1. Contribution from the Institute of Organic Chemistry, University Halle, Kurt-Mothes-Strasse 2, D-06120 Halle, Germany, Institute of Physical Chemistry, University Halle, Mühlpforte 1, D-06108 Halle, Germany, and Centre for Molecular Materials, University of Sheffield, Robert Hadfield Building Mappin Street, Sheffield S1 3JD, Great Britain