Protonation of free 2,9-bis(p-biphenylyl)-1,10-phenanthrolinesites in a 56-membered macrocycle and in its ReIand CuIcomplexes Absorption spectra, luminescence properties, and excited state interactions

Abstract

We have investigated the absorption and luminescence properties, and the ¹H NMR spectra of (i) a macrocycle (L–L) made of a 56-membered ring incorporating two 2,9-bis(p-biphenylyl)-1,10-phenanthroline units and (ii) its metal complexes (L–L)Re(CO)₃Cl and [(L–L)Cu(A)]⁺, where the latter is a catenate species (A is a 33-membered macrocycle containing a 2,9-p-biphenylyl-1,10-phenanthroline unit). The free macrocycle and its complexes contain, respectively, two and one free phenanthroline sites which can be protonated. Addition of trifluoroacetic acid to CH₂Cl₂ solutions of L–L, (L–L)Re(CO)₃Cl, and [(L–L)Cu(A)]⁺ causes strong changes in the absorption spectra, with maintenance of the same family of isosbestic points in all cases. The ¹H NMR spectrum of L–L is identical to that of the uncomplexed phenanthroline site of [(L–L)Cu(A)]⁺ and (L–L)Re(CO)₃Cl. Such absorption and NMR data show that the two phenanthroline units of L–L do not interact in the ground state. For this macrocycle, the changes in the absorption spectrum upon protonation are accompanied by the disappearance of the very strong (emission quantum yield Φ_em = 0.89), short lived (τ = 1.7 ns) fluorescence band (λ_max = 419 nm) of the 2,9-bis(p-biphenylyl)-1,10-phenanthroline units, with the appearance of a weak (Φ_em = 0.019), short lived (τ = 1.9 ns), strongly red-shifted (λ_max = 572 nm) band. The results obtained show that the two equivalent basic sites of L–L are independently protonated. In the monoprotonated L · H⁺–L species, energy transfer from the unprotonated to the protonated site occurs. The phosphorescence band of L–L (λ_max = 538 nm, τ = 0.8 s), which can be observed on cooling the solution to 77 K, moves slightly to the red (λ_max = 571 nm, τ = 1.1 s) upon protonation. In the [(L–L)Cu(A)]⁺ and (L–L)Re(CO)₃Cl complexes, protonation occurs at the free coordination site, as shown by the changes observed in the absorption and emission spectra. In the [(L · H⁺–L)Cu(A)]⁺ complex, the lowest triplet metal-to-ligand charge-transfer (³MLCT) level of the metal-based moiety lies below the lowest singlet excited state and the lowest triplet excited state of the protonated moiety. Therefore, both the fluorescence and (at 77 K) the phosphorescence of the protonated moiety are, at least in part, quenched. In Re(L · H⁺–L)(CO)₃Cl, the lowest ³MLCT of the Re-based moiety is very close in energy to the fluorescent and phosphorescent levels of the protonated moiety with a consequent more complex luminescence behaviour.