Assembling model tris(bipyridine)ruthenium(ii) photosensitizers into ordered monolayers in the presence of the polyoxometallate anion [Co4(H2O)2(α-PW9O34)2]10−

Abstract

The complexes cis-[Ru(1)₂Cl₂] and [Ru(1)₂(bpy)][PF₆]₂ in which 1 is dioctadecyl (2,2′-bipyridine)-4,4′-dicarboxylate have been synthesized and fully characterized; the single crystal structures of the syn,syn- and anti,anti-conformers of 1 have been determined. Pressure–area isotherms for monolayers of cis-[Ru(1)₂Cl₂] on water, aqueous Co₄POM (Co₄POM = K₁₀[Co₄(H₂O)₂(α-PW₉O₃₄)₂]) or aqueous KCl subphases exhibit collapse pressures of 25–27 mN m⁻¹ and mean molecular areas of 220 ± 10 Ų. The similarities between these isotherms confirm that there are no significant interactions between neutral cis-[Ru(1)₂Cl₂] and the anionic Co₄POM cluster as the monolayer is formed. In contrast, use of the cationic [Ru(1)₂(bpy)]²⁺ complex results in higher collapse pressures on pure water (54 mN m⁻¹) or aqueous KPF₆ (48 mN m⁻¹) subphases, but a collapse pressure of only 17 mN m⁻¹ on an aqueous Co₄POM subphase. The data are consistent with the monolayer forming at the air–Co₄POM interface being significantly less stable than that at the air–water interface, and point to substantial electrostatic interactions between [Ru(1)₂(bpy)]²⁺ and the anionic Co₄POM which can lead to a reduction in the integrity of the film. The introduction of DODA (DODA = dimethyldioctyldecylammonium bromide) stabilizes the monolayers on aqueous Co₄POM; mole ratios of [Ru(1)₂(bpy)][PF₆]₂–DODA of 1 : 5 and 1 : 20 lead to collapse pressures of 41 and 53 nM m⁻¹, respectively. Brewster angle microscopy has been used to image the monolayers and to monitor the effects of the presence of DODA. Langmuir–Blodgett (LB) films of cis-[Ru(1)₂Cl₂] and [Ru(1)₂(bpy)][PF₆]₂ with and without Co₄POM have been produced on mica substrates. Atomic force microscopy reveals that LB films formed in a single dipping cycle of cis-[Ru(1)₂Cl₂] from a water subphase are distinct from those formed on aqueous Co₄POM. The former consists of islands of height ≈3, 6 or 9 nm; these values compare with a modelled molecular diameter of cis-[Ru(1)₂Cl₂] of ≈3 nm and are consistent with the formation of mono-, bi-, or trilayers of cis-[Ru(1)₂Cl₂]. In contrast, LB films formed from cis-[Ru(1)₂Cl₂] on aqueous Co₄POM consist of small aggregates of variable height. LB films formed from [Ru(1)₂(bpy)][PF₆]₂ on aqueous subphase exhibit small aggregates but there is a very low surface coverage of the complex on mica (2 domains per μm²); the coverage increases (18 domains per μm²) when the films are formed in the presence of Co₄POM but is significantly lower than for cis-[Ru(1)₂Cl₂] (75 domains per μm²). No significant difference in the morphology of the LB films containing [Ru(1)₂(bpy)][PF₆]₂ is observed in the presence of DODA.