Amplification of a cyclic mixed-metalloporphyrin tetramer from a dynamic combinatorial library through orthogonal metal coordinationElectronic supplementary information (ESI) available: Fig. S1–3: 1H NMR, 1H–1H COSY and 1H–1H NOESY spectra of (4,4′-bipy)⊂[(Zn-1)2/(Rh-2)2] at 300 K. See http://www.rsc.org/suppdata/cc/b1/b111019p/