DNA-binding property and antitumor activity of bismuth(iii) complex with 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazacyclododecaneElectronic supplementary information (ESI) available: 1H-NMR, ES-MS and CD spectra. See http://www.rsc.org/suppdata/dt/b3/b305290g/