| Autor: |
Cekli S; Department of Chemistry and Center for Macromolecular Science and Engineering , University of Florida , Gainesville , Florida 32611-7200 , USA . Email: kschanze@chem.ufl.edu., Winkel RW; Department of Chemistry and Center for Macromolecular Science and Engineering , University of Florida , Gainesville , Florida 32611-7200 , USA . Email: kschanze@chem.ufl.edu., Alarousu E; Solar and Photovoltaics Engineering Research Center , Division of Physical Sciences and Engineering , King Abdullah University of Science and Technology , Thuwal 23955-6900 , Kingdom of Saudi Arabia., Mohammed OF; Solar and Photovoltaics Engineering Research Center , Division of Physical Sciences and Engineering , King Abdullah University of Science and Technology , Thuwal 23955-6900 , Kingdom of Saudi Arabia., Schanze KS; Department of Chemistry and Center for Macromolecular Science and Engineering , University of Florida , Gainesville , Florida 32611-7200 , USA . Email: kschanze@chem.ufl.edu. |
| Abstrakt: |
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. |
