Complexes Containing 2,9-Bis(p-biphenylyl)-1,10-phenanthroline Units Incorporated into a 56-Membered Ring. Synthesis, Electrochemistry, and Photophysical Properties

We synthesized a new macrocycle ( 1 ) which is made of a 56-membered ring and contains two 2,9-bis( p- biphenylyl)-1,10-phenanthroline ligands. Then we prepared the complexes [Cu( 1 )( 2 )] + (a catenate species), [Cu( 3 ) 2 ] +, [Cu( 2 )( 3 )] + (a threaded species), Re( 1 )(CO) 3 Cl, and Re( 3 )(CO) 3 Cl, where 2 is a 33-membered macrocycle containing a 2,9-di- p -phenyl-1,10-phenanthroline ligand and 3 is an acyclic ligand containing the same 2,9-bis( p- biphenylyl)-1,10-phenanthroline unit which is present in 1 . The absorption spectra of the free ligands in CH 2 Cl 2 solution show very intense bands in the near-UV region, characteristic of their aromatic moieties. In the Cu(I) complexes, besides the perturbed ligand-centered bands, weak metal-to-ligand charge-transfer (MLCT) bands can be observed in the 400−700 nm region. In the Re(I) complexes the MLCT bands lie at higher energy because the oxidation potential of Re(I) is much more positive than that of Cu(I). At room temperature, ligands 1 and 3, which contain an extensively conjugated aromatic unit, show a very strong (Φ = 0.89), short-lived (1.7 ns) fluorescence band with λ max = 419 nm. The fluorescence of 2 is blue shifted (λ max = 387 nm) and less intense (Φ = 0.29) because of the less extended conjugation and rigidity of its chromophoric unit. Besides fluorescence, also a long-lived (second time scale) phosphorescence is displayed by the three ligands in a rigid matrix at 77 K. At room temperature the [Cu( 1 )( 2 )] +, [Cu( 3 ) 2 ] +, and [Cu( 2 )( 3 )] + species exhibit a weak (Φ ∼ 10 - 3 ), relatively long-lived (10 2 ns time scale) emission with λ max around 750 nm, which can be assigned to a triplet metal-to-ligand charge-transfer ( 3 MLCT) excited state. This emission is slightly red shifted and considerably longer lived (τ ∼ 2 μs) at 77 K. The Re( 1 )(CO) 3 Cl and Re( 3 )(CO) 3 Cl complexes at room temperature show a moderately weak (Φ ∼ 10 - 2 ), long-lived (10 2 ns time scale) 3 MLCT emission with λ max around 610 nm. At 77 K this emission is blue shifted (λ max = 560 nm) and considerably longer lived (τ ∼ 10 μs). In the [Cu( 1 )( 2 )] + catenate and in the Re( 1 )(CO) 3 Cl complex one of the two chelating sites of the macrocycle 1 is coordinated to the metal and the other one is free. Besides a 3 MLCT emission, these species exhibit the fluorescence band of the 2,9-bis( p- biphenylyl)-1,10-phenanthroline unit, strongly quenched by the nearby metal-based chromophoric unit ( k q = 1.6 × 10 10 and 1.4 × 10 10 s - 1 for the Cu(I) and the Re(I) complex, respectively). In aerated solution all of the complexes sensitize the 1 Δ g (O 2 ) luminescence at 1269 nm.