Redox States of Long Oligothiophenes: Two Polarons on a Single Chain

Abstract

A detailed investigation is presented of the redox states of three oligothiophenes (nT) with 6, 9, and 12 thiophene units. While open-shell radical cations (polarons) and closed-shell dications (bipolarons) are usually invoked as the primary redox species in these systems, we have obtained evidence that the dication of the longest oligothiophene (12T) has an electronic structure with two individual polarons. The redox states of 6T, 9T, and 12T have been fully characterized using UV/visible/near-IR and ESR spectroscopy in combination with electrospray mass spectrometry. For 6T and 9T, singleelectron oxidation in dichloromethane produces the corresponding radical cations, which form spinless p dimers at lower temperatures. A second oxidation step forms dications which possess a bipolaronic electronic structure. However, the redox behavior of the longest oligothiophene, 12T, is entirely different. Radical cations of 12T disproportionate into neutral oligomers and dications, except at the lowest oxidation levels. The spectral data for doubly oxidized 12T are incompatible with those expected for a bipolaronic structure but are consistent with the formation of two individual polarons on a single chain; this interpretation is also supported by the results from correlated quantum chemical calculations.

Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitution

“…Moreover, for thiophene oligomers, AM1 35 calculations have predicted a polaron confined to approximately five thiophene rings, while changes in the radical cation spectra up to a thiophene oligomer containing twelve rings have been observed experimentally. 42 This indicates that HF calculations underestimate the spatial extent of charge carriers on conjugated chains considerably, probably due to the absence of ͑dynamic͒ electron correlation. DFT calculations have been performed for singly charged thiophene oligomers by Moro et al 34 and by Brocks 38 and both found results similar to those obtained here for phenylene vinylenes; i.e., the charge distribution and geometry deformations are fully delocalized over the entire chain.…”

Systematic elongation of thienyl linkers and their effect on optical and electrochemical properties in carbazole–BODIPY donor–acceptor systems

“…As reported in, 27,28 unsubstituted BODIPY features a reversible redox couple at À1.15 V and an irreversible oxidation peak at +1.17 V. Our investigated dyes undergo oxidation as well as reduction processes as seen in their cyclic voltammograms presented in Fig. 8.…”

Physical electrochemistry of nanostructured devices

“…97 It has been pointed out that the relative stability of one B versus two P may depend on the experimental conditions such as solvent and type of counterions. 98 The statistics of P and B in conjugated polymers are well known, 99,100 and from these expressions the chemical capacitance can be calculated. In addition, the effect of the Gaussian disorder on the statistics of carriers has been described.…”