Peripheral halo-functionalization in [Cu(NN)(PP)](+) emitters: influence on the performances of light-emitting electrochemical cells

A series of heteroleptic [Cu(N<^>N)(P<^>P)][PF6] complexes is described in which P<^>P = bis(2-(diphenylphosphino) phenyl) ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N<^>N = 4,4'-diphenyl-6,6'-dimethyl-2,2'-bipyridine substituted in the 4-position of the phenyl groups with atom X (N<^>N = 1 has X = F, 2 has X = Cl, 3 has X = Br, 4 has X = I; the benchmark N<^>N ligand with X = H is 5). These complexes have been characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analyses and cyclic voltammetry; representative single crystal structures are also reported. The solution absorption spectra are characterized by high energy bands (arising from ligand-centred transitions) which are red-shifted on going from X = H to X = I, and a broad metal-to-ligand charge transfer band with lambda(max) in the range 387-395 nm. The ten complexes are yellow emitters in solution and yellow or yellow-orange emitters in the solid-state. For a given N<^>N ligand, the solution photoluminescence (PL) spectra show no significant change on going from [Cu(N<^>N)(POP)](+) to [Cu(N<^>N)(xantphos)](+); introducing the iodo-functionality into the N<^>N domain leads to a red-shift in lambda(max) em compared to the complexes with the benchmark N<^>N ligand 5. In the solid state, [Cu(1)(POP)][PF6] and [Cu(1)(xantphos)][PF6] (fluoro-substituent) exhibit the highest PL quantum yields (74 and 25%, respectively) with values of tau(1/2) = 11.1 and 5.8 mu s, respectively. Light-emitting electrochemical cells (LECs) with [Cu(N<^>N)(P<^>P)][PF6] complexes in the emissive layer have been tested. Using a block-wave pulsed current driving mode, the best performing device employed [Cu(1)(xantphos)](+) and this showed a maximum luminance (Lum(max)) of 129 cd m(-2) and a device lifetime (t(1/2)) of 54 h; however, the turn-on time (time to reach Lum(max)) was 4.1 h. Trends in performance data reveal that the introduction of fluoro-groups is beneficial, but that the incorporation of heavier halo-substituents leads to poor devices, probably due to a detrimental effect on charge transport; LECs with the iodo-functionalized N<^>N ligand 4 failed to show any electroluminescence after 50 h.