Complexes of substituted derivatives of 2-(2-pyridyl)benzimidazole with Re(i), Ru(ii) and Pt(ii): structures, redox and luminescence properties

Abstract

N,N′-Chelating ligands based on the 2-(2-pyridyl)benzimidazole (PB) core have been prepared with a range of substituents (phenyl, pentafluorophenyl, naphthyl, anthracenyl, pyrenyl) connected to the periphery via alkylation of the benzimidazolyl unit at one of the N atoms. These PB ligands have been used to prepare a series of complexes of the type [Re(PB)(CO)₃Cl], [Pt(PB)(CCR)₂] (where –CCR is an acetylide ligand) and [Ru(bpy)₂(PB)][PF₆]₂ (bpy = 2,2′-bipyridine). Six of the complexes have been structurally characterised. Electrochemical and luminescence studies show that all three series of complexes behave in a similar manner to the analogous complexes with 2,2′-bipyridine in place of PB. In particular, all three series of complexes show luminescence in the range 553–605 nm (Pt series), 620–640 nm (Re series) and 626–645 nm (Ru series) arising from the ³MLCT state, with members of the Pt(II) series being the most strongly emissive with lifetimes of up to 500 ns and quantum yields of up to 6% in air-saturated CH₂Cl₂ at room temperature. In the Re and Ru series there was clear evidence for inter-component energy-transfer processes in both directions between the ³MLCT state of the metal centre and the singlet and triplet states of the pendant organic luminophores (naphthalene, pyrene, anthracene). For example the pyrene singlet is almost completely quenched by energy transfer to a Re-based MLCT excited state, which in turn is completely quenched by energy transfer to the lower-lying pyrene triplet state. For the analogous Ru(II) complexes the inter-component energy transfer is less effective, with ¹anthracene → Ru(³MLCT) energy transfer being absent, and Ru(³MLCT) → ³anthracene energy transfer being incomplete. This is rationalised on the basis of a greater effective distance for energy transfer in the Ru(II) series, because the MLCT excited states are localised on the bpy ligands which are remote from the pendant aromatic group; in the Re series in contrast, the MLCT excited states involve the PB ligand to which the pendant aromatic group is directly attached, giving more efficient energy transfer.