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

Abstract

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)3Cl, and Re(3)(CO)3Cl, 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 CH2Cl2 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 λmax = 419 nm. The fluorescence of 2 is blue shifted (λmax = 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 (102 ns time scale) emission with λmax around 750 nm, which can be assigned to a triplet metal-to-ligand charge-transfer (3MLCT) excited state. This emission is slightly red shifted and considerably longer lived (τ ∼ 2 μs) at 77 K. The Re(1)(CO)3Cl and Re(3)(CO)3Cl complexes at room temperature show a moderately weak (Φ ∼ 10-2), long-lived (102 ns time scale) 3MLCT emission with λmax around 610 nm. At 77 K this emission is blue shifted (λmax = 560 nm) and considerably longer lived (τ ∼ 10 μs). In the [Cu(1)(2)]+ catenate and in the Re(1)(CO)3Cl complex one of the two chelating sites of the macrocycle 1 is coordinated to the metal and the other one is free. Besides a 3MLCT 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 (kq = 1.6 × 1010 and 1.4 × 1010 s-1 for the Cu(I) and the Re(I) complex, respectively). In aerated solution all of the complexes sensitize the 1Δg(O2) luminescence at 1269 nm.