Properties of distorted tetraphenylporphyrins

Citation

Hodge, Julia A. (1995) Properties of distorted tetraphenylporphyrins. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bwee-h202. https://resolver.caltech.edu/CaltechETD:etd-10092007-082429

Abstract

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X-ray crystallographic structure determinations reveal that tetrakis(pentafluorophenyl)-porphyrin derivatives substituted with bulky [beta]-substituents undergo a tetrahedral distortion that reduces the macrocyclic symmetry from [...] to [...]. The nickel complex of [2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin], [...], has nitrogen atoms displaced ±0.192 [...] and meso-carbons ±0.211 [...] from the plane of the porphyrin. Despite large conformational changes, bond distances and angles are essentially conserved for all tetraphenylporphyrins. Electrochemical and UV-vis spectroelectrochemical experiments revealed a novel, ligand-centered two-electron (2e) oxidation for Zn and Mg derivatives. Values for the 1e potentials were calculated from [...], the constant for the disproportionation of the singly oxidized product into the doubly oxidized and the neutral species, [...], and the Nernst equation. The redox potentials are consistent with semiempirical AM1 calculations that indicate that [...] distortion destabilizes the HOMOs [...] preferentially over the [...] LUMOs whereas halogen substitution lowers the energy of both the HOMO and the LUMO approximately equally. Consistent with the meso-substituent influencing relative ordering, EPR spectra of [...], and [...], and [...] reveal [...] and [...] HOMOs, respectively. Furthermore, the [beta]-octaethyl and octamethyl tetrakis(pentafluorophenyl)porphyrins undergo a similar net 2e oxidation. Thus, the distortion observed in the neutral molecule appears to mirror an electronically favorable geometry for the doubly oxidized product. An understanding of the sterics and electronics that influence frontier orbital energy levels creates the possibility of tuning porphyrins for a variety of applications: light harvesting, electron transfer, and catalytic reactions involving 2e reductions.

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