Synthesis, structure and spectroscopic properties of chloranilate-bridged 4f–4f dinuclear complexes: a comparative study of the emission properties with Cr–Ln complexes

New 4f–4f chloranilate-bridged dinuclear LnIII complexes, [(HBpz3)2Ln(μ-C6Cl2O4)Ln(HBpz3)2] (Ln(CA)Ln: Ln=Eu, Tb, Yb), were synthesized and characterized by the X-ray analysis. Their structure and spectroscopic properties were compared with those of dinuclear 3d–4f assembled molecular systems [(acac)2CrIII(μ-ox)LnIII(HBpz3)2] (Cr(ox)Ln: acac−=acetylacetonate, ox2−=oxalate, HBpz3 −=hydrotris(pyrazol-1-yl)borate) and [(acac)2Cr(μ-bpypz)Ln(hfac)3] (Cr(bpypz)Ln: bpypz−=3,5-di(2-pyridyl)pyrazolate, hfac=hexafluoroacetylacetonate). The complex Yb(CA)Yb shows strong 4f–4f emission due to the ligand to metal energy transfer from the triplet state of the CA2− to the excited 4f state of YbIII. On the other hand, the observation of only the 4f–4f emission in the Cr(bpypz)Yb complex is in contrast to the simultaneous observation of the low temperature 3d–3d and 4f–4f emissions associated with the energy transfer from CrIII to YbIII in the Cr(ox)Yb complexes. This indicates that the energy transfer from CrIII to YbIII is faster in the Cr(bpypz)Yb as compared to that in the Cr(ox)Yb even at low temperatures leading to the stronger 4f–4f luminescence in the former complex. No observation of either TbIIIor EuIII emission in the Tb(CA)Tb or Eu(CA)Eu complexes suggests the energy transfer or back-transfer from the Tb or Eu ions to the CA2− moiety. Conversely, the Cr(bpypz)Eu and Cr(bpypz)Tb complexes show 3d–3d emissions as similarly to the corresponding Cr(ox)Eu and Cr(ox)Tb complexes; indicating the energy transfer from the Eu or Tb to the CrIII moiety.