Ultrathin luminescent films of rigid dinuclear ruthenium(II) trisbipyridine complexes

Two asymmetric, luminescent, bimetallic ruthenium trisbipyridine complexes with the general formula [Ru(bpy)3-ph4-Ru(bpy) L 2](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 imagea 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.