Photoinduced energy transfer between Re(I) and Ru(II) termini connected through a new exo-ditopic bis-phenanthroline ligand fused to a central macrocycle spacer: Synthesis, structure, and electrochemical and photophysical properties of a heterodinuclear complex

A new ligand L¹ has been prepared in which two 1,10-phenanthroline fragments are separated by an 18-crown-6 macrocyclic spacer. This was used to prepare the heterodinuclear complex [(bipy)₂Ru(μ-L¹)Re(CO)₃Cl][PF₆]₂ [Ru(L¹)Re] in which the {Ru(bipy)₂(phen)}²⁺ and {Re(CO)Cl(phen)} chromophores are separated by a saturated and fairly flexible crown-ether fragment. On the basis of photophysical studies on Ru(L¹)Re and associated mononuclear Ru(II) and Re(I) complexes, Re → Ru photoinduced energy-transfer occurs with a rate constant of 1.9 × 10⁸ s⁻¹ in solution room temperature leading to near-complete quenching of the Re(I)-based luminescence. At 77 K the Re(I)-based luminescence component is completely quenched. Calculations on the efficiency of both Förster and Dexter energy-transfer as a function of Re?Ru distance in this system suggest that a folded conformation of the complex, in which the Re?Ru separation is much shorter than that implied by the extended conformation detected crystallographically, is responsible for the energy-transfer, since neither Förster nor Dexter Re → Ru energy-transfer should be possible with the complex in an extended conformation. Addition of K⁺ or Ba²⁺ salts to solutions of Ru(L¹)Re had no effect on the photophysical properties, probably because the association constants are too low to give significant metal-ion binding in the macrocycle at the low concentrations employed.