Solution-Based Self-Assembly and Stability of Ruthenium(II) Tris-bipyridyl Monolayers on Gold

Abstract

Ruthenium(II) polypyridyl complexes are commonly applied as photosensitizers in the fields of artificial photosynthesis and light harvesting. Their immobilization on gold surfaces is also of interest for sensing and biological applications. Here, we report the self-assembly of [Ru-(dmbpy) 2 (dcbpy)](PF 6 ) 2 complexes on gold substrates from solution (dmbpy: 4,4′-dimethyl-2,2′-bipyridine; dcbpy: 2,2′-bipyridine-4,4′-dicarboxylic acid). Applying X-ray photoelectron spectroscopy, we demonstrate the formation of self-assembled monolayers (SAMs) of the Ru(II) complexes upon loss of counterions with carboxylate groups oriented toward the gold surface. We investigate the stability of the formed SAMs toward the substitution in solvents with competing aliphatic and aromatic thiols such as 4′-nitro[1,1′-biphenyl]-4-thiol, [1,1′-biphenyl]-4-thiol, and 1-hexadecanethiol. We show that the exchange reactions may lead to both complete replacement of the Ru(II) complexes and controlled formation of mixed SAMs. Moreover, we demonstrate that thiol-based SAMs can also be replaced completely from gold via their immersion into solutions of [Ru(dmbpy) 2 (dcbpy)](PF 6 ) 2 , indicating a relatively high stability for the Ru(II) complex SAMs. Our findings open up a variety of opportunities for applications of carboxylate-based SAMs on gold in nanotechnology.

Two‐Dimensional Photosensitizer Nanosheets via Low‐Energy Electron Beam Induced Cross‐Linking of Self‐Assembled RuII Polypyridine Monolayers

“…These characteristics of Ru II SAM 2 a correspond very well to the results of Ref. [10] where the self-assembly of complex 1 on gold was studied at various experimental conditions in detail. The low-energy electron beam irradiation induced cross-linking to Ru II CNM 2 b leads to several changes in the XP spectra.…”

“…Experimental details are presented as well in Scheme 1 and the Supporting Information. First, a SAM of the complex 1 [Ru(dmbpy) 2 (dcbpy)](PF 6 ) 2 (dmbpy: 4,4′‐dimethyl‐2,2′‐bipyridine; dcbpy: 2,2′‐bipyridine‐4,4′‐dicarboxylic acid) is formed by immersion of Au/mica substrates in a dimethylformamide (DMF) solution of complex 1 [10] . Formation of Ru II SAM 2 a takes place upon deprotonation of the carboxyl groups and their attachment to the substrate; by adjusting temperature, the layer thickness can be tuned [10] .…”

“…First, a SAM of the complex 1 [Ru(dmbpy) 2 (dcbpy)](PF 6 ) 2 (dmbpy: 4,4′‐dimethyl‐2,2′‐bipyridine; dcbpy: 2,2′‐bipyridine‐4,4′‐dicarboxylic acid) is formed by immersion of Au/mica substrates in a dimethylformamide (DMF) solution of complex 1 [10] . Formation of Ru II SAM 2 a takes place upon deprotonation of the carboxyl groups and their attachment to the substrate; by adjusting temperature, the layer thickness can be tuned [10] . As we report next, irradiation of Ru II SAM 2 a with low‐energy electrons (electron energy 50 eV, dose 50 mC cm −2 ) leads to the cross‐linking between adjacent molecules, as shown for small linear aromatic molecules, e.g., 4′‐nitro[1,1′‐biphenyl]‐4‐thiol ( 3 , NBPT) [11] and [1,1′‐biphenyl]‐4‐thiol (BPT) [12] .…”

“…[10] Formation of Ru II SAM 2 a takes place upon deprotonation of the carboxyl groups and their attachment to the substrate; by adjusting temperature, the layer thickness can be tuned. [10] As we report next, irradiation of Ru II SAM 2 a with low-energy electrons (electron energy 50 eV, dose 50 mC cm À 2 ) leads to the cross-linking between adjacent molecules, as shown for small linear aromatic molecules, e.g., 4'-nitro[1,1'-biphenyl]-4-thiol (3, NBPT) [11] and [1,1'biphenyl]-4-thiol (BPT). [12] In this way a continuous and mechanically stable nanosheet is formed (Ru II CNM 2 b) from the parent Ru II polypyridine complexes.…”

“…To improve the mechanical stability, we change the self-assembly conditions to increase the thickness of the initial SAM either by rising the self-assembly temperature to 50 °C (Ru II SAM 5 a) or by using mixed SAMs with NBPT 3 (mixed Ru II SAM 6 a). [10] According to the XPS analysis, the thickness of the formed nanosheets (Ru II CNM 5 b and Ru II CNM 6 b) on Au corresponds to � 1.2 nm (see Scheme 1). Figures 2e, f present SEM images of the Ru II CNMs 5 b and 6 b (see also Figures S2 and S3).…”