**Acknowledgement**

Dr Fahmina Zafar (Pool Officer) and Dr Eram Sharmin (Pool Officer) acknowledge Council of Scientific and Industrial Research, New Delhi, India for Senior Research Associateships against grant nos. 13(8385‐A)/2010‐POOL and 13(8464‐A)/2011‐10 POOL, respectively. They are also thankful to the Head, Department of Chemistry, Jamia Millia Islamia (A Central University), for providing support to carry out the work.
