent desorption or for understanding the time dependent desorption from a partially covered surface. This method is tested using a case study of coronene adsorbed on Au(111) and HOPG substrates with periodic DFT calculations. Calculations show that coronene exhibits coverage and substrate dependence in molecular desorption. We found that intermolecular contact energies in the coronene monolayer are not strongly influenced by the HOPG substrate, while coronene desorption on Au(111) exhibits strong cooperativity where the additive model fails.
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- Award ID(s):
- 1800070
- PAR ID:
- 10096275
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 21
- Issue:
- 20
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 10505 to 10513
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9CP01774G
