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Title: Charge transfer fluorescence and 34 nm exciton diffusion length in polymers with electron acceptor end traps

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 to 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, andmore » that the exciton diffusion length LD =34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. As a result, the efficiency of exciton capture depends on chain length, but not on trap depth, solvent polarity or which trap group is present.« less
 [1] ;  [1] ;  [2] ;  [3] ;  [4] ;  [2] ;  [2] ;  [2]
  1. Dowling College, Oakdale, NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
  2. Brookhaven National Lab. (BNL), Upton, NY (United States)
  3. Dowling College, Oakdale, NY (United States)
  4. Kyoto Inst. of Technology, Kyoto (Japan)
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 1520-6106; R&D Project: CO004; KC0304030
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry
Additional Journal Information:
Journal Volume: 119; Journal Issue: 24; Journal ID: ISSN 1520-6106
American Chemical Society
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY exciton trapping; charge transfer state; conjugated polymer; polyfluorene; exciton diffusion; diffusion length; charge transfer exciton