1. a Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences , H-1525 , Budapest , P.O. Box 49, Hungary
2. b Institute of Physics, Academy of Sciences of the Czech Republic , Na Slovance 2, 18221 , Prague 8 , Czech Republic
3. c Organic Materials Research Group, Department of Chemistry , Vanderbilt University , Nashville , TN , 37235 , USA