Role of spacer in single- or two-step FRET: studies in the presence of two connected cryptands with properly chosen fluorophores

Two cryptand molecules are connected via p-xyloyl, benzene-1,4-dicarbolyol and 9,10-dimethylene anthracene. Each cryptand is further derivatized with fluorophores such that electronic absorption of one fluorophore overlaps emission of the other. This way, three different systems L(1), L(2) and L(3) have been synthesized to get a better view of the effect on the distance in single- and two-step fluorescence resonance energy transfer (FRET) process. These molecules are probed for FRET in the presence of a metal ion as input. L(1) and L(2) exhibit poor performance in single step FRET while in the case of L(3), a large two-step FRET process is operational with Cu(II) or Hg(II) as input.