title: Charge Transfer Fluorescence and 34 nm Exciton Diffusion Length in Polymers with Electron Acceptor End Traps.
abstract: Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17-127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence, and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps, the trap depths are 0.06 (p-xylene), 0.13 (THF), and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization, and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ∼50% of the excitons, and that the exciton diffusion length is LD = 34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. The efficiency of exciton capture depends on chain length but not on trap depth, solvent polarity, or which trap group is present.
authors.affiliation: †Chemistry and Physics Department, Dowling College, Oakdale, New York 11769, United States.|‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Lori
authors.initials: L
authors.lastname: Zaikowski
authors.affiliation: †Chemistry and Physics Department, Dowling College, Oakdale, New York 11769, United States.|‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Gina
authors.initials: G
authors.lastname: Mauro
authors.affiliation: ‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Matthew
authors.initials: M
authors.lastname: Bird
authors.affiliation: †Chemistry and Physics Department, Dowling College, Oakdale, New York 11769, United States.
authors.forename: Brianne
authors.initials: B
authors.lastname: Karten
authors.affiliation: ∥Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan.
authors.forename: Sadayuki
authors.initials: S
authors.lastname: Asaoka
authors.affiliation: §Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Qin
authors.initials: Q
authors.lastname: Wu
authors.affiliation: ‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Andrew R
authors.initials: AR
authors.lastname: Cook
authors.affiliation: ‡Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: John R
authors.initials: JR
authors.lastname: Miller
mesh_terms: D000880:Anthraquinones / Q000737:chemistry*; D004058:Diffusion; D004583:Electrons; D005449:Fluorenes / Q000737:chemistry*; D005453:Fluorescence; D015394:Molecular Structure; D053644:Naphthalimides / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D000880:Anthraquinones; D005449:Fluorenes; D053644:Naphthalimides; C000605652:poly(2,7-(9,9-dihexyl)fluorene)