Absorption and emission properties of a 2-catenand, its protonated forms, and its complexes with Li+, Cu+, Ag+, Co2+, Ni2+, Zn2+, Pd2+ and Cd2+: tuning of the luminescence over the whole visible spectral region

Abstract

The absorption spectra (at 298 K), luminescence spectra and lifetimes (at 77 and 298 K) and luminescence quantum yields (at 298 K) of a catenand, its mono- and diprotonated forms, and its complexes with Li⁺, Cu⁺, Ag⁺, Co²⁺, Ni²⁺, Zn²⁺, Pd²⁺ and Cd²⁺ in CH₂Cl₂ solutions are obtained. The catenand (L) consists of two interlocked 30-membered rings, each one containing a 2,9-diphenyl-1,10-phenanthroline-type co-ordinating moiety. Upon addition of trifluoroacetic acid to CH₂Cl₂ solutions of L, significant changes in the absorption and luminescence properties are observed, that are fully reversed on addition of base. Comparison with the behaviour of a reference compound, 2,9-di(p-methoxyphenyl)-1,10-phenanthroline suggests that the catenand protonation does not occur at each phenanthroline-type moiety, but involves co-ordination of both moieties around one or two protons (in HL⁺ or H₂L²⁺ respectively). The free catenand shows ¹(ππ*) fluorescence (λ_max= 400 nm, τ= 2.0 ns, Φ= 0.42) at 298 K, and both ¹(ππ*) fluorescence (λ_max= 382 nm, τ= 2.2 ns) and ³(ππ*) phosphorescence (λ_max= 524 nm, τ= 0.79 s) in a rigid matrix at 77 K. The metal complexes of L are forced to have a tetrahedral-type co-ordination geometry because of the entwined arrangement of the two phenanthroline ligands. The only exception is for the Pd²⁺ complex in which orthometallation is observed to avoid a tetrahedral co-ordination. The complexes with Li⁺, Cd²⁺ and Zn²⁺ exhibit ligand-centred (l.c.) fluorescence and phosphorescence, this being considerably more perturbed from that of L for the divalent ions. The complexes of Co²⁺ and Ni²⁺ are not luminescent, as expected because of the presence of low energy metal-centred levels which offer a pathway to a fast radiationless decay of the l.c. levels. The copper(I) complex exhibits an emission band in the red spectral region (298 K: λ_max= 730 nm, τ= 175 ns, Φ= 0.0011; 77 K: λ_max= 690 nm, τ= 1.1 µs), that can be assigned to the lowest energy triplet metal-to-ligand charge transfer (m.l.c.t.) excited state. For the silver(I) complex no emission can be observed at room temperature, whereas in a rigid matrix at 77 K a very intense, long-lived band is present at about 498 nm (τ= 0.012 s) that can be assigned to the lowest ³l.c. level. The palladium(II) complex, in which the first co-ordination sphere of the metal ion is constituted by C^–-phenyl and two phenanthroline nitrogens of a diphenylphenanthroline unit and a nitrogen of the other diphenylphenanthroline unit, displays an emission band at 555 nm (τ= 3.0 × 10^–4 s) at 77 K that can be assigned to a perturbed ³m.l.c.t. level. The emission maxima of L, HL⁺ and H₂L²⁺ and its six luminescent metal complexes cover the range 378–730 nm, showing that it is possible to tune the luminescence of the L derivatives over the whole visible spectral region.