Whitney A. Webre

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin-Copper Corrole Donor-Acceptor Conjugates.

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10(10)  s(-1) and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.

Charge separation in supramolecular ferrocene(s)-zinc porphyrin-fullerene triads: A femtosecond transient absorption study

Mechanistic aspects of photoinduced charge separation in supramolecular triads, constructed using covalently linked zinc porphyrin-ferrocene(s) dyads — self-assembled via axial coordination to either pyridine or phenylimidazole appended fulleropyrrolidine (Fcx-ZnP:PyC60 or Fcx-ZnP:ImC60; x = 1 or 2), has been investigated using femtosecond pump-probe transient spectroscopy. Upon photoexcitation of ZnP, charge separation from ferrocene to 1ZnP* to yield the initial Fc+-ZnP•-:C60 radical ion-pair or charge separation from 1ZnP* to C60 to yield the initial Fc-ZnP•+:C60•- radical ion-pair, depending upon the ferrocene-zinc porphyrin intermolecular distance, was observed. These radical ion-pairs resulted in the formation of ultimate distantly separated Fc+-ZnP:C60•- radical ion-pairs either via an electron migration (former case) or hole shift (latter case) process. Kinetics of charge separation as a function of spacer connecting the ferrocene and porphyrin, and spacer between the porphyrin and fullerene is reported. In agreement with our earlier study (J. Phys. Chem. B 2004; 108: 11333–11343), the Fc+-ZnP:C60•- radical ion-pair persisted beyond the monitoring time window of our instrument, suggesting charge stabilization in these supramolecular triads.

Selective octabromination of tetraarylporphyrins based on meso-substituent identity: Structural and electrochemical studies

Synthesis and isolation of selectively brominated tetraarylporphyrin derivatives is reported. Treatment with bromine of meso-5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin (1) or meso-5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphinatocopper(II) (2-Cu) yields products octabrominated at the 2,6-positions of meso-aryl substituents [5,10,15,20-tetrakis(2,6-dibromo-3,4,5-trimethoxyphenyl)porphyrin, 2,6Br81] or macrocyclic β-positions. The latter of these (β-brominated) was identified as the oxoporphyrinogen 2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen 3 obtained due to the adventitious oxidation and demetalation of 2-Cu, which could be alkylated at its macrocyclic nitrogen atoms yielding N21,N22,N23,N24-tetrakis(4-bromobenzyl)-2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(3,5-di-t-butyl-4-oxo-cyclo hexa-2,5-dienylidene)porphyrinogen 4. The former compound 2,6Br81 was complexed with Zn(II) (2,6Br81-Zn) or Cu(II) (2,6Br81-Cu)...