Photochemical, photophysical and electrochemical properties of six dansyl-based dyads

We have prepared six dyads containing a fluorescent propyldansylamide (PD) unit covalently linked to nitrobenzene (NB), naphthalene (NA), trans- and cis-azobenzene (tAZ and cAZ), 2,2'-bipyridine (BPY), and [Ru(bpy)(3)](2+) (RU) moieties. The photochemical, photophysical, and electrochemical properties of the dyads have been investigated in acetonitrile solution. In the PD-NB dyad, the fluorescence of the PD unit is quenched by electron transfer to the NB unit at 298 K, but it is not quenched in a rigid matrix at 77 K. In the PD-NA dyad, the fluorescence of the naphthalene unit is completely quenched by competing energy (80% efficiency) and electron transfer processes. In the case of PD-tAZ, the dansyl fluorescence is quenched without sensitization of the trans-->cis photoisomerization reaction of the tAZ moiety, whereas in the PD-cAZ dyad the dansyl fluorescence quenching is accompanied by the sensitisation of the cis-->trans photoisomerization of the cAZ moiety. In the PD-BPY dyad the fluorescence of the dansyl moiety is sensitized by the BPY unit; upon protonation or Zn2+ complexation of the BPY unit, however, the dansyl fluorescence is quenched by electron transfer. In the PD-RU dyad, the fluorescence of the dansyl unit is quenched by competing energy (25% efficiency) and electron transfer processes, whereas the emission of the RU moiety is unaffected.