Protonation of free 2,9-bis(p-biphenylyl)-1,10-phenanthroline sites in a 56-membered macrocycle and in its Re-I and Cu-I complexes - Absorption spectra, luminescence properties, and excited state interactions

We have investigated the absorption and luminescence properties, and the H-1 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)(3)Cl 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)(3)Cl, 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 H-1 NMR spectrum of L-L is identical to that of the uncomplexed phenanthroline site of [(L-L)Cu(A)](+) and (L-L)Re(CO)(3)Cl. 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 Phi(em) = 0.89), short lived (tau = 1.7 ns) fluorescence band (lambda(max) = 419 ma) of the 2,9-bis(p-biphenylyl)-1,10-phenanthroline units, with the appearance of a weak (Phi(em) = 0.019), short lived (tau = 1.9 ns), strongly red-shifted (lambda(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 (lambda(max) = 538 nm, tau = 0.8 s), which can be observed on cooling the solution to 77 K, moves slightly to the red (lambda(max) = 571 nm, tau = 1.1 s) upon protonation. In the [(L-L)Cu(A)](+) and (L-L)Re(CO)(3)Cl complexes, protonation occurs at the free coordination site, as shown by the changes observed in the absorption and emission spectre. In the [(L . H+-L)Cu(A)](+) complex, the lowest triplet metal-to-ligand charge-transfer ((MLCT)-M-3) 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)(3)Cl, the lowest (MLCT)-M-3 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.