Nature and Energies of Electrons and Holes in a Conjugated Polymer, Polyfluorene

Abstract

Electrons and holes were injected selectively into poly-2,7-(9,9-dihexylfluorene) (pF) dissolved in a tetrahydrofuran (THF) and a 1,2-dichloroethane (DCE) solution, respectively, using pulse radiolysis. Transient absorption spectra of monoions of both signs revealed two bands attributable to formation of polarons, one in the visible region (pF+* at 580 nm, pF-* at 600 nm) and another in the near-IR region. Additional confirmation for the identification of pF+* and pF-* comes from bimolecular charge-transfer reactions, such as bithiophene-* + pF --> pF-* or pF+* + TTA --> +TTA+* (TTA = tri-p-tolylamine), in which known radical ions transfer charge to pF or from pF. Difference absorption spectra of pF chemically reduced by sodium in THF provided a ratio of absorbance of anions formed to bleaching of the neutral band at 380 nm. In conjunction with pulse-radiolysis results, the data show that each polaron occupies 4.5 +/- 0.5 fluorene units, most probably contiguous units. Extensive reduction of pF by sodium also revealed resistance to formation of bipolarons: excess electrons reside as separate polarons when two or more electrons were injected. Redox equilibria with pyrene and terthiophene by pulse radiolysis established reversible one-electron redox potentials of E0(pF+/0) = +0.66 V and E0(pF0/-) = -2.65 V vs Fc+/0. Together with the excited-state energy, these results predict a singlet exciton binding energy of 0.2 eV for pF in the presence of 0.1 M tetrabutylammonium tetrafluoroborate. This binding energy would increase substantially without an electrolyte.

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 anions were determined from the absorptions ( Supplementary Fig. S3) along with known absorption coefficients 26,27,29 . Care was taken to add one or less reducing or oxidizing equivalent to each oligomer, or for polymers, to produce polarons well spaced from each other in the polymer chains.…”

“…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 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%.…”