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Title: Computational Study of Solvent Incorporation into a Porphyrin Monolayer
Density functional theory (DFT) is used to investigate the conversion from a solvent incorporated pseudo-polymorph into a single component monolayer. Calculations of thermodynamic properties both for the surfaces in contact with gas phase and with solvent are reported. In the case of wetted surfaces, a simple bond-additivity model, first proposed by Campbell and modified here, is used to augment the DFT calculations. The model predicts a dramatic reduction in desorption energies in solvent as compared to gas phase. Eyring’s reaction rate theory is used to predict limiting desorption rates for guest (solvent) molecules from the pockets in the pseudo-polymorph and for cobalt octaethylporphyrin (COEP) molecules in all structures. The pseudo-polymorph studied here is a nearly rectangular lattice (REC) composed of two CoOEP and 2 molecules of either 1,2,4-trichlorobenzene (TCB) or toluene (TOL) supported on 63 atoms of Au(111). At sufficiently high initial concentrations of CoOEP, only a hexagonal unit cell (HEX) with two molecules of CoOEP, supported on 50 atoms of gold is observed. Experimentally, the TCB-REC structure is more stable than the TOL-REC structure existing in solution at initial mM concentrations of CoOEP in TCB as opposed to initial M concentration of CoOEP in toluene. Calculations here show that the HEX structure is the thermodynamically stable structure at all practical concentrations of CoOEP. Once the REC structure forms kinetically at low concentration because of the vast excess of solvent on the surface, it is difficult to convert to the more stable HEX structure. The difference in stability is primarily due to the difference in electronic adsorption energy of the solvents (TOL or TCB) and to the very low desorption rate of CoOEP. The adsorption energy of TCB has two important contributors: the adsorption energy onto Au alone, and the intermolecular interactions between TCB and the CoOEP host lattice. Neither factor can be neglected. We also find that planar adsorption of both TOL and TCB on Au(111) is the energetically preferred orientation when space is available on the surface. Rates of desorption are very sensitive to the solvent free activation energy and to the thermodynamic parameters required to convert the solvent free activation energy to one for the solvated surface. Small changes in the computed energy (of the order of 5%) can lead to one order of magnitude change in rates. Further, the solvation model used does not provide the barrier to adsorption in solution needed to determine values for the desorption activation energy. Thus, the rates computed here for desorption into solvent are limiting values.  more » « less
Award ID(s):
2306316
PAR ID:
10506700
Author(s) / Creator(s):
;
Publisher / Repository:
The American Chemical Society
Date Published:
Journal Name:
The Journal of Physical Chemistry C
Volume:
128
Issue:
4
ISSN:
1932-7447
Page Range / eLocation ID:
1827 to 1839
Subject(s) / Keyword(s):
adsorption polymorph STM solvent effects liquid-solid interface equilibrium kinetic control
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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