title: Electron-transfer oxidation properties of substituted bi-, ter-, and quaterpyrroles.
abstract: A set of open-chain fully substituted bi-, ter-, and quaterpyrroles bearing analogous substituents in the alpha- and beta-pyrrolic positions were studied as a function of their chain length, subunit number, and size of potential conjugation pathway by means of cyclic voltammetry, EPR, and UV-vis spectroelectrochemistry. A comparison of E1/2 values for the first one-electron abstraction of bipyrrole 1 (1.07 V), terpyrrole 2 (0.67 V), and quaterpyrrole 3 (0.44 V) demonstrate clearly that the longer oligopyrroles are easier to oxidize. A strong absorption band is observed in the visible region when terpyrrole 2 is subject to one-electron oxidation, growing in at 856 nm accompanied by a shoulder at 778 nm. These strong absorptions in the visible region of the spectrum are in sharp contrast with the absence of bands in the red region when the bipyrrole 1 is subject to a similar one-electron oxidation and this can be explained by the presence of a longer conjugation pathway in the singly oxidized forms of 2 as was confirmed by EPR spectroscopy. The EPR spectra of 1*+, 2*+, and 3*+ indicate that the unpaired electron is more delocalized on the pyrroles with a longer conjugation and that the more the unpaired electron is delocalized, the faster is the electron exchange rate.
authors.affiliation: Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
authors.forename: Wenbo
authors.initials: W
authors.lastname: E
authors.forename: Kei
authors.initials: K
authors.lastname: Ohkubo
authors.forename: David
authors.initials: D
authors.lastname: Sanchez-Garcia
authors.forename: Min
authors.initials: M
authors.lastname: Zhang
authors.forename: Jonathan L
authors.initials: JL
authors.lastname: Sessler
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
authors.forename: Karl M
authors.initials: KM
authors.lastname: Kadish
mesh_terms: D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011758:Pyrroles / Q000737:chemistry*; D013053:Spectrophotometry
chemical_list: D011758:Pyrroles