and 17 is estimated to be at least 25% shorter than the Förster radius of the green/red FRET pair. Obviously, the interaction between the two fluorophores is sufficiently long range to be suitable for the observations of FRET in molecules of this size. The chemical transformation appears to be complete within ca. 30 min and after that both emission signals in the red and in the green channel remain constant -despite continuous irradiation -thus providing evidence for the photostability of the respective fluorophores. In order to verify this approach, the complex 16 was also synthesized (scheme 4), isolated and fully characterized by NMR-spectroscopy and high-resolution mass spectrometry. and the green (535 nm) channel (fig. ). As evidenced by the rapid change in the fluorescence signal this reaction is significantly faster and within a few minutes an emission plateau in the red and the green channel is reached. The reaction leads to a sulfur-bridged species with an Au-S-Au core. Again, the outcome of the reaction was verified by synthesizing and characterizing the sulfur-bridged dimer 17. The fast formation of the sulfur-bridged dimer is followed by a much slower secondary reaction as evidenced by correspondingly slow changes (over the course of several hours) in the red and the green emission (fig. ).