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Cationic (Charge Shift) Exciplexes.

Joseph P Dinnocenzo1, Paul B Merkel1, Samir Farid1

  • 1Department of Chemistry, University of Rochester , Rochester, New York 14627, United States.

The Journal of Physical Chemistry. A
|October 10, 2017
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Summary
This summary is machine-generated.

Novel cationic exciplexes formed from excited N-methyl isoquinolinium cations (NMiQ+) and alkyl benzene donors were discovered. These charge-shift exciplexes exhibit behavior similar to conventional charge-generation exciplexes.

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Area of Science:

  • Photochemistry
  • Supramolecular Chemistry
  • Organic Electronics

Background:

  • Exciplexes are excited-state complexes crucial in photochemistry and organic electronics.
  • Conventional exciplexes involve charge generation between excited donors and acceptors.
  • Cationic exciplexes formed via bimolecular encounter were previously unreported in fluid media.

Purpose of the Study:

  • To report the first observation of emissive cationic exciplexes formed from excited N-methyl isoquinolinium cation (NMiQ+) and alkyl benzene donors.
  • To investigate the photophysical properties and electronic structure of these novel cationic exciplexes.
  • To compare the behavior of cationic exciplexes with conventional exciplexes.

Main Methods:

  • Spectroscopic analysis (UV-Vis absorption, fluorescence emission) in acetonitrile.
  • Varying alkyl benzene donors with different oxidation potentials.
  • Photophysical parameter determination (fluorescence quantum yield, radiative rate constant).
  • Analysis of electronic coupling matrix elements.

Main Results:

  • Emissive cationic exciplexes of NMiQ+ and alkyl benzenes were successfully formed and detected.
  • These cationic exciplexes exhibit a charge shift (A+* + D → A•D•+), unlike conventional charge-generation exciplexes (A* + D → A•-D•+).
  • Similar trends observed between cationic and conventional exciplexes, including bathochromic shifts and decreased fluorescence quantum yields with decreasing donor oxidation potentials.
  • Fluorescence quantum yield decreased by 30-fold and radiative rate constant by 6.6-fold as charge-transfer character increased.
  • Electronic coupling matrix elements were comparable to conventional exciplexes despite lacking Coulombic stabilization.

Conclusions:

  • The formation and characterization of novel cationic exciplexes expand the scope of exciplex chemistry.
  • Cationic exciplexes offer a new paradigm for excited-state complex formation with potential applications in materials science.
  • The observed similarities in photophysical trends suggest shared fundamental principles governing exciplex formation, irrespective of charge dynamics.