Towards new chiroptical switches

We have synthesized a new chiroptical ligand system, where a 4,5-diaza-fluorene type ligand and a methoxy-benzo-xanthene unit are coupled by a double bond. The ligand exists in two enantiomeric forms, namely P- and M-helical arrangement, as shown by X-ray analysis. By irradiation, the ligand can change its helicity (M <----> P) by rotation around the double bond. A complexation between the new ligand and a photoactive metal center (ruthenium, osmium. or rhenium) has extended the molecular structure by a chemical bounded photosensitizer. A calculation using MOPAC 97 and the semi-empirical AM I method estimated the thermodynamic stability of the pure enantiomeric form of the ligand 3. The estimated rotational barrier found was 24 kcal mol(-1). Such an energy barrier is high enough to separate the two enantiomeric forms at room temperature. The preparation of the ligand system as well as the metal complexes is described. None of the metal complexes prepared shows any detectable emission at room temperature in aerated acetonitrile solutions. This is an indication that the different sensitizers in its excited state are quenched effectively by the coupled switching unit. (C) 2002 Elsevier Science B.V. All rights reserved.