Electron-transfer oxidation properties of substituted bi-, ter-, and quaterpyrroles

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.