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 Recdots, three dots, centeredRu distance in this system suggest that a folded conformation of the complex, in which the Recdots, three dots, centeredRu 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.