1. Department of Physics, Ecole Normale Supérieure , University of Yaoundé I , P.O. Box 47 , Yaoundé , Cameroon
2. Department of Chemistry , University of Buea , P.O. Box 63 , Buea , Cameroon
3. Department of Inorganic Chemistry, Faculty of Science , University of Yaoundé I , BP 812 , Yaoundé , Cameroon
4. CEPAMOQ, Faculty of Science , University of Douala , P.O. Box 8580 , Douala , Cameroon
5. Laboratoire de Physique Fondamentale et Appliquée (LPFA) , University of Abobo-Adjamé (now Nangui Abrogoua) , Abidjan 02 , Côte d’Ivoire
6. Department of Pharmaceutical Chemistry , Martin-Luther University Halle-Wittenberg , Kurt-Mothes Str. 4, 06120 Halle (Saale) , Germany
7. Institut für Botanik , Technische Universität Dresden , Zellescher Weg 20b, 01062 Dresden , Germany