Ultrathin Luminescent Films of Rigid Dinuclear Ruthenium(II) Trisbipyridine 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.

On the Effect of Coordination and Protonation Preferences in the Amphiphilic Behavior of Metallosurfactants with Asymmetric Headgroups

“…5.0 kcal mol -1 using a comparable level of theory. Similar values were obtained for 6 suggesting that the 3d 8 ion favors trans,fac coordination. This is similar to the results observed for 3d 7 ions, and in clear opposition to the preferred cis,fac observed for 3d 5 ions.…”

“…The ORTEP diagrams are displayed in Figure 2, and selected bond lengths and angles summarized in Table 1 [24] Finally, it is noteworthy that the ligand (L tBuI ) -in 5 is definitely an imine, as evidenced by the short bond length between C7-N1 (1.30 Å). Even though the ligand (L tBuODA ) -in 1 is more flexible than that in 5 and could possibly adopt a facial conformation in a tetrahedral coordination sphere, the preferential coordination modes of a low-spin 3d 8 ion like nickel(II) should favor a square-planar geometry. Therefore, 5 provides an acceptable model for the coordination environment for the surfactant 1.…”

“…The six-coordinate geometry and absorption spectra (vide infra) indicate the presence of a high spin 3d 8 ion. [23,26] On the basis of the structures of these two archetypes, an accurate picture of the coordination modes present in 2 can be drawn.…”

“…Assessment of the differences in coordination geometry revealed by mass spectrometry and elemental analysis was not possible. The square-planar 3d 8 low-spin (S = 0) nickel(II) complex 1 reveals its nature with a medium-intensity band at 422 nm, attributed to a 1 A 1g Ǟ 1 A 2g spin-allowed d-d transition. [34] The archetype 5 displays a similar, albeit slightly more intense, band at 425 nm, along with a ill-defined shoulder at 348 nm.…”

“…The approach has been used with impressive results in the design of soft materials with tixothropic, [5] mesogenic, [6] ionophoric, [7] luminescent, [8] and micellar [9] properties. This fact points out to the potential role of discrete metallosurfactant materials in the emerging field of responsive thin films.…”