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    Triplet excited state properties in variable gap π-conjugated donor–acceptor–donor chromophores

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    Type
    Article
    Authors
    Cekli, Seda
    Winkel, Russell W.
    Alarousu, Erkki
    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
    2016
    Permanent link to this record
    http://hdl.handle.net/10754/617306
    
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    Abstract
    A series of variable band-gap donor–acceptor–donor (DAD) chromophores capped with platinum(II) acetylide units has been synthesized and fully characterized by electrochemical and photophysical methods, with particular emphasis placed on probing triplet excited state properties. A counter-intuitive trend of increasing fluorescence quantum efficiency and lifetime with decreasing excited state energy (optical gap) is observed across the series of DAD chromophores. Careful study of the excited state dynamics, including triplet yields (as inferred from singlet oxygen sensitization), reveals that the underlying origin of the unusual trend in the fluorescence parameters is that the singlet–triplet intersystem crossing rate and yield decrease with decreasing optical gap. It is concluded that the rate of intersystem crossing decreases as the LUMO is increasingly localized on the acceptor unit in the DAD chromophore, and this result is interpreted as arising because the extent of spin–orbit coupling induced by the platinum heavy metal centers decreases as the LUMO is more localized on the acceptor. In addition to the trend in intersystem crossing, the results show that the triplet decay rates follow the Energy Gap Law correlation over a 1.8 eV range of triplet energy and 1000-fold range of triplet decay rates. Finally, femtosecond transient absorption studies for the DAD chromophores reveals a strong absorption in the near-infrared region which is attributed to the singlet excited state. This spectral band appears to be general for DAD chromophores, and may be a signature of the charge transfer (CT) singlet excited state.
    Citation
    Triplet excited state properties in variable gap π-conjugated donor–acceptor–donor chromophores 2016, 7 (6):3621 Chem. Sci.
    Sponsors
    This work was supported by the National Science Foundation (Grant No. CHE-115164 and CHE-1504727). The authors acknowledge the University of Florida Research Computing (http://researchcomputing.ufl.edu) for providing computational resources and support that have contributed to the research results reported in this publication.
    Publisher
    Royal Society of Chemistry (RSC)
    Journal
    Chemical Science
    DOI
    10.1039/C5SC04578A
    Additional Links
    http://xlink.rsc.org/?DOI=C5SC04578A
    ae974a485f413a2113503eed53cd6c53
    10.1039/C5SC04578A
    Scopus Count
    Collections
    Articles; Physical Science and Engineering (PSE) Division; Chemical Science Program; KAUST Solar Center (KSC)

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