Synthesis and Electrochemical, Photophysical, and Anion Binding Properties of Self-Assembly Heterometallic Cyclophanes

A series of heterometallic cyclophanes have been prepared from self-assembly of cis-(dppf)M(H2O)2(OTf)2 (M = Pd or Pt) and BrRe(CO)3(L)2 with the general formula [cis-(dppf)M(μ-L)2Re(CO)3Br]2(OTf)4 (4: M = Pd, L = 4,4‘-bpy; 5: M = Pt, L = 4,4‘-bpy; 6: M = Pd, L = 1,2-(4‘-dipyridyl)acetylene (DPA); 7: M = Pd, L = 1,4-(4‘-dipyridyl)butadiyne (DPB); 8: M = Pt, L = DPB) or cis-(dppf)Pt(4-ethynylpyridine)2 and BrRe(CO)5 with the formula {[cis-(dppf)Pt(4-ethynylpyridyl)2](fac-Re(CO)3Br)}2 (10). These complexes have been characterized by a variety of analytical techniques including IR, NMR, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction. The photophysical properties of these complexes have been examined in detail. Squares 4−8 exhibit room-temperature luminescence from lowest-lying triplet-centered metal to ligand charge transfer (3MLCT) excited states, albeit significantly weaker than that predicted on the basis of their excited state energies and the energy gap law. No luminescence was observed from the neutral square 10. Pd(II)- or Pt(II)-based oxidative quenching and energy transfer pathways from the emitting 3MLCT state to lower nonemissive excited states localized on the ferrocenyl moieties are invoked to explain the luminescence observations. Inorganic anion binding studies for squares 4−8 again reveal two different quenching pathways.