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    Effect of Conjugation Length on Photoinduced Charge-Transfer in π-Conjugated Oligomer-Acceptor Dyads

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    Type
    Article
    Authors
    Jiang, Junlin
    Alsam, Amani A. cc
    Wang, Shanshan
    Aly, Shawkat Mohammede cc
    Pan, Zhenxing
    Mohammed, Omar F. cc
    Schanze, Kirk S.
    KAUST Department
    Chemical Science Program
    KAUST Solar Center (KSC)
    Physical Science and Engineering (PSE) Division
    Ultrafast Laser Spectroscopy and Four-dimensional Electron Imaging Research Group
    Date
    2017-06-21
    Online Publication Date
    2017-06-21
    Print Publication Date
    2017-07-06
    Permanent link to this record
    http://hdl.handle.net/10754/623760
    
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    Abstract
    A series of -conjugated oligomer-acceptor dyads were synthesized that feature oligo(phenylene ethynylene) (OPE) conjugated backbones end-capped with a naphthalene diimide (NDI) acceptor. The OPE segments vary in length from 4 to 8 phenylene ethynene units (PEn-NDI, where n = 4, 6 and 8). Fluorescence and transient absorption spectroscopy reveals that intramolecular OPE NDI charge transfer dominates the deactivation of excited states of the PEn-NDI oligomers. Both charge separation (CS) and charge recombination (CR) are strongly exothermic (G0CS ~ -1.1 and G0CR ~ -2.0 eV), and the driving forces do not vary much across the series because the oxidation and reduction potentials and singlet energies of the OPEs do not vary much with their length. Bimolecular photoinduced charge transfer between model OPEs that do not contain the NDI acceptors with methyl viologen was studied, and the results reveal that the absorption of the cation radical state (OPE+•) remains approximately constant ( ~ 575 nm) regardless of oligomer length. This finding suggests that the cation radical (polaron) of the OPE is relatively localized, effectively occupying a confined segment of n 4 repeat units in the longer oligomers. Photoinduced intramolecular electron transfer dynamics in the PEn-NDI series was investigated by UV-visible femtosecond transient absorption spectroscopy with visible and mid-infrared probes. Charge separation occurs on the 1 – 10 ps timescale, with the rates decreasing slightly with increased oligomer length (βCS ~ 0.15 Å-1). The rate for charge-recombination decreases in the sequence PE4-NDI > PE6-NDI ~ PE8-NDI. The discontinuous distance dependence in the rate for charge recombination may be related to the spatial localization of the positive polaron state in the longer oligomers.
    Citation
    Jiang J, Alsam AA, Wang S, Aly SM, Pan Z, et al. (2017) Effect of Conjugation Length on Photoinduced Charge-Transfer in π-Conjugated Oligomer-Acceptor Dyads. The Journal of Physical Chemistry A. Available: http://dx.doi.org/10.1021/acs.jpca.7b03547.
    Sponsors
    This work was partially supported by the National Science Foundation (Grant No. CHE-1504727). KSS also acknowledges the Welch Foundation for support. We also acknowledge many insightful discussions with John R. Miller (Brookhaven National Laboratory) which in part stimulated this work.
    Publisher
    American Chemical Society (ACS)
    Journal
    The Journal of Physical Chemistry A
    DOI
    10.1021/acs.jpca.7b03547
    Additional Links
    http://pubs.acs.org/doi/abs/10.1021/acs.jpca.7b03547
    ae974a485f413a2113503eed53cd6c53
    10.1021/acs.jpca.7b03547
    Scopus Count
    Collections
    Articles; Physical Science and Engineering (PSE) Division; Chemical Science Program; KAUST Solar Center (KSC)

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