If you wish to cite UniChem, please use the following reference...
Chambers, J., Davies, M., Gaulton, A., Hersey, A., Velankar, S., Petryszak, R., Hastings, J., Bellis, L., McGlinchey, S. and Overington, J.P.
UniChem: A Unified Chemical Structure Cross-Referencing and Identifier Tracking System.
Journal of Cheminformatics 2013, 5:3 (January 2013).
DOI: 10.1186/1758-2946-5-3
...a copy of which may be downloaded from here...
http://www.jcheminf.com/content/5/1/3
If you wish to specifically cite UniChem Connectivity Search funtionality, then please use the following reference...
Chambers, J., Davies, M., Gaulton, A., Papadatos, G., Hersey and Overington, J.P.
UniChem: extension of InChI-based compound mapping to salt, connectivity and stereochemistry layers.
Journal of Cheminformatics 2014, 6:43 (September 2014).
DOI: 10.1186/s13321-014-0043-5
...a copy of which may be downloaded from here...
http://www.jcheminf.com/content/6/1/43