Thin films of a dimeric ruthenium phthalocyanine complex on gold

Abstract

a b s t r a c tThin films of a new dimeric ruthenium phthalocyanine complex bearing a thioester-functionalized axial ligand were formed on gold surfaces. Characterization of the thin films by laser ablation-inductively coupled-mass spectrometry and scanning tunneling microscopy revealed that the films do not have any long-range order.Ó 2009 Elsevier B.V. All rights reserved. Ruthenium phthalocyanine (PcRu) complexes have been the focus of sustained interest since the first reported synthesis in 1963 [1]. However, only a handful of examples of dimeric PcRu complexes have been reported. The simplest PcRu dimer is that of pure ruthenium phthalocyanine, [(PcRu) 2 ] [2-6]. Other examples include a nitrido-bridged dimer, [(PcRu) 2 N] [3], a l-carbido dimer, [{PcRu} 2 (l-C)] [7,8] and an oxo-bridged dimer, [{(NaSO 3 ) 4 PcRuO-H} 2 O]Á8H 2 O [9] while a Ru@Ru double bond was identified in the dimeric species [({2,9,16, 4 Pc}Ru(iso-quinoline)) 2 ] [10]. In addition, thin films of PcRu complexes on surfaces have been generating increased interest [1,11] with applications in areas such as photovoltaics [12][13][14][15] and sensors [16]. We have recently reported that ruthenium phthalocyanine complexes bearing axial ligands with thioacetate groups can form thin films on gold surfaces [17]. The peripheral and axial ligand substituents on the complexes had a significant effect on their surface coverage and ordering. For example, thin films of a PcRu complex bearing no peripheral substituents contained ordered regions but no ordering was apparent in films of a complex where the Pc ring was substituted with four peripheral tertiary butyl groups. In each case, however, the complexes contained only a single PcRu unit. Here, we describe the synthesis and characterization of a new PcRu dimer with a thioacetate functional group that allows binding of the complex to gold surfaces.The synthesis of the new dimeric complex, 2, is shown in Scheme 1 (see Supplementary material for details of the synthesis and characterization). The first step involves the synthesis of the new axially unsymmetrical PcRu complex, 1, that bears a pyridine ligand functionalized with a thioacetate group to enable binding to gold surfaces, and a pyrazine ligand to enable coordination to a second PcRu unit. Complex 1 was prepared by the axial ligand exchange reaction of [PcRu(PhCN) 2 ] [18] with the pyridine and pyrazine ligands. Both ligands were added at the same time and so the reaction resulted in a mixture of three complexes; the bis-S-(pyridin-4-ylmethyl)ethanethioate and bis-pyrazine complexes, as well as complex 1. The individual complexes were subsequently separated using silica gel column chromatography. A 10-fold excess of the axial ligands was used to minimize the degree of polymerization that can occur when using pyrazine as an axial ligand, which may coordinate through both nitrogen atoms and has been employed as a bridging ligand in ruthenium phthalocyanine oligomers [2,19]. To prepare complex 2, complex 1 was used as a ligand and the same axial li...