Complexes of substituted derivatives of 2-(2-pyridyl)benzimidazole with Re(I),RU(II)and Pt(II):structures,redox and luminescence properties

摘要

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)][PF_6]_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 CH_2Cl_2 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 'anthracene-> 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.