From old porphyrins to novel materials

textThe fascinating role that porphyrins play in natural processes such as photosynthesis and respiration, continues to provide a compelling motivation to study these chromophores and to design new analogues with improved functions. This dissertation describes the interdisciplinary study of several classes of compounds that include π-extended porphyrins, expanded porphyrins, porphycenes, polypyrroles and porphyrins. Dictated by the need to capture efficiently red and near-infrared light, the so-called π-extension approach has been established as a powerful tool in the synthesis of large aromatic chromophores. Many of these artificial systems demonstrate properties similar to natural porphyrins. Often, however, a reduced bandgap is seen due to a greater π-system. Synthetic challenges associated with the preparation of the linearly annulated porphyrins have long been recognized. Many of these have now beed overcome as the result of a newly developed synthetic protocol described in Chapter 1. This protocol allows for the synthesis of a pyrrolic building block under milder conditions. As a continuation of this work, naphthobipyrroles were obtained via the π-extension strategy applied to a benzobipyrrole. The utility of this new building block was demonstrated with the syntheses of an electrochromic polynaphthobipyrrolic polymer (Chapter 3) and a dinaphthoporphycene (Chapter 2), a chromophore isomeric to porphyrin. Chapter 4 describes a different approach to porphyrin functionalization. Here, the goal was to effect substitution at the so-called β pyrrolic positions with using two bithiophene groups. Although, the resulting product is not completely rigid, enhancement in the sensitizing properties of the basic porphyrin chromophore was seen in a dye sensitized solar cell (DSSC) assembly, as studied by scanning electrochemical microscopy (SECM). The synthetic contributions concerning this work, carried in collaboration with Bard group, are expected to lay the groundwork for the development future photovoltaic materials. Expanded porphyrins are the more diverse group of porphyrinic derivatives. In effort to develop new conjugated expanded porphyrin systems and to understand their spectroscopic behavior in greater detail, a set of expanded porphyrins, based on the direct electrochemical oxidation of terpyrrole-like fragments, was developed in conjunction with the Bucher group (Grenoble, France). This effort is described in Chapter 5. Specifically it is shown that trithiacyclo[9]pyrrole may be prepared by means of an electrochemical synthesis.Chemistr

Radical Anions of Trifluoromethylated Perylene and Naphthalene Imide and Diimide Electron Acceptors

A series of electron-deficient perylene and naphthalene imides and diimides (<b>1</b>–<b>4</b>) with varying degrees of trifluoromethylation were synthesized. Single crystal X-ray analysis afforded detailed structural information, while spectroelectrochemical and EPR spectroscopy provided characterization of the radical anions of <b>1</b>–<b>4</b>. This study reveals that trifluoromethylation of the imides and diimides makes their one-electron reduction potentials substantially more positive relative to the unsubstituted counterparts, while their other properties remain largely unchanged

Bias-Switchable Permselectivity and Redox Catalytic Activity of a Ferrocene-Functionalized, Thin-Film Metal–Organic Framework Compound

The installation of ferrocene molecules within the wide-channel metal–organic framework (MOF) compound, NU-1000, and subsequent configuration of the modified MOF as thin-film coatings on electrodes renders the MOF electroactive in the vicinity of the ferrocenium/ferrocene (Fc⁺/Fc) redox potential due to redox hopping between anchored Fc^+/0 species. The observation of effective site-to-site redox hopping points to the potential usefulness of the installed species as a redox shuttle in photoelectrochemical or electrocatalytic systems. At low supporting electrolyte concentration, we observe bias-tunable ionic permselectivity; films are blocking toward solution cations when the MOF is in the ferrocenium form but permeable when in the ferrocene form. Additionally, with ferrocene-functionalized films, we observe that the MOF’s pyrene-based linkers, which are otherwise reversibly electroactive, are now redox-silent. Linker electroactivity is fully recovered, however, when the electrolyte concentration is increased 10-fold, that is, to a concentration similar to or exceeding that of an anchored shuttle molecule. The findings have clear implications for the design and use of MOF-based sensors, electrocatalysts, and photoelectrochemical devices

Electrochemical Synthesis of a Thiophene-Containing Cyclo[9]pyrrole

International audienc

Photoinduced charge and energy transfer within meta- and para-linked chlorophyll a-perylene-3,4:9,10-bis(dicarboximide) donor-acceptor dyads

Connecting electron donors and acceptors to a benzene ring in a meta or para relationship results in quantum interference effects that can strongly influence charge separation (CS) and charge recombination (CR) processes in these systems. We report on the energy and electron transfer behavior of chlorophyll-based para- and meta-linked donor–bridge–acceptor (D-B-A) dyads, where the semisynthetic chlorophyll a derivative, zinc methyl 3-ethyl-pyrochlorophyllide a (D), is covalently attached at its 20-position to the para position of one phenyl of diphenylacetylene (B). The meta or para position of the phenyl in B distal to the donor is in turn attached to perylene-3,4:9,10-bis(dicarboximide) (PDI) (A). Photoexcitation of the D-B-A dyads produces long-lived radical ion pairs D•+-B-A•–, which recombine to the ground state and to both 3*D-B-A and D-B-3*A. Time-resolved optical and electron paramagnetic resonance spectroscopies were used to monitor the charge transfer and triplet energy transfer (TEnT) processes. At longer times, TEnT occurs from 3*D-B-A to D-B-3*A. Surprisingly, the D-B-A molecules linked via the meta linkage exhibit faster CS, CR, and TEnT rates than do those with the para linkage in contrast to most other meta/para-linked D-B-A molecules previously examined

Cyclo[<i>m</i>]pyridine[<i>n</i>]pyrroles: Hybrid Macrocycles That Display Expanded π-Conjugation upon Protonation

Novel hybrid cyclo­[<i>m</i>]­pyridine­[<i>n</i>]­pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies

Cyclo[<i>m</i>]pyridine[<i>n</i>]pyrroles: Hybrid Macrocycles That Display Expanded π-Conjugation upon Protonation

Novel hybrid cyclo­[<i>m</i>]­pyridine­[<i>n</i>]­pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies

Cyclo[<i>m</i>]pyridine[<i>n</i>]pyrroles: Hybrid Macrocycles That Display Expanded π-Conjugation upon Protonation

Novel hybrid cyclo­[<i>m</i>]­pyridine­[<i>n</i>]­pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies

Cyclo[<i>m</i>]pyridine[<i>n</i>]pyrroles: Hybrid Macrocycles That Display Expanded π-Conjugation upon Protonation

Novel hybrid cyclo­[<i>m</i>]­pyridine­[<i>n</i>]­pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies