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Keywords:

  • Light-emitting devices;
  • Ruthenium complexes

Abstract

Two asymmetric, luminescent, bimetallic ruthenium trisbipyridine complexes with the general formula [Ru(bpy)3-ph4-Ru(bpy) L2](PF6)4 (bpy = 2,2′-bipyridine, ph = phenyl, L = 4,4′-di-n-undecyl-2,2′-bipyridine (1); 4,4′-di-non-1-enyl-2,2′-bipyridine (2)) have been synthesized and characterized. The introduction of two 4,4′-dialkyl-2,2′-bipyridine ligands on one of the ruthenium centers does not influence the electronic structure of the overall complexes to a large extent. Owing to the hydrophobic and hydrophilic nature of the two terminal metal complexes, the compounds 1 and 2 are expected to form Langmuir monolayers at the air/water interface. The film-forming properties of the amphiphilic complexes have been investigated by measuring surface-pressure–molecular-area (π–A) isotherms and recording Brewster-angle microscopy images. Complexes 1 and 2 were shown to form monolayer films at the air/water interface, which have subsequently been transferred to solid substrates using the Langmuir–Blodgett (LB) technique. The homogeneity of the resulting LB films has been investigated using atomic force microscopy and has been compared with that of LB films of the reference compound [Ru(bpy)3-ph4-Ru(bpy)3](PF6)4 (3), which lacks the alkyl chains. The presence of the hydrocarbon chains on one side of the rigid bimetallic complexes was shown to be a prerequisite for the formation of homogeneous monolayers, as with 3 only multilayer formation was obtained. Confocal laser scanning microscopy measurements proved that the LB films of complexes 1 and 2 display a homogeneous red emission upon photoexcitation. Such important results represent the first step towards the fabrication of mono- or few-molecular-layer electroluminescent devices.