The invention is directed to a compound having the general formula (1): wherein each of La and Lb is an independently selected from a linking moiety comprising 0 to 10 main chain atoms, optionally substituted; each of Za and Zb is an independently selected complexing moiety comprising at least one nitrogen atom; either both or one of Za and/or Zb is coordinatively bonded to a respective transition metal complex Ma Va and Mb Vb through said nitrogen atom, wherein each of Ma and Mb is an independently selected transition metal, and each of Va and Vb is an independently selected valence group.