Polarons, Bipolarons, and Side-By-Side Polarons in Reduction of Oligofluorenes

Abstract

The nature of charge carriers in conjugated polymers was elucidated through optical spectroscopy following single- and multielectron reduction of 2,7-(9,9-dihexylfluorene) oligomers, F(n), n = 1-10, yielding spectra with the two bands typical of polarons upon single reduction. For short oligomers addition of a second electron gave a single band demonstrating the classic polaron-bipolaron transition. However, for long oligomers double reductions yielded spectra with two bands, better described as two polarons, possibly residing side-by-side in the F(n) chains. The singly reduced anions do not appear to delocalize over the entire length of the longer conjugated systems; instead they are polarons occupying approximately four fluorene repeat units. The polarons of F(3) and F(4) display sharp absorption bands, but for longer oligomers the bands broaden, possibly due to fluctuations of the lengths of these unconfined polarons. DFT calculations with long-range-corrected functionals were fully consistent with the experiments describing polarons in anions, bipolarons in dianions of short oligomers, and side-by-side polarons in dianions of long oligomers, while results from standard functionals were not compatible with the experimental results. The computations found F(10)(2-), for example, to be an open-shell singlet (<S(2)> ≈ 1), with electrons in two side-by-side orbitals, while dianions of shorter oligomers experienced a gradual transition to bipolarons with states of intermediate character at intermediate lengths. The energies and extinction coefficients of each anionic species were measured by ultraviolet-visible-near-infrared absorption spectroscopy with chemical reduction and pulse radiolysis. Reduction potentials determined from equilibria mirrored oxidation potentials reported by Chi and Wegner. Anions of oligomers four or more units in length contained vestigial neutral (VN) absorption bands that arise from neutral parts of the chain. Energies of the VN bands correspond to those of oligomers shorter by four units.

Giant infrared absorption bands of electrons and holes in conjugated molecules

“…At high frequencies, usually B2,000 cm À 1 , broad continuous absorptions appear because of polaron bands that are well known for holes and electrons in conjugated polymers and oligomers. For the molecules studied here, the polaron bands were reported earlier in the near IR 26,27,29 . To estimate the IR intensities of CH stretches in Fig.…”

“…For ions of F 2 and F 3 , the polarons are confined by the short length of the oligomers, so less displacement of the polaronic charge occurs on vibration. As polarons in F n oligomers have a length of B4.3 repeat units 26,27 , the IR enhancements are optimal at lengths slightly longer than four repeat units. For this reason, F 5 was chosen for the illustration in Fig.…”

“…Concentrations of the anions were determined to be 0.27, 0.11, 0.4, 0.14 and 0.18 mM for F 2 , F 3 , F 4 , F 8 and pF 30 , respectively, based on measurements of visible bands ( Supplementary Fig. S4) in the solutions of anions and the known absorption coefficients of the anions 26,27 where determinations of absolute absorption coefficients were aided by the use of pulse radiolysis 26 . Figure 1 | IR absorption spectra of radical anions.…”

“…The calculations used the oPBE (o ¼ 0.1) functional that was found to give a good description of the optical transitions of anions of fluorene oligomers and the spatial extent of their polarons, which typically spread over four fluorene repeat units 26 . Computed IR bands in the CH stretch region (not shown) are small, and will be discussed below.…”

“…The CDCl 3 solvent permitted acquisition of spectra of the cations over a slightly larger frequency range. Absorption coefficients for the pF 30 cation were calibrated using e(pF þ ) reported by Takeda et al 27 For F 4 and F 8 cations, absorption coefficients of the visible bands were assumed to be similar to those of the corresponding anions 26 . We estimate that this approximation raises the uncertainty of the IR absorption coefficients of the cations to ± 30%.…”