Nature and energies of electrons and holes in a conjugated polymer, polyfluorene.
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.
Takeda
Norihiko
N
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
Asaoka
Sadayuki
S
Miller
John R
JR
D001695:Biology; D004583:Electrons; D005449:Fluorenes / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D007477:Ions / Q000737:chemistry; D007700:Kinetics; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011108:Polymers / Q000737:chemistry*; D012964:Sodium; D012996:Solutions; D013057:Spectrum Analysis
D005449:Fluorenes; D005609:Free Radicals; D007477:Ions; D011108:Polymers; D012996:Solutions; C041509:fluorene; D012964:Sodium