1. Department of Chemistry and Polymer Science , University of Stellenbosch , Private Bag X1, Matieland , Stellenbosch 7602 , South Africa
2. Department of Chemical Sciences , Olabisi Onabanjo University , P. M. B. 2002 , Ago-Iwoye , Nigeria
3. School of Chemistry and Physics , University of KwaZulu-Natal , Private Bag X54001, Westville Campus , Westville 4000 , Durban , South Africa