Solvent-Porphyrin Pseudo-Polymorphs: A DFT and Statistical Mechanical Study

Abstract

Well defined monolayers of solvent incorporated cobalt(II) octaethylporphyrin (CoOEP) on Au(111) can be formed in toluene (TOL) and in 1,2,4 trichlorobenzene (TCB). These structures are pseudo rectangular and one-to-one in composition (REC). These structures only form when the initial exposure of Au is to very dilute solutions of CoOEP. When the initial Au exposure is to high CoOEP concentration solution, only the porphyrin adsorbs in the well-known pseudo hexagonal (HEX) structure. Attempts to convert the REC structure to HEX by the addition of concentrated CoOEP at 298K were never observed for the TCB case, and occurred slowly in the TOL case. Density functional theory (DFT) and statistical mechanics are used to investigate the rates of desorption and the equilibrium concentrations for the TOL-REC, TCB-REC, and HEX system. Two different DFT functional types (GGA and meta-GGA), both with corrections for van der Waals forces, were used. In order to address both the energies of the activated states and the kinetics of desorption in contact with liquid solvent, we will use a method proposed by Charlie Campbell for estimating the difference in adsorption energy of a molecule in the presence and absence of a liquid solvent, which assumes pairwise interfacial bond additivity. The results of these calculations indicate that the REC forms only exist because of the rapid adsorption and desorption of the solvents and the slow desorption and diffusion of the porphyrin. We note that computed desorption rates are extremely sensitive to the actual activation energy and, based on the difference in values produced by the two computational methods, are not quantitively reliable. The computed equilibrium state at 298K, on the other hand, is predicted to be the HEX form independent of computational method and solution concentration for all practical values of the concentration. Thus the REC structures are kinetically controlled while the HEX structure is the equilibrium structure.