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Keywords:

  • dual fluorescence;
  • iminocoumarin;
  • internal charge transfer;
  • kasha’s rule;
  • zinc

Abstract

3-(2,2′-Bipyridyl)-substituted iminocoumarin molecules (compounds 1 and 2) exhibit dual fluorescence. Each molecule has one electron donor and two electron acceptors that are in conjugation, which leads to fluorescence from two independent charge transfer (CT) states. To account for the dual fluorescence, we subscribe to a kinetic model in which both CT states form after rapid decays from the directly accessed S1 and S2 excited states. Due to the slow internal conversion from S2 to S1, or more likely the slow interconversion between the two subsequently formed CT states, dual emission is allowed to occur. This hypothesis is supported by the following evidence: 1) the emission at short and long ends of the spectrum originates from two different excitation spectra, which eliminates the possibility that dual emission occurs after an adiabatic reaction at the S1 level. 2) The fluorescence quantum yield of compound 2 grows with increasing excitation wavelength, which indicates that the high-energy excitation elevates the molecule to a weakly emissive state that does not internally convert to the low-energy, highly emissive state. The intensity of the two emission bands of 1 is tunable through the specific interactions between either of the two electron acceptors with another species, such as Zn2+ in the current demonstration. Therefore, the development of ratiometric fluorescent indicators based on the dual-emitting iminocoumarin system is conceivable. Further fundamental studies on this series of compounds using time-resolved spectroscopic techniques, and explorations of their applications will be carried out in the near future.